MyPlate, a revised set of dietary guidelines introduced by the U.S. Department of Agriculture in 2011, divides the four basic food groups (fruits, grains, protein, and vegetables) into sections on a plate, with the size of each section representing the relative dietary proportions of each food group. The small blue circle shown at the upper right illustrates the inclusion and recommended proportion of dairy products in the diet.
Human nutrition, process by which substances in food are transformed into body tissues and provide energy for the full range of physical and mental activities that make up human life.
In a style that reflects the culinary traditions of China, the Food Guide Pagoda recommends a liberal daily intake of grain products (represented by the wide base of the pagoda) and a sparing intake of fats and oils (represented by the tip of the structure).
The study of human nutrition is interdisciplinary in character, involving not only physiology, biochemistry, and molecular biology but also fields such as psychology and anthropology, which explore the influence of attitudes, beliefs, preferences, and cultural traditions on food choices. Human nutrition further touches on economics and political science as the world community recognizes and responds to the suffering and death caused by malnutrition. The ultimate goal of nutritional science is to promote optimal health and reduce the risk of chronic diseases such as cardiovascular disease and cancer as well as to prevent classic nutritional deficiency diseases such as kwashiorkor and pellagra.
This article covers the major issues of human nutrition, such as energy generation and balance, essential nutrients, and recommended dietary guidelines. For a full-length treatment of health problems created by failure in nutrition, see nutritional disease. The utilization of food materials by all living things is described in nutrition, and specific biochemical processes are described in metabolism.
Utilization of food by the body
Calories and kilocalories: energy supplyThe human body can be thought of as an engine that releases the energy present in the foods that it digests. This energy is utilized partly for the mechanical work performed by the muscles and in the secretory processes and partly for the work necessary to maintain the body’s structure and functions. The performance of work is associated with the production of heat; heat loss is controlled so as to keep body temperature within a narrow range. Unlike other engines, however, the human body is continually breaking down (catabolizing) and building up (anabolizing) its component parts. Foods supply nutrients essential to the manufacture of the new material and provide energy needed for the chemical reactions involved.
Carbohydrate, fat, and protein are, to a large extent, interchangeable as sources of energy. Typically, the energy provided by food is measured in kilocalories, or Calories. One kilocalorie is equal to 1,000 gram-calories (or small calories), a measure of heat energy. However, in common parlance, kilocalories are referred to as “calories.” In other words, a 2,000-calorie diet actually has 2,000 kilocalories of potential energy. One kilocalorie is the amount of heat energy required to raise one kilogram of water from 14.5 to 15.5 °C at one atmosphere of pressure. Another unit of energy widely used is the joule, which measures energy in terms of mechanical work. One joule is the energy expended when one kilogram is moved a distance of one metre by a force of one newton. The relatively higher levels of energy in human nutrition are more likely to be measured in kilojoules (1 kilojoule = 103 joules) or megajoules (1 megajoule = 106 joules). One kilocalorie is equivalent to 4.184 kilojoules.
The energy provided by a well-digested food can be estimated if the gram amounts of energy-yielding substances (non-fibre carbohydrate, fat, protein, and alcohol) in that food are known. For example, a slice of white bread containing 12 grams of carbohydrate, 2 grams of protein, and 1 gram of fat supplies 67 kilocalories (280 kilojoules) of energy. Food composition tables (see table) and food labels provide useful data for evaluating energy and nutrient intake of an individual diet. Most foods provide a mixture of energy-supplying nutrients, along with vitamins, minerals, water, and other substances. Two notable exceptions are table sugar and vegetable oil, which are virtually pure carbohydrate (sucrose) and fat, respectively.
BMR and REE: energy balanceEnergy is needed not only when a person is physically active but even when the body is lying motionless. Depending on an individual’s level of physical activity, between 50 and 80 percent of the energy expended each day is devoted to basic metabolic processes (basal metabolism), which enable the body to stay warm, breathe, pump blood, and conduct numerous physiological and biosynthetic activities, including synthesis of new tissue in growing children and in pregnant and lactating women. Digestion and subsequent processing of food by the body also uses energy and produces heat. This phenomenon, known as the thermic effect of food (or diet-induced thermogenesis), accounts for about 10 percent of daily energy expenditure, varying somewhat with the composition of the diet and prior dietary practices. Adaptive thermogenesis, another small but important component of energy expenditure, reflects alterations in metabolism due to changes in ambient temperature, hormone production, emotional stress, or other factors. Finally, the most variable component in energy expenditure is physical activity, which includes exercise and other voluntary activities as well as involuntary activities such as fidgeting, shivering, and maintaining posture. Physical activity accounts for 20 to 40 percent of the total energy expenditure, even less in a very sedentary person and more in someone who is extremely active.
Basal or resting energy expenditure is correlated primarily with lean body mass (fat-free mass and essential fat, excluding storage fat), which is the metabolically active tissue in the body. At rest, organs such as the liver, brain, heart, and kidney have the highest metabolic activity and, therefore, the highest need for energy, while muscle and bone require less energy, and body fat even less. Besides body composition, other factors affecting basal metabolism include age, sex, body temperature, and thyroid hormone levels.
The basal metabolic rate (BMR), a precisely defined measure of the energy expenditure necessary to support life, is determined under controlled and standardized conditions—shortly after awakening in the morning, at least 12 hours after the last meal, and with a comfortable room temperature. Because of practical considerations, the BMR is rarely measured; the resting energy expenditure (REE) is determined under less stringent conditions, with the individual resting comfortably about 2 to 4 hours after a meal. In practice, the BMR and REE differ by no more than 10 percent—the REE is usually slightly higher—and the terms are used interchangeably.
Energy expenditure can be assessed by direct calorimetry, or measurement of heat dissipated from the body, which employs apparatuses such as water-cooled garments or insulated chambers large enough to accommodate a person. However, energy expenditure is usually measured by the less cumbersome techniques of indirect calorimetry, in which heat produced by the body is calculated from measurements of oxygen inhaled, carbon dioxide exhaled, and urinary nitrogen excreted. The BMR (in kilocalories per day) can be roughly estimated using the following formula: BMR = 70 × (body weight in kilograms)3/4.
The energy costs of various activities have been measured (see table). While resting may require as little as 1 kilocalorie per minute, strenuous work may demand 10 times that much. Mental activity, though it may seem taxing, has no appreciable effect on energy requirement. A 70-kg (154-pound) man, whose REE over the course of a day might be 1,750 kilocalories, could expend a total of 2,400 kilocalories on a very sedentary day and up to 4,000 kilocalories on a very active day. A 55-kg (121-pound) woman, whose daily resting energy expenditure might be 1,350 kilocalories, could use from 1,850 to more than 3,000 total kilocalories, depending on level of activity.
The law of conservation of energy applies: If one takes in more energy than is expended, over time one will gain weight; insufficient energy intake results in weight loss, as the body taps its energy stores to provide for immediate needs. Excess food energy is stored in small amounts as glycogen, a short-term storage form of carbohydrate in muscle and liver, and as fat, the body’s main energy reserve found in adipose tissue. Adipose tissue is mostly fat (about 87 percent), but it also contains some protein and water. In order to lose 454 grams (one pound) of adipose tissue, an energy deficit of about 3,500 kilocalories (14.6 megajoules) is required.
Body mass, body fat, and body waterThe human body consists of materials similar to those found in foods; however, the relative proportions differ, according to genetic dictates as well as to the unique life experience of the individual. The body of a healthy lean man is composed of roughly 62 percent water, 16 percent fat, 16 percent protein, 6 percent minerals, and less than 1 percent carbohydrate, along with very small amounts of vitamins and other miscellaneous substances. Females usually carry more fat (about 22 percent in a healthy lean woman) and slightly less of the other components than do males of comparable weight.
The body’s different compartments—lean body mass, body fat, and body water—are constantly adjusting to changes in the internal and external environment so that a state of dynamic equilibrium (homeostasis) is maintained. Tissues in the body are continuously being broken down (catabolism) and built up (anabolism) at varying rates. For example, the epithelial cells lining the digestive tract are replaced at a dizzying speed of every three or four days, while the life span of red blood cells is 120 days, and connective tissue is renewed over the course of several years.
Although estimates of the percentage of body fat can be made by direct inspection, this approach is imprecise. Body fat can be measured indirectly using fairly precise but costly methods, such as underwater weighing, total body potassium counting, and dual-energy X-ray absorptiometry (DXA). However, more practical, albeit less accurate, methods are often used, such as anthropometry, in which subcutaneous fat at various sites is measured using skinfold calipers; bioelectrical impedance, in which resistance to a low-intensity electrical current is used to estimate body fat; and near infrared interactance, in which an infrared light aimed at the biceps is used to assess fat and protein interaction. Direct measurement of the body’s various compartments can only be performed on cadavers.
The composition of the body tends to change in somewhat predictable ways over the course of a lifetime—during the growing years, in pregnancy and lactation, and as one ages—with corresponding changes in nutrient needs during different phases of the life cycle (see the section Nutrition throughout the life cycle). Regular physical exercise can help attenuate the age-related loss of lean tissue and increase in body fat.
Essential nutrientsThe six classes of nutrients found in foods are carbohydrates, lipids (mostly fats and oils), proteins, vitamins, minerals, and water. Carbohydrates, lipids, and proteins constitute the bulk of the diet, amounting together to about 500 grams (just over one pound) per day in actual weight. These macronutrients provide raw materials for tissue building and maintenance as well as fuel to run the myriad of physiological and metabolic activities that sustain life. In contrast are the micronutrients, which are not themselves energy sources but facilitate metabolic processes throughout the body: vitamins, of which humans need about 300 milligrams per day in the diet, and minerals, of which about 20 grams per day are needed. The last nutrient category is water, which provides the medium in which all the body’s metabolic processes occur.
A nutrient is considered “essential” if it must be taken in from outside the body—in most cases, from food. (See table.) These nutrients are discussed in this section. Although they are separated into categories for purposes of discussion, one should keep in mind that nutrients work in collaboration with each other in the body, not as isolated entities.
CarbohydratesCarbohydrates, which are composed of carbon, hydrogen, and oxygen, are the major supplier of energy to the body, providing 4 kilocalories per gram. In most carbohydrates, the elements hydrogen and oxygen are present in the same 2:1 ratio as in water, thus “carbo” (for carbon) and “hydrate” (for water).
GlucoseThe simple carbohydrate glucose is the principal fuel used by the brain and nervous system and by red blood cells. Muscle and other body cells can also use glucose for energy, although fat is often used for this purpose. Because a steady supply of glucose is so critical to cells, blood glucose is maintained within a relatively narrow range through the action of various hormones, mainly insulin, which directs the flow of glucose into cells, and glucagon and epinephrine, which retrieve glucose from storage. The body stores a small amount of glucose as glycogen, a complex branched form of carbohydrate, in liver and muscle tissue, and this can be broken down to glucose and used as an energy source during short periods (a few hours) of fasting or during times of intense physical activity or stress. If blood glucose falls below normal (hypoglycemia), weakness and dizziness may result. Elevated blood glucose (hyperglycemia), as can occur in diabetes, is also dangerous and cannot be left untreated.
Glucose can be made in the body from most types of carbohydrate and from protein, although protein is usually an expensive source of energy. Some minimal amount of carbohydrate is required in the diet—at least 50 to 100 grams a day. This not only spares protein but also ensures that fats are completely metabolized and prevents a condition known as ketosis, the accumulation of products of fat breakdown, called ketones, in the body. Although there are great variations in the quantity and type of carbohydrates eaten throughout the world, most diets contain more than enough.
Other sugars and starchThe simplest carbohydrates are sugars, which give many foods their sweet taste but at the same time provide food for bacteria in the mouth, thus contributing to dental decay. Sugars in the diet are monosaccharides, which contain one sugar or saccharide unit, and disaccharides, which contain two saccharide units linked together. Monosaccharides of nutritional importance are glucose, fructose, and galactose; disaccharides include sucrose (table sugar), lactose (milk sugar), and maltose. A slightly more complex type of carbohydrate is the oligosaccharide (e.g., raffinose and stachyose), which contains three to 10 saccharide units; these compounds, which are found in beans and other legumes and cannot be digested well by humans, account for the gas-producing effects of these foods. Larger and more complex storage forms of carbohydrate are the polysaccharides, which consist of long chains of glucose units. Starch, the most important polysaccharide in the human diet—found in grains, legumes, potatoes, and other vegetables—is made up of mainly straight glucose chains (amylose) or mainly branching chains (amylopectin). Finally, nondigestible polysaccharides known as dietary fibre are found in plant foods such as grains, fruits, vegetables, legumes, seeds, and nuts.
In order to be utilized by the body, all complex carbohydrates must be broken down into simple sugars, which, in turn, must be broken down into monosaccharides—a feat, accomplished by enzymes, that starts in the mouth and ends in the small intestine, where most absorption takes place. Each dissacharide is split into single units by a specific enzyme; for example, the enzyme lactase breaks down lactose into its constituent monosaccharides, glucose and galactose. In much of the world’s population, lactase activity declines during childhood and adolescence, which leads to an inability to digest lactose adequately. This inherited trait, called lactose intolerance, results in gastrointestinal discomfort and diarrhea if too much lactose is consumed. Those who have retained the ability to digest dairy products efficiently in adulthood are primarily of northern European ancestry.
Dietary fibreDietary fibre, the structural parts of plants, cannot be digested by the human intestine because the necessary enzymes are lacking. Even though these nondigestible compounds pass through the gut unchanged (except for a small percentage that is fermented by bacteria in the large intestine), they nevertheless contribute to good health. Insoluble fibre does not dissolve in water and provides bulk, or roughage, that helps with bowel function (regularity) and accelerates the exit from the body of potentially carcinogenic or otherwise harmful substances in food. Types of insoluble fibre are cellulose, most hemicelluloses, and lignin (a phenolic polymer, not a carbohydrate). Major food sources of insoluble fibre are whole grain breads and cereals, wheat bran, and vegetables. Soluble fibre, which dissolves or swells in water, slows down the transit time of food through the gut (an undesirable effect) but also helps lower blood cholesterol levels (a desirable effect). Types of soluble fibre are gums, pectins, some hemicelluloses, and mucilages; fruits (especially citrus fruits and apples), oats, barley, and legumes are major food sources. Both soluble and insoluble fibre help delay glucose absorption, thus ensuring a slower and more even supply of blood glucose. Dietary fibre is thought to provide important protection against some gastrointestinal diseases and to reduce the risk of other chronic diseases as well. (See nutritional disease.)
LipidsLipids also contain carbon, hydrogen, and oxygen but in a different configuration, having considerably fewer oxygen atoms than are found in carbohydrates. Lipids are soluble in organic solvents (such as acetone or ether) and insoluble in water, a property that is readily seen when an oil-and-vinegar salad dressing separates quickly upon standing. The lipids of nutritional importance are triglycerides (fats and oils), phospholipids (e.g., lecithin), and sterols (e.g., cholesterol). Lipids in the diet transport the four fat-soluble vitamins (vitamins A, D, E, and K) and assist in their absorption in the small intestine. They also carry with them substances that impart sensory appeal and palatability to food and provide satiety value, the feeling of being full and satisfied after eating a meal. Fats in the diet are a more concentrated form of energy than carbohydrates and have an energy yield of 9 kilocalories per gram. Adipose (fatty) tissue in the fat depots of the body serves as an energy reserve as well as helping to insulate the body and cushion the internal organs.
TriglyceridesThe major lipids in food and stored in the body as fat are the triglycerides, which consist of three fatty acids attached to a backbone of glycerol (an alcohol). Fatty acids are essentially hydrocarbon chains with a carboxylic acid group (COOH) at one end, the alpha (α) end, and a methyl group (CH3) at the other, omega (ω), end. They are classified as saturated or unsaturated according to their chemical structure. A point of unsaturation indicates a double bond between two carbon atoms, rather than the full complement of hydrogen atoms that is present in saturated fatty acids. A monounsaturated fatty acid has one point of unsaturation, while a polyunsaturated fatty acid has two or more.
The common fatty acids in foods are listed in the table. Fatty acids found in the human diet and in body tissues range from a chain length of 4 carbons to 22 or more, each chain having an even number of carbon atoms. Of particular importance for humans are the 18-carbon polyunsaturated fatty acids alpha-linolenic acid (an omega-3 fatty acid) and linoleic acid (an omega-6 fatty acid); these are known as essential fatty acids because they are required in small amounts in the diet. The omega designations (also referred to as n-3 and n-6) indicate the location of the first double bond from the methyl end of the fatty acid. Other fatty acids can be synthesized in the body and are therefore not essential in the diet. About a tablespoon daily of an ordinary vegetable oil such as safflower or corn oil or a varied diet that includes grains, nuts, seeds, and vegetables can fulfill the essential fatty acid requirement. Essential fatty acids are needed for the formation of cell membranes and the synthesis of hormone-like compounds called eicosanoids (e.g., prostaglandins, thromboxanes, and leukotrienes), which are important regulators of blood pressure, blood clotting, and the immune response. The consumption of fish once or twice a week provides an additional source of omega-3 fatty acids that appears to be healthful.
A fat consisting largely of saturated fatty acids, especially long-chain fatty acids, tends to be solid at room temperature; if unsaturated fatty acids predominate, the fat is liquid at room temperature. Fats and oils usually contain mixtures of fatty acids, although the type of fatty acid in greatest concentration typically gives the food its characteristics. Butter and other animal fats are primarily saturated; olive and canola oils, monounsaturated; and fish, corn, safflower, soybean, and sunflower oils, polyunsaturated. Although plant oils tend to be largely unsaturated, there are notable exceptions, such as coconut fat, which is highly saturated but nevertheless semiliquid at room temperature because its fatty acids are of medium chain length (8 to 14 carbons long).Saturated fats tend to be more stable than unsaturated ones. The food industry takes advantage of this property during hydrogenation, in which hydrogen molecules are added to a point of unsaturation, thereby making the fatty acid more stable and resistant to rancidity (oxidation) as well as more solid and spreadable (as in margarine). However, a result of the hydrogenation process is a change in the shape of some unsaturated fatty acids from a configuration known as cis to that known as trans. Trans-fatty acids, which behave more like saturated fatty acids, may also have undesirable health consequences.
PhospholipidsA phospholipid is similar to a triglyceride except that it contains a phosphate group and a nitrogen-containing compound such as choline instead of one of the fatty acids. In food, phospholipids are natural emulsifiers, allowing fat and water to mix, and they are used as food additives for this purpose. In the body, phospholipids allow fats to be suspended in fluids such as blood, and they enable lipids to move across cell membranes from one watery compartment to another. The phospholipid lecithin is plentiful in foods such as egg yolks, liver, wheat germ, and peanuts. However, the liver is able to synthesize all the lecithin the body needs if sufficient choline is present in the diet.
SterolsSterols are unique among lipids in that they have a multiple-ring structure. The well-known sterol cholesterol is found only in foods of animal origin—meat, egg yolk, fish, poultry, and dairy products. Organ meats (e.g., liver, kidney) and egg yolks have the most cholesterol, while muscle meats and cheeses have less. There are a number of sterols in shellfish but not as much cholesterol as was once thought. Cholesterol is essential to the structure of cell membranes and is also used to make other important sterols in the body, among them the sex hormones, adrenal hormones, bile acids, and vitamin D. However, cholesterol can be synthesized in the liver, so there is no need to consume it in the diet.
Cholesterol-containing deposits may build up in the walls of arteries, leading to a condition known as atherosclerosis, which contributes to myocardial infarction (heart attack) and stroke. Furthermore, because elevated levels of blood cholesterol, especially the form known as low-density lipoprotein (LDL) cholesterol, have been associated with an increased risk of cardiovascular disease, a limited intake of saturated fat—particularly medium-chain saturated fatty acids, which act to raise LDL cholesterol levels—is advised. Trans-fatty acids also raise LDL cholesterol, while monounsaturated and polyunsaturated (cis) fats tend to lower LDL cholesterol levels. Because of the body’s feedback mechanisms, dietary cholesterol has only a minor influence on blood cholesterol in most people; however, since some individuals respond strongly to cholesterol in the diet, a restricted intake is often advised, especially for those at risk of heart disease. The complex relationships between various dietary lipids and blood cholesterol levels, as well as the possible health consequences of different dietary lipid patterns, are discussed in the article nutritional disease.
ProteinsProteins, like carbohydrates and fats, contain carbon, hydrogen, and oxygen, but they also contain nitrogen, a component of the amino chemical group (NH2), and in some cases sulfur. Proteins serve as the basic structural material of the body as well as being biochemical catalysts and regulators of genes. Aside from water, protein constitutes the major part of muscles, bones, internal organs, and the skin, nails, and hair. Protein is also an important part of cell membranes and blood (e.g., hemoglobin). Enzymes, which catalyze chemical reactions in the body, are also protein, as are antibodies, collagen in connective tissue, and many hormones, such as insulin.
Tissues throughout the body require ongoing repair and replacement, and thus the body’s protein is turning over constantly, being broken down and then resynthesized as needed. Tissue proteins are in a dynamic equilibrium with proteins in the blood, with input from proteins in the diet and losses through urine, feces, and skin. In a healthy adult, adjustments are made so that the amount of protein lost is in balance with the amount of protein ingested. However, during periods of rapid growth, pregnancy and lactation, or recuperation after illness or depletion, the body is in positive nitrogen balance, as more protein is being retained than excreted. The opposite is true during illness or wasting, when there is negative nitrogen balance as more tissue is being broken down than synthesized.
Amino acidsThe proteins in food—such as albumin in egg white, casein in dairy products, and gluten in wheat—are broken down during digestion into constituent amino acids, which, when absorbed, contribute to the body’s metabolic pool. Amino acids are then joined via peptide linkages to assemble specific proteins, as directed by the genetic material and in response to the body’s needs at the time. Each gene makes one or more proteins, each with a unique sequence of amino acids and precise three-dimensional configuration. Amino acids are also required for the synthesis of other important nonprotein compounds, such as peptide hormones, some neurotransmitters, and creatine.
Food contains approximately 20 common amino acids, 9 of which are considered essential, or indispensable, for humans; i.e., they cannot be synthesized by the body or cannot be synthesized in sufficient quantities and therefore must be taken in the diet. The essential amino acids for humans are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Conditionally indispensable amino acids include arginine, cysteine, and tyrosine, which may need to be provided under special circumstances, such as in premature infants or in people with liver disease, because of impaired conversion from precursors.
The relative proportions of different amino acids vary from food to food (see table). Foods of animal origin—meat, fish, eggs, and dairy products—are sources of good quality, or complete, protein; i.e., their essential amino acid patterns are similar to human needs for protein. (Gelatin, which lacks the amino acid tryptophan, is an exception.) Individual foods of plant origin, with the exception of soybeans, are lower quality, or incomplete, protein sources. Lysine, methionine, and tryptophan are the primary limiting amino acids; i.e., they are in smallest supply and therefore limit the amount of protein that can be synthesized. However, a varied vegetarian diet can readily fulfill human protein requirements if the protein-containing foods are balanced such that their essential amino acids complement each other. For example, legumes such as beans are high in lysine and low in methionine, while grains have complementary strengths and weaknesses. Thus, if beans and rice are eaten over the course of a day, their joint amino acid patterns will supplement each other and provide a higher quality protein than would either food alone. Traditional food patterns in native cultures have made good use of protein complementarity. However, careful balancing of plant proteins is necessary only for those whose protein intake is marginal or inadequate. In affluent populations, where protein intake is greatly in excess of needs, obtaining sufficient good quality protein is usually only a concern for young children who are not provided with animal proteins.
Protein intakeThe World Health Organization recommends a daily intake of 0.75 gram of good quality protein per kilogram of body weight for adults of both sexes. Thus, a 70-kg (154-pound) man would need 52.5 grams of protein, and a 55-kg (121-pound) woman would need about 41 grams of protein. This recommendation, based on nitrogen balance studies, assumes an adequate energy intake. Infants, children, and pregnant and lactating women have additional protein needs to support synthesis of new tissue or milk production. Protein requirements of endurance athletes and bodybuilders may be slightly higher than those of sedentary individuals, but this has no practical significance because athletes typically consume much more protein than they need.
Protein consumed in excess of the body’s needs is degraded; the nitrogen is excreted as urea, and the remaining keto acids are used for energy, providing 4 kilocalories per gram, or are converted to carbohydrate or fat. During conditions of fasting, starvation, or insufficient dietary intake of protein, lean tissue is broken down to supply amino acids for vital body functions. Persistent protein inadequacy results in suboptimal metabolic function with increased risk of infection and disease.
VitaminsVitamins are organic compounds found in very small amounts in food and required for normal functioning—indeed, for survival. Humans are able to synthesize certain vitamins to some extent. For example, vitamin D is produced when the skin is exposed to sunlight; niacin can be synthesized from the amino acid tryptophan; and vitamin K and biotin are synthesized by bacteria living in the gut. However, in general, humans depend on their diet to supply vitamins. When a vitamin is in short supply or is not able to be utilized properly, a specific deficiency syndrome results. When the deficient vitamin is resupplied before irreversible damage occurs, the signs and symptoms are reversed. The amounts of vitamins in foods and the amounts required on a daily basis are measured in milligrams and micrograms.
Unlike the macronutrients, vitamins do not serve as an energy source for the body or provide raw materials for tissue building. Rather, they assist in energy-yielding reactions and facilitate metabolic and physiologic processes throughout the body. Vitamin A, for example, is required for embryonic development, growth, reproduction, proper immune function, and the integrity of epithelial cells, in addition to its role in vision. The B vitamins function as coenzymes that assist in energy metabolism; folic acid (folate), one of the B vitamins, helps protect against birth defects in the early stages of pregnancy. Vitamin C plays a role in building connective tissue as well as being an antioxidant that helps protect against damage by reactive molecules (free radicals). Now considered to be a hormone, vitamin D is involved in calcium and phosphorus homeostasis and bone metabolism. Vitamin E, another antioxidant, protects against free radical damage in lipid systems, and vitamin K plays a key role in blood clotting. Although vitamins are often discussed individually, many of their functions are interrelated, and a deficiency of one can influence the function of another.
Vitamin nomenclature is somewhat complex, with chemical names gradually replacing the original letter designations created in the era of vitamin discovery during the first half of the 20th century. Nomenclature is further complicated by the recognition that vitamins are parts of families with, in some cases, multiple active forms. Some vitamins are found in foods in precursor forms that must be activated in the body before they can properly fulfill their function. For example, beta(β)-carotene, found in plants, is converted to vitamin A in the body.
The 13 vitamins known to be required by human beings are categorized into two groups according to their solubility. The four fat-soluble vitamins (soluble in nonpolar solvents) are vitamins A, D, E, and K. Although now known to behave as a hormone, the activated form of vitamin D, vitamin D hormone (calcitriol), is still grouped with the vitamins as well. The nine water-soluble vitamins (soluble in polar solvents) are vitamin C and the eight B-complex vitamins: thiamin, riboflavin, niacin, vitamin B6, folic acid, vitamin B12, pantothenic acid, and biotin. Choline is a vitamin-like dietary component that is clearly required for normal metabolism but that can be synthesized by the body. Although choline may be necessary in the diet of premature infants and possibly of those with certain medical conditions, it has not been established as essential in the human diet throughout life.
Different vitamins are more or less susceptible to destruction by environmental conditions and chemical agents. For example, thiamin is especially vulnerable to prolonged heating, riboflavin to ultraviolet or fluorescent light, and vitamin C to oxidation (as when a piece of fruit is cut open and the vitamin is exposed to air). In general, water-soluble vitamins are more easily destroyed during cooking than are fat-soluble vitamins.
The solubility of a vitamin influences the way it is absorbed, transported, stored, and excreted by the body as well as where it is found in foods. With the exception of vitamin B12, which is supplied by only foods of animal origin, the water-soluble vitamins are synthesized by plants and found in both plant and animal foods. Strict vegetarians (vegans), who eat no foods of animal origin, are therefore at risk of vitamin B12 deficiency. Fat-soluble vitamins, on the other hand, are found in association with fats and oils in foods and in the body and typically require protein carriers for transport through the water-filled compartments of the body.
Water-soluble vitamins are not appreciably stored in the body (except for vitamin B12) and thus must be consumed regularly in the diet. If taken in excess they are readily excreted in the urine, although there is potential toxicity even with water-soluble vitamins; especially noteworthy in this regard is vitamin B6. Because fat-soluble vitamins are stored in the liver and fatty tissue, they do not necessarily have to be taken in daily, so long as average intakes over time—weeks, months, or even years—meet the body’s needs. However, the fact that these vitamins can be stored increases the possibility of toxicity if very large doses are taken. This is particularly of concern with vitamins A and D, which can be toxic if taken in excess. Under certain circumstances, pharmacological (“megadose”) levels of some vitamins—many times higher than the amount typically found in food—have accepted medical uses. Niacin, for example, is used to lower blood cholesterol levels; vitamin D is used to treat psoriasis; and pharmacological derivatives of vitamin A are used to treat acne and other skin conditions as well as to diminish skin wrinkling. However, consumption of vitamins or other dietary supplements in amounts significantly in excess of recommended levels is not advised without medical supervision.
Vitamins synthesized in the laboratory are the same molecules as those extracted from food, and they cannot be distinguished by the body. However, various forms of a vitamin are not necessarily equivalent. In the particular case of vitamin E, supplements labeled d-α-tocopherol (or “natural”) generally contain more vitamin E activity than those labeled dl-α-tocopherol. Vitamins in food have a distinct advantage over vitamins in supplement form because they come associated with other substances that may be beneficial, and there is also less potential for toxicity. Nutritional supplements cannot substitute for a healthful diet.
MineralsUnlike the complex organic compounds (carbohydrates, lipids, proteins, vitamins) discussed in previous sections, minerals are simple inorganic elements—often in the form of salts in the body—that are not themselves metabolized, nor are they a source of energy. Minerals constitute about 4 to 6 percent of body weight—about one-half as calcium and one-quarter as phosphorus (phosphates), the remainder being made up of the other essential minerals that must be derived from the diet. Minerals not only impart hardness to bones and teeth but also function broadly in metabolism—e.g., as electrolytes controlling the movement of water in and out of cells, as components of enzyme systems, and as constituents of many organic molecules.
As nutrients, minerals are traditionally divided into two groups according to the amounts present in and needed by the body. The major minerals (macrominerals)—those required in amounts of 100 milligrams or more per day—are calcium, phosphorus (phosphates), magnesium, sulfur, sodium, chloride, and potassium. The trace elements (microminerals or trace minerals), required in much smaller amounts of about 15 milligrams per day or less, include iron, zinc, copper, manganese, iodine (iodide), selenium, fluoride, molybdenum, chromium, and cobalt (as part of the vitamin B12 molecule). Fluoride is considered a beneficial nutrient because of its role in protecting against dental caries, although an essential function in the strict sense has not been established in human nutrition.
The term ultratrace elements is sometimes used to describe minerals that are found in the diet in extremely small quantities (micrograms each day) and are present in human tissue as well; these include arsenic, boron, nickel, silicon, and vanadium. Despite demonstrated roles in experimental animals, the exact function of these and other ultratrace elements (e.g., tin, lithium, aluminum) in human tissues and indeed their importance for human health are uncertain.
Minerals have diverse functions, including muscle contraction, nerve transmission, blood clotting, immunity, the maintenance of blood pressure, and growth and development. The major minerals, with the exception of sulfur, typically occur in the body in ionic (charged) form: sodium, potassium, magnesium, and calcium as positive ions (cations) and chloride and phosphates as negative ions (anions). Mineral salts dissolved in body fluids help regulate fluid balance, osmotic pressure, and acid-base balance.
Sulfur, too, has important functions in ionic forms (such as sulfate), but much of the body’s sulfur is nonionic, serving as an integral part of certain organic molecules, such as the B vitamins thiamin, biotin, and pantothenic acid and the amino acids methionine, cysteine, and cystine. Other mineral elements that are constituents of organic compounds include iron, which is part of hemoglobin (the oxygen-carrying protein in red blood cells), and iodine, a component of thyroid hormones, which help regulate body metabolism. Additionally, phosphate groups are found in many organic molecules, such as phospholipids in cell membranes, genetic material (DNA and RNA), and the high-energy molecule adenosine triphosphate (ATP).
The levels of different minerals in foods are influenced by growing conditions (e.g., soil and water composition) as well as by how the food is processed. Minerals are not destroyed during food preparation; in fact, a food can be burned completely and the minerals (ash) will remain unchanged. However, minerals can be lost by leaching into cooking water that is subsequently discarded.
Many factors influence mineral absorption and thus availability to the body. In general, minerals are better absorbed from animal foods than from plant foods. The latter contain fibre and other substances that interfere with absorption. Phytic acid, found principally in cereal grains and legumes, can form complexes with some minerals and make them insoluble and thereby indigestible. Only a small percentage of the calcium in spinach is absorbed because spinach also contains large amounts of oxalic acid, which binds calcium. Some minerals, particularly those of a similar size and charge, compete with each other for absorption. For example, iron supplementation may reduce zinc absorption, while excessive intakes of zinc can interfere with copper absorption. On the other hand, the absorption of iron from plants (nonheme iron) is enhanced when vitamin C is simultaneously present in the diet, and calcium absorption is improved by adequate amounts of vitamin D. Another key factor that influences mineral absorption is the physiological need for the mineral at the time.
Unlike many vitamins, which have a broader safety range, minerals can be toxic if taken in doses not far above recommended levels. This is particularly true for the trace elements, such as iron and copper. Accidental ingestion of iron supplements has been a major cause of fatal poisoning in young children.
WaterAlthough often overlooked as a nutrient, water (H2O) is actually the most critical nutrient of all. Humans can survive weeks without food but only a matter of days without water.
Water provides the medium in which nutrients and waste products are transported throughout the body and the myriad biochemical reactions of metabolism occur. Water allows for temperature regulation, the maintenance of blood pressure and blood volume, the structure of large molecules, and the rigidity of body tissues. It also acts as a solvent, a lubricant (as in joints), and a protective cushion (as inside the eyes and in spinal fluid and amniotic fluid). The flow of water in and out of cells is precisely controlled by shifting electrolyte concentrations on either side of the cell membrane. Potassium, magnesium, phosphate, and sulfate are primarily intracellular electrolytes; sodium and chloride are major extracellular ones.
Water makes up about 50 to 70 percent of body weight, approximately 60 percent in healthy adults and an even higher percentage in children. Because lean tissue is about three-quarters water, and fatty tissue is only about one-fifth water, body composition—the amount of fat in particular—determines the percentage of body water. In general, men have more lean tissue than women, and therefore a higher percentage of their body weight is water.
Water is consumed not only as water itself and as a constituent of other beverages but also as a major component of many foods, particularly fruits and vegetables, which may contain from 85 to 95 percent water. Water also is manufactured in the body as an end product of metabolism. About 2.5 litres (about 2.6 quarts) of water are turned over daily, with water excretion (primarily in urine, water vapour from lungs, sweat loss from skin, and feces) balancing intake from all sources. Because water requirements vary with climate, level of activity, dietary composition, and other factors, there is no one recommendation for daily water intake. However, adults typically need at least 2 litres (8 cups) of water a day, from all sources. Thirst is not reliable as a register for dehydration, which typically occurs before the body is prompted to replace fluid. Therefore, water intake is advised throughout the day, especially with increased sweat loss in hot climates or during vigorous physical activity, during illness, or in a dehydrating situation such as an airplane flight.
Food groupsThe following nine food groups reflect foods with generally similar nutritional characteristics: (1) cereals, (2) starchy roots, (3) legumes, (4) vegetables and fruits, (5) sugars, preserves, and syrups, (6) meat, fish, and eggs, (7) milk and milk products, (8) fats and oils, and (9) beverages.
CerealsThe cereals are all grasses that have been bred over millennia to bear large seeds (i.e., grain). The most important cereals for human consumption are rice, wheat, and corn (maize). Others include barley, oats, and millet. The carbohydrate-rich cereals compare favourably with the protein-rich foods in energy value; in addition, the cost of production (per calorie) of cereals is less than that of almost all other foods and they can be stored dry for many years. Therefore, most of the world’s diets are arranged to meet main calorie requirements from the cheaper carbohydrate foods. The major component of all grains is starch. Cereals contain little fat, with oats having an exceptional 9 percent. The amount of protein in cereals ranges from 6 to 16 percent but does not have as high a nutritive value as that of many animal foods because of the low lysine content.
Controversy exists as to the relative merits of white bread and bread made from whole wheat flour. White flour consists of about 72 percent of the grain but contains little of the germ (embryo) and of the outer coverings (bran). Since the B vitamins are concentrated mainly in the scutellum (covering of the germ), and to a lesser extent in the bran, the vitamin B content of white flour, unless artificially enriched, is less than that of brown flour. Dietary fibre is located mostly in the bran, so that white flour contains only about one-third of that in whole wheat flour. White flour is compulsorily enriched with synthetic vitamins in a number of countries, including the United States and the United Kingdom, so that the vitamin content is similar to that of the darker flours. White flour, of course, still lacks fibre and any yet unidentified beneficial factors that may be present in the outer layers of the wheat.
B vitamins are also lost when brown rice is polished to yield white rice. People living on white rice and little else are at risk for developing the disease beriberi, which is caused by a deficiency of thiamin (vitamin B1). Beriberi was formerly common in poor Asian communities in which a large proportion of the diet consisted of polished rice. The disease has almost completely disappeared from Asia with the advent of greater availability of other foods and, in some areas, fortification of the rice with thiamin.
Yellow corn differs from other cereals in that it contains carotenoids with vitamin A activity. (Another exception is a genetically modified so-called golden rice, which contains carotene, the precursor for vitamin A.) Corn is also lower in the amino acid tryptophan than other cereals. The niacin in corn is in a bound form that cannot be digested or absorbed by humans unless pretreated with lime (calcium hydroxide) or unless immature grains are eaten at the so-called milky stage (usually as sweet corn). Niacin is also formed in the body as a metabolite of the amino acid tryptophan, but this alternative source is not available when the tryptophan content is too low.
Starchy rootsStarchy roots consumed in large quantities include potatoes, sweet potatoes, yams, taro, and cassava. Their nutritive value in general resembles that of cereals. The potato, however, provides some protein (2 percent) and also contains vitamin C. The yellow-fleshed varieties of sweet potato contain the pigment beta-carotene, convertible in the body into vitamin A. Cassava is extremely low in protein, and most varieties contain cyanide-forming compounds that make them toxic unless processed correctly.
LegumesBeans and peas are the seeds of leguminous crops that are able to utilize atmospheric nitrogen via parasitic microorganisms attached to their roots. Legumes contain at least 20 percent protein, and they are a good source of most of the B vitamins and of iron. Like cereals, most legumes are low in fat; an important exception is the soybean (17 percent), a major commercial source of edible oil. Tofu, or bean curd, is made from soybeans and is an important source of protein in China, Japan, Korea, and Southeast Asia. Peanuts (groundnuts) are also the seeds of a leguminous plant, although they ripen underground; much of the crop is processed for its oil.
Vegetables and fruitsVegetables and fruits have similar nutritive properties. (See the table of nutrient composition of vegetables and the table of nutrient composition of fruits.) Because 70 percent or more of their weight is water, they provide comparatively little energy or protein, but many contain vitamin C and carotene. However, cooked vegetables are an uncertain source of vitamin C, as this vitamin is easily destroyed by heat. The dark-green leafy vegetables are particularly good sources of vitamin A activity. Vegetables also provide calcium and iron but often in a form that is poorly absorbed. The more typical fruits, such as apples, oranges, and berries, are rich in sugar. Bananas are a good source of potassium. Vegetables and fruits also contain fibre, which adds bulk to the intestinal content and is useful in preventing constipation. (For more on the health benefits of a diet rich in fruit, see Sidebar: A Kiwi a Day: Fruit, the Doctor, and You.)
Botanically, nuts are actually a kind of fruit, but they are quite different in character with their hard shell and high fat content. The coconut, for example, contains some 60 percent fat when dried. Olives are another fruit rich in fat and are traditionally grown for their oil.
Sugars, preserves, and syrupsOne characteristic of diets of affluent societies is their high content of sugar. This is due in part to sugar added at the table or as an ingredient in candy, preserves, and sweetened colas or other beverages. The sugars, mostly sucrose and high-fructose corn syrup, together provide 12 percent of the average total calories in adults and a little more in children. There are also naturally occurring sugars in foods (lactose in milk and fructose, glucose, and sucrose in fruits and some vegetables). The intake of these in the United States is about 8 percent of total caloric intake in adults and much more in young children due to their greater intake of lactose in milk. Sugar, however, contains no protein, minerals, or vitamins and thus has been called the source of “empty calories.”
Because sugar adsorbs water and prevents the growth of microorganisms, it is an excellent preservative. Making jam or marmalade is a way of preserving fruit, but most of the vitamin C is destroyed, and the products contain up to 70 percent sugar. Honey and natural syrups (e.g., maple syrup) are composed of more than 75 percent sugar.
Meat, fish, and eggsGenerally meats consist of about 20 percent protein, 20 percent fat, and 60 percent water. The amount of fat present in a particular portion of meat varies greatly, not only with the kind of meat but also with the quality; the “energy value” varies in direct proportion with the fat content (see table). Meat is valuable for its protein, which is of high biological value. Pork is an excellent source of thiamin. Meat is also a good source of niacin, vitamin B12, vitamin B6, and the mineral nutrients iron, zinc, phosphorus, potassium, and magnesium. Liver is the storage organ for, and is very rich in, vitamin A, riboflavin, and folic acid. In many cultures the organs (offal) of animals—including the kidneys, the heart, the tongue, and the liver—are considered delicacies. Liver is a particularly rich source of many vitamins.
The muscular tissue of fishes consists of 13 to 20 percent protein, fat ranging from less than 1 to more than 20 percent, and 60 to 82 percent water that varies inversely with fat content (see table). Many species of fish, such as cod and haddock, concentrate fat in the liver and as a result have extremely lean muscles. The tissues of other fish, such as salmon and herring, may contain 15 percent fat or more. However, fish oil, unlike the fat in land animals, is rich in essential long-chain fatty acids and is regarded as nutritionally advantageous. Large amounts of one of the major fatty acids, eicosapentaenoic acid, reduces the tendency to thrombosis.
The egg has a deservedly high reputation as a food. Its white contains protein, and its yolk is rich in both protein and vitamin A (see table). An egg also provides calcium and iron. Egg yolk, however, has a high cholesterol content.
Milk and milk productsThe milk of each species of animal is a complete food for its young. Moreover, one pint of cow’s milk contributes about 90 percent of the calcium, 30 to 40 percent of the riboflavin, 25 to 30 percent of the protein, 10 to 20 percent of the calories and vitamins A and B, and up to 10 percent of the iron and vitamin D needed by a human adult. (See table.)
Human breast milk is the perfect food for infants, provided it comes from a healthy, well-nourished mother and the infant is full-term. Breast milk contains important antibodies, white blood cells, and nutrients. In communities where hygiene is poor, breast-fed babies have fewer infections than formula-fed babies. In the past, infants who could not be breast-fed were given cow’s milk that was partially “humanized” with the addition of water and a small amount of sugar or wheat flour. However, this was far from an ideal substitute for breast milk, being lower in iron and containing undenatured proteins that could produce allergic reactions with bleeding into the gut and, in some cases, eczema.
Lactose, the characteristic sugar of milk, is a disaccharide made of the monosaccharides glucose and galactose. Some adults can break down the lactose of large quantities of milk into galactose and glucose, but others have an inherited lactose intolerance as a result of the lactase enzyme no longer being secreted into the gut after the age of weaning. As a result, unabsorbed lactose is fermented by bacteria and produces bloating and gas. People who have little lactase in their bodies can still consume large amounts of milk if it has been allowed to go sour, if lactobacilli have split most of the lactose into lactic acid (as in yogurt), or if the lactose has been treated with commercially available lactase. People originating in northern Europe usually retain full intestinal lactase activity into adult life.
Most commercially available milk has been pasteurized with heat to kill bovine tuberculosis organisms and other possible pathogens. The most widely used method for pasteurizing milk is the high-temperature, short-time (HTST) sterilization treatment. If products are to be stored under refrigeration, or even at room temperature, for long periods of time, they may be processed by ultrahigh temperature (UHT) pasteurization. Another method of preserving milk without refrigeration involves the removal of water to form condensed milk, which can be exposed to air for several days without deterioration. Milk, either whole or defatted, can also be dried to a powder. In some countries, such as the United States, milk is homogenized so that fat particles are broken up and evenly distributed throughout the product.
Cow’s milk is good food for human adults, but the cream (i.e., the fat) contains 52 percent saturated fatty acids as compared with only 3 percent polyunsaturated fat. This fat is either drunk with the milk or eaten in butter or cream. Because milk fat is regarded as undesirable by people who want to reduce their energy intake or cholesterol level, the dairy industry has developed low-fat cow’s milk (with 2 percent fat instead of the almost 4 percent of whole milk), very low-fat skim milk, and skim milk with extra nonfat milk solids (lactose, protein, and calcium) that give more body to the milk. Buttermilk, originally the watery residue of butter making, is now made from either low-fat or skim milk that has been inoculated with nonpathogenic bacteria.
Cheese making is an ancient art formerly used on farms to convert surplus milk into a food that could be stored without refrigeration. Rennet, an enzyme found in a calf’s stomach, is added to milk, causing the milk protein casein to coagulate into a semisolid substance called curd, thus trapping most of the fat. The remaining watery liquid (whey) is then drained, and the curd is salted, inoculated with nonpathogenic organisms, and allowed to dry and mature. Cheese is rich in protein and calcium and is a good source of vitamin A and riboflavin. Most cheeses, however, contain about 25 to 30 percent fat (constituting about 70 percent of the calories of the cheese), which is mostly saturated, and they are usually high in sodium. (See also dairy product.)
Fats and oilsThe animal fats used by humans are butter, suet (beef fat), lard (pork fat), and fish oils. Important vegetable oils include olive oil, peanut (groundnut) oil, coconut oil, cottonseed oil, sunflower seed oil, soybean oil, safflower oil, rape oil, sesame (gingelly) oil, mustard oil, red palm oil, and corn oil. Fats and oils provide more calories per gram than any other food, but they contain no protein and few micronutrients. Only butter and the previously mentioned fish-liver oils contain any vitamin A or D, though red palm oil does contain carotene, which is converted to vitamin A in the body. Vitamins A and D are added to margarines. All natural fats and oils contain variable amounts of vitamin E, the fat-soluble vitamin antioxidant.
The predominant substances in fats and oils are triglycerides, chemical compounds containing any three fatty acids combined with a molecule of glycerol. When no double bonds are present, a fatty acid is said to be saturated; with the presence of one or more double bonds, a fatty acid is said to be unsaturated (see the section Essential nutrients: Lipids). Fats with a high percentage of saturated fatty acids, e.g., butter and lard, tend to be solid at room temperature. Those with a high percentage of unsaturated fatty acids are usually liquid oils, e.g., sunflower, safflower, and corn oils. The process of hydrogenation is used by the food industry to convert unsaturated oils to saturated solid fats, which are more resistant to rancidity. However, hydrogenation also causes the formation of trans-fatty acids. These appear to have some of the same undesirable effects on blood cholesterol as saturated fatty acids.
A small group of fatty acids is essential in the diet. They occur in body structures, especially the different membranes inside and around cells, and cannot be synthesized in the body from other fats. Linoleic acid is the most important of these fatty acids because it is convertible to other essential fatty acids. Linoleic acid has two double bonds and is a polyunsaturated fatty acid. As well as being an essential fatty acid, it tends to lower the cholesterol level in the blood. Linoleic acid occurs in moderate to high proportions in many of the seed oils, e.g., corn, sunflower, cottonseed, and safflower oils. Some margarines (polyunsaturated margarines) use a blend of oils selected to provide a moderately high linoleic acid content.
BeveragesAlthough most adults drink one to two litres (about one to two quarts) of water a day, much of this is in the form of liquids such as coffee, tea, fruit juices, and soft drinks. In general, these are appreciated more for their taste or for their effects than for their nutritive value. Fruit juices are, of course, useful for their vitamin C content and are good sources of potassium. Coffee and tea by themselves are of no nutritive value, except that coffee contains some niacin and tea contains fluoride and manganese; these beverages also contain natural caffeine, which has a stimulating effect. Caffeine is added to colas, and so-called diet soft drinks contain small quantities of artificial sweeteners in place of sugars so that their overall calorie value is reduced.
Since ethyl alcohol (ethanol) has an energy value of 7 kilocalories per gram, very significant amounts of energy can be obtained from alcoholic drinks (see table). Beer contains 2 to 6 percent alcohol, wines 10 to 13 percent, and most spirits up to 40 percent. Fermented drinks also include significant amounts of residual sugars, and champagne and dessert wines may have sugar added to them. With one or two exceptions, alcoholic beverages contain no nutrients and are only a source of “empty calories.” The only vitamin present in significant amounts in beer is riboflavin. Wines are devoid of vitamins but sometimes contain large amounts of iron, probably acquired from iron vessels used in their preparation. Heavy alcohol consumption is known to lead to a greater risk of malnutrition, in part because it can damage the absorptive power of the gut and also because heavy drinkers commonly neglect to follow a normal pattern of meals. On the other hand, evidence from a number of studies shows that persons consuming one to two drinks per day are healthier than are those who abstain from drinking alcohol. This might be due in part to substances in red wine, such as flavonoids and tannins, which may protect against heart disease.
Dietary and nutrient recommendationsNotions of what constitutes a healthful diet vary with geography and custom as well as with changing times and an evolving understanding of nutrition. In the past, people had to live almost entirely on food that was locally produced. With industrialization and globalization, however, food can now be transported over long distances. Researchers must be careful in making generalizations about a national diet from a relatively small sample of the population; the poor cannot afford to eat the same diet as the rich, and many countries have large immigrant groups with their own distinctive food patterns. Even within a culture, some people abstain on moral or religious grounds from eating certain foods. In general, persons living in more affluent countries eat more meat and other animal products. By comparison, the diets of those living in poorer, agricultural countries rely primarily on cereals in the form of wheat flour, white rice, or corn, with animal products providing less than 10 percent of energy. Another difference between cultures is the extent to which dairy products are consumed. The Chinese, for example, obtain about 2 percent of their energy from dairy products. In contrast, in Pakistan dairy products contribute almost 10 percent of energy. Among Western diets, the lowest in saturated fat is the so-called Mediterranean diet. In the 1950s it was found that Europeans living in rural areas near the Mediterranean Sea had a greater life expectancy than those living elsewhere in Europe, despite poor medical services and a lower standard of living. The traditional diet of Mediterranean peoples is low in animal products; instead, olive oil is a major source of monounsaturated fat. Also, tomatoes and green leafy vegetables, which are regularly consumed in large quantities in the region, contain a variety of antioxidant compounds that are thought to be healthful.
Following the publication of dietary goals for the Nordic countries in 1968 and for the United States in 1977, dietary goals and guidelines have been set forth by a number of countries and revised periodically as a way of translating scientific recommendations into simple and practical dietary suggestions. These authoritative statements—some published by scientific bodies and some by government agencies—aim to promote long-term health and to prevent or reduce the chances of developing chronic and degenerative diseases. Although the guidelines of different countries may vary in important ways, most recent dietary recommendations include variations on the following fundamental themes: eat a variety of foods; perform regular physical activity and maintain a healthy weight; limit consumption of saturated fat, trans fat, sugar, salt (more specifically, sodium), and alcohol; and emphasize vegetables, fruits, and whole grains.
Food guide pyramids and other aids
The original USDA Food Guide Pyramid recommended a liberal daily intake of grain products (represented by the wide base of the pyramid) and a sparing intake of fats, oils, and sugary foods (represented by the tip of the structure).
Different formats for dietary goals and guidelines have been developed over the years as educational tools, grouping foods of similar nutrient content together to help facilitate the selection of a balanced diet. In the United States, the four food-group plan of the 1950s—which suggested a milk group, a meat group, a fruit and vegetable group, and a breads and cereals group as a basic diet—was replaced in 1992 by the five major food groups of the Food Guide Pyramid. This innovative visual display was introduced by the United States Department of Agriculture (USDA) as a tool for helping the public cultivate a daily pattern of wise food choices, ranging from liberal consumption of grain products, as represented in the broad base of the pyramid, to sparing use of fats, oils, and sugary foods, as represented in the apex. Subsequently, similar devices were developed for particular cultural and ethnic food patterns such as Asian, Latin American, Mediterranean, and even vegetarian diets—all emphasizing grains, vegetables, and fruits. While an adaptation of the 1992 USDA pyramid was used by Mexico, Chile, the Philippines, and Panama, a rainbow was used by Canada, a square by Zimbabwe, plates by Australia and the United Kingdom, a bean pot by Guatemala, the number 6 by Japan, and a pagoda by South Korea and China.
MyPyramid, introduced by the U.S. Department of Agriculture in 2005, represented the major food groups in vertical bands. It was replaced in 2011 by a revised food guide graphic known as MyPlate.
In the early 21st century, many countries altered the pictorial representation of their food guides. For example, in 2005 Japan introduced a spinning-top food guide that essentially was an inverted version of the U.S. pyramid graphic. That same year, the USDA released new dietary guidelines and redesigned its original Food Guide Pyramid, which was known as MyPyramid and featured colourful vertical stripes of varying widths to reflect the relative proportions of different food groups. Similar to Japan’s spinning-top graphic, which depicted a figure running on the top’s upper level, the MyPyramid graphic used a figure climbing steps to illustrate the importance of daily exercise. Unlike the original Food Guide Pyramid, the abstract geometry of MyPyramid did not offer specific dietary guidance at a glance; rather, individuals were directed to an interactive Web site for customized eating plans based on their age, sex, and activity level.
In 2011 the USDA abandoned MyPyramid and introduced MyPlate, which divided the four basic food groups (fruits, grains, protein, and vegetables) into sections on a plate, with the size of each section representing the relative dietary proportions of each food group. A small circle shown at the edge of the plate was used to illustrate the dietary inclusion and proportion of dairy products. Unlike MyPyramid, MyPlate did not include an exercise component, nor did it include a section for fats and oils. The two were similar, however, in that the guidance they offered was nonspecific and was supported by a Web site.
Adapting guidelines to cultureDietary guidelines have been largely the province of more affluent countries, where correcting imbalances due to overconsumption and inappropriate food choices has been key. Not until 1989 were proposals for dietary guidelines published from the perspective of low-income countries, such as India, where the primary nutrition problems stemmed from the lack of opportunity to acquire or consume needed food. But even in such countries, there was a growing risk of obesity and chronic disease among the small but increasing number of affluent people who had adopted some of the dietary habits of the industrialized world. For example, the Chinese Dietary Guidelines, published by the Chinese Nutrition Society in 1997, made recommendations for that part of the population dealing with nutritional diseases such as those resulting from iodine and vitamin A deficiencies, for people in some remote areas where there was a lack of food, as well as for the urban population coping with changing lifestyle, dietary excess, and increasing rates of chronic disease. The Food Guide Pagoda, a graphic display intended to help Chinese consumers put the dietary recommendations into practice, rested on the traditional cereal-based Chinese diet. Those who could not tolerate fresh milk were encouraged to consume yogurt or other dairy products as a source of calcium. Unlike dietary recommendations in Western countries, the pagoda did not include sugar, as sugar consumption by the Chinese was quite low; however, children and adolescents in particular were cautioned to limit sugar intake because of the risk of dental caries.
Nutrient recommendationsThe relatively simple dietary guidelines discussed above provide guidance for meal planning. Standards for evaluating the adequacy of specific nutrients in an individual diet or the diet of a population require more detailed and quantitative recommendations. Nutrient recommendations are usually determined by scientific bodies within a country at the behest of government agencies. The World Health Organization and other agencies of the United Nations have also issued reports on nutrients and food components. The Recommended Dietary Allowances (RDAs), first published by the U.S. National Academy of Sciences in 1941 and revised every few years until 1989, established dietary standards for evaluating nutritional intakes of populations and for planning food supplies. The RDAs reflected the best scientific judgment of the time in setting amounts of different nutrients adequate to meet the nutritional needs of most healthy people.
Dietary Reference IntakesDuring the 1990s a paradigm shift took place as scientists from the United States and Canada joined forces in an ambitious multiyear project to reframe dietary standards for the two countries. In the revised approach, known as the Dietary Reference Intakes (DRIs), classic indicators of deficiency, such as scurvy and beriberi, were considered an insufficient basis for recommendations. Where warranted by a sufficient research base, the guidelines rely on indicators with broader significance, those that might reflect a decreased risk of chronic diseases such as osteoporosis, heart disease, hypertension, or cancer. DRIs are intended to help individuals plan a healthful diet as well as avoid consuming too much of a nutrient. The comprehensive approach of the DRIs has served as a model for other countries. A DRI report was published in 1997, and subsequent updates were published for specific nutrients and for some food components such as flavonoids that are not considered nutrients but have an impact on health.
The collective term Dietary Reference Intakes encompasses four categories of reference values. The Estimated Average Requirement (EAR) is the intake level for a nutrient at which the needs of 50 percent of the population will be met. Because the needs of the other half of the population will not be met by this amount, the EAR is increased by about 20 percent to arrive at the RDA. The RDA is the average daily dietary intake level sufficient to meet the nutrient requirement of nearly all (97 to 98 percent) healthy persons in a particular life stage. When the EAR, and thus the RDA, cannot be set due to insufficient scientific evidence, another parameter, the Adequate Intake (AI), is given, based on estimates of intake levels of healthy populations. Lastly, the Tolerable Upper Intake Level (UL) is the highest level of a daily nutrient intake that will most likely present no risk of adverse health effects in almost all individuals in the general population (see table).
Nutrition information is commonly displayed on food labels, but this information is generally simplified to avoid confusion. Because only one nutrient reference value is listed, and because sex and age categories usually are not taken into consideration, the amount chosen is generally the highest RDA value. In the United States, for example, the Daily Values, determined by the Food and Drug Administration, are generally based on RDA values published in 1968. The different food components are listed on the food label as a percentage of their Daily Values.
Confidence that a desirable level of intake is reasonable for a particular group of people can be bolstered by multiple lines of evidence pointing in the same direction, an understanding of the function of the nutrient and how it is handled by the body, and a comprehensive theoretical model with strong statistical underpinnings. Of critical importance in estimating nutrient requirements is explicitly defining the criterion that the specified level of intake is intended to satisfy. Approaches that use different definitions of adequacy are not comparable. For example, it is one thing to prevent clinical impairment of bodily function (basal requirement), which does not necessarily require any reserves of the nutrient, but it is another to consider an amount that will provide desirable reserves (normative requirement) in the body. Yet another approach attempts to evaluate a nutrient intake conducive to optimal health, even if an amount is required beyond that normally obtainable in food—possibly necessitating the use of supplements. Furthermore, determining upper levels of safe intake requires evidence of a different sort. These issues are extremely complex, and the scientists who collaborate to set nutrient recommendations face exceptional challenges in their attempts to reach consensus.
Nutrition throughout the life cycleNutritional needs and concerns vary during different stages of life. Selected issues are discussed below.
Pregnancy and lactationA woman’s nutritional status before and during pregnancy affects not only her own health but also the health and development of her baby. If a woman is underweight before becoming pregnant or fails to gain sufficient weight during pregnancy, her chance of having a premature or low-birth-weight infant is increased. Overweight women, on the other hand, have a high risk of complications during pregnancy, such as high blood pressure (hypertension) and gestational diabetes, and of having a poorly developed infant or one with birth defects. Weight loss during pregnancy is never recommended. Recommended weight gain during pregnancy is 11.5 to 16 kg (25 to 35 pounds) for a woman of normal weight—slightly more for an underweight woman and slightly less for an overweight woman.
At critical periods in the development of specific organs and tissues, there is increased vulnerability to nutrient deficiencies, nutrient excesses, or toxins. For example, excess vitamin A taken early in pregnancy can cause brain malformations in the fetus. One important medical advance of the late 20th century was the recognition that a generous intake of folic acid (also called folate or folacin) in early pregnancy reduces the risk of birth defects, specifically neural tube defects such as spina bifida and anencephaly (partial or complete absence of the brain), which involve spinal cord damage and varying degrees of paralysis, if not death. For this reason, supplementation with 400 micrograms (0.4 milligram) of folic acid is recommended for all women who have a chance of becoming pregnant. Good food sources of folic acid include green leafy vegetables, citrus fruit and juice, beans and other legumes, whole grains, fortified breakfast cereals, and liver.
Overall nutritional requirements increase with pregnancy. In the second and third trimesters, pregnant women need additional food energy—about 300 kilocalories above nonpregnant needs. Most additional nutrient needs can be met by selecting food wisely, but an iron supplement (30 milligrams per day) is usually recommended during the second and third trimesters, in addition to a folic acid supplement throughout pregnancy. Other key nutrients of particular concern are protein, vitamin D, calcium, and zinc.
Heavy alcohol consumption or “binge drinking” during pregnancy can cause fetal alcohol syndrome, a condition with irreversible mental and physical retardation. Even lighter social drinking during pregnancy may result in milder damage—growth retardation, behavioral or learning abnormalities, or motor impairments—sometimes described as fetal alcohol effects. Until a completely safe level of intake can be determined, pregnant women are advised not to drink at all, especially during the first trimester. Caffeine consumption is usually limited as a precautionary measure, and cigarette smoking is not advised under any circumstances. Limiting intake of certain fish, such as swordfish and shark, which may be contaminated with methylmercury, is also recommended.
An extra 500 kilocalories of food per day is needed to meet the energy demands of lactation. Because pregnancy depletes maternal iron stores, iron supplementation during lactation may be advised. Breast-fed infants may be sensitive to the constituents and flavours of foods and beverages consumed by the mother. In general, lactating women are advised to consume little, if any, alcohol.
Infancy, childhood, and adolescenceBreast-fed infants, in general, have fewer infections and a reduced chance of developing allergies and food intolerances. For these and other reasons, breast-feeding is strongly recommended for at least the first four to six months of life. However, if a woman is unable to breast-feed or chooses not to, infant formulas (altered forms of cow’s milk) can provide safe and adequate nourishment for an infant. Goat’s milk, evaporated milk, and sweetened condensed milk are inappropriate for infants. Soy formulas and hydrolyzed protein formulas can be used if a milk allergy is suspected. In developing countries with poor sanitation, over-diluted formulas or those prepared with contaminated water can cause malnutrition and infection, resulting in diarrhea, dehydration, and even death. Breast-fed infants may need supplements of iron and vitamin D during the first six months of life and fluoride after six months. A vitamin B12 supplement is advised for breast-fed infants whose mothers are strict vegetarians (vegans). (See infancy.)
Solid foods, starting with iron-fortified infant cereals, can be introduced between four and six months to meet nutrient needs that breast milk or infant formulas can no longer supply alone. Other foods can be introduced gradually, one every few days. Infants should not be given honey (which may contain bacteria that can cause botulism), foods that are too salty or sweet, foods that may cause choking, or large amounts of fruit juice.
Starting at one year of age, whole cow’s milk can be an excellent source of nutrients for children. However, because cow’s milk is associated with gastrointestinal blood loss, iron deficiency, and an allergic response in some young infants, some medical societies do not recommend giving unmodified whole cow’s milk to children less than one year old. Low-fat or nonfat milk is inappropriate for children less than two years of age.
The rapid growth rate of infancy slows down in early childhood. During childhood—but not before the age of two—a gradual transition to lower-fat foods is recommended, along with regular exercise. Establishing healthful practices in childhood will reduce the risk of childhood obesity as well as obesity in adulthood and related chronic diseases (e.g., heart disease, diabetes, and high blood pressure).
Vegetarian children can be well nourished but care is needed for them to receive sufficient energy (calories), good-quality protein, vitamins B12 and D, and the minerals iron, zinc, and calcium. It is difficult for children who do not drink milk to obtain enough calcium from their food, and supplements may be required. Because of possible toxicity, iron supplements should be taken only under medical supervision.
A small percentage of school-age children who have difficulty sitting still and paying attention are diagnosed with attention-deficit/ hyperactivity disorder (ADHD). Studies have found no convincing evidence that ADHD is caused by sugar or food additives in the diet or that symptoms can be alleviated by eliminating these substances.
Because of unusual eating practices, skipped meals, and concerns about body image, many teenagers, especially girls, have a less than optimal diet. Teenage girls, in particular, need to take special care to obtain adequate amounts of calcium so that bones can be properly mineralized. Iron-deficiency anemia is a concern not only for teenage girls, who lose iron periodically in menstrual blood, but also for teenage boys.
AdulthoodNo matter which nutritional and health practices are followed, the body continues to age, and there appears to be a strong genetic component to life expectancy. Nevertheless, healthful dietary practices and habits such as limited alcohol use, avoidance of tobacco products, and regular physical activity can help reduce the chance of premature death and increase the chance of vitality in the older years. For the most part, a diet that is beneficial for adults in general is also beneficial for people as they age, taking into account possible changes in energy needs.
In elderly people, common problems that contribute to inadequate nutrition are tooth loss, decreased sense of taste and smell, and a sense of isolation—all of which result in decreased food intake and weight loss. The elderly may have gastrointestinal ailments, such as poor absorption of vitamin B12, and digestion difficulties, such as constipation. Inadequate fluid intake may lead to dehydration. Nutritional deficiency may further compromise declining immune function. Prescription and over-the-counter drugs may interact with nutrients and exacerbate the nutritional deficits of the elderly. In addition, decreasing physical activity, loss of muscle tissue, and increasing body fat are associated with type 2 diabetes, hypertension, and risk of cardiovascular disease and other diseases. Older people, especially those with reduced sun exposure or low intakes of fatty fish or vitamin D-fortified food, may need supplemental vitamin D to help preserve bone mass. Adequate calcium intake and weight-bearing exercise are also important, but these measures cannot completely stop the decline in bone density with age that makes both men and women vulnerable to bone fractures (due to osteoporosis), which could leave them bedridden and could even be life-threatening. Treatment with various bone-conserving drugs has been found to be effective in slowing bone loss. Staying physically fit as one ages can improve strength and balance, thereby preventing falls, contributing to overall health, and reducing the impact of aging.
There is evidence that intake of the antioxidants vitamin C, vitamin E, and beta-carotene as well as the mineral zinc may slow the progression of age-related macular degeneration, a leading cause of blindness in people older than 65 years. Two carotenoids, lutein and zeaxanthin, also are being studied for their possible role in protecting against age-related vision loss. Research suggests that the dietary supplement glucosamine, a substance that occurs naturally in the body and contributes to cartilage formation, may be useful in lessening the pain and disability of osteoarthritis. Aerobic exercise and strength training, as well as losing excess weight, also may provide some relief from arthritis pain.
Preliminary evidence suggests that fish oil, rich in omega-3 fatty acids, helps reduce the joint inflammation of rheumatoid arthritis. Fish oil also reduces blood clotting and exerts other effects that may protect the heart and blood vessels. However, in large quantities it may contribute to hemorrhagic stroke and have other undesirable side effects. Although consumption of fish once or twice a week may be beneficial, supplementation with fish oil capsules is advised only with medical supervision.
Elevated blood levels of the amino acid homocysteine have been associated with an increased risk of cardiovascular disease and with Alzheimer disease, the most common form of dementia; certain B vitamins, particularly folic acid, may be effective in lowering homocysteine levels. High concentrations of aluminum in the brains of persons with Alzheimer disease are most likely a result of the disease and not a cause, as correspondingly high levels of aluminum are not found in blood and hair. There is ongoing research into the possible value of dietary supplements for the normal memory problems that beset healthy older people.
Eating a healthful diet, obtaining sufficient sleep, avoiding smoking, keeping physically fit, and maintaining an active mind are among the practices that may increase not only life expectancy but also the chance of a full and productive life in one’s later years. The so-called free-radical theory of aging—the notion that aging is accelerated by highly reactive substances that damage cellular components, and that intake of various antioxidants can repair free-radical damage and thereby slow aging—has generated much interest and is a promising area of research, but it has not been scientifically established. On the contrary, the life spans of various mammalian species have not been extended significantly by antioxidant therapy. Ongoing studies are investigating whether the consumption of 30 percent fewer calories (undernutrition, not malnutrition) slows aging and age-related disease and extends life spans in nonhuman primates. There is no evidence that severe energy restriction would extend the human life span beyond its current maximum of 115 to 120 years.
References of this information and pictures at: http://www.britannica.com/EBchecked/topic/422896/human-nutrition