Nutrition is the science that interprets the relationship of food to the functioning of the living organism. It is concerned with the intake of food, digestive processes, the liberation of energy, and the elimination of wastes, as well as with all the syntheses that are essential for maintenance, growth, and reproduction. These fundamental activities are characteristic of all living organisms--from the simplest to the most complex plants and animals. Nutrients are substances, either naturally occurring or synthesized, that are necessary for maintenance of the normal function of organisms. These include \Lcarbohydrate\ls, \Llipid\ls, \Tproteins\t, \Tvitamins and minerals\t, water, and some unknown substances. The nutritionist, a scientist working in the field of nutrition, differs from the dietitian, who translates the science of nutrition into the skill of furnishing optimal nourishment to people (see \Tdiet\t). Dietetics is a profession concerned with the science and art of human nutrition care, an essential component of the health sciences. The treatment of disease by modification of the diet lies within the province of the physician and the dietitian. Such modification can be, and often is, effected without the use of special foods, simply by changing the methods of food preparation or by restricting the diet. Special foods for particular dietary uses differ from ordinary foods by their specific composition or by physical, chemical, or other modifications resulting from processing. If there is an inability to metabolize one of the normal constituents of a diet, then this must be removed from the diet (see \Tdiet, human\t). Therapeutic diets fall into a number of categories. The foods consumed by humans must contain, in adequate amounts, about 45 to 50 highly important substances. Water and oxygen are equally essential. Starting only with these essential nutrients obtained from food, the body makes literally thousands of substances necessary for life and physical fitness. Most of these substances are far more complicated in structure than the original nutrients. Energy \Tmetabolism\t and requirements are customarily expressed in terms of the calorie, a heat unit. Adoption of the calorie by nutritionists followed quite naturally from the original methods of measuring energy metabolism. The magnitude of human energy metabolism, however, made it awkward to record the calorie measured, so the convention of the large calorie, or kilocalorie (kcal), was accepted. Atwater factors, also called physiologic fuel factors, are based on the corrections for losses of unabsorbed nutrients in the feces and for the calorie equivalent of the nitrogenous products in the urine. These factors are as follows: 1 g of pure protein will yield 4 calories, 1 g of pure fat will yield 9 calories, and 1 g of pure carbohydrate will yield 4 calories. ESSENTIAL NUTRIENTS Proteins Proteins are widely distributed in nature, and no life forms are without them. They are made up of relatively simple organic compounds, the amino acids, which contain nitrogen and sometimes sulfur. Humans and animals build the protein they need for growth and repair of tissues by breaking down the proteins obtained in food into their component parts, the amino acids, and then building up these components into proteins of the type needed. The protein-rich foods from animal sources contain complete proteins, which supply all the amino acids in the proper proportions necessary in the human diet. Proteins of plant origin must be combined to get the proper complement of amino acids, a matter of concern to vegetarians. The world's most prevalent nutritional deficiency is lack of protein. This deficiency affects physical growth and mental development, causes emotional and psychological disturbances, and reduces resistance to and subsequent recovery from diseases and infections, consequently shortening the life span. \Tkwashiorkor\t is the typical protein malnutrition found in children. The disease syndrome is variable and is characterized by growth retardation, loss of body fat, and muscle wasting (see also \Tnutritional-deficiency diseases\t). Vitamins and Minerals Most foods contain several \Tvitamins and minerals\t. Vitamins are organic food substances, needed only in minute quantities but essential for the normal metabolism of other nutrients to promote proper growth and maintenance of health. Many act as catalysts or help form catalysts in the body. Minerals--such as calcium, iodine, and iron--are essential to life and health. Minerals are a necessary part of all cells and body fluids and enter into many physiological and structural functions. Fats and Carbohydrates Fats, which are widely distributed in nature, are a concentrated food source of energy. Fats are glyceryl esters of fatty acids and yield glycerol and many different fatty acids when broken down by hydrolysis. Carbohydrates are the most abundant and least expensive food sources of energy. Important dietary carbohydrates are divided into two groups--starches and sugars. The starches, which may be converted into utilizable sugars in plants or in the human body, are supplied in the grains, the pulses, the tubers, and some rhizomes and roots. The sugars occur in many plants and fruits, the most important as a food commodity being sucrose, which is obtained from sugarcane or the sugar beet. DIETARY FIBER Dietary fiber, also known as bulk and roughage, is also an essential element in the diet even though it provides no nutrients. It consists of plant \Tcellulose\t and other indigestible materials in foods, along with \Lpectin\ls and gums. The chewing it requires stimulates saliva flow, and the bulk it adds in the stomach and intestines during digestion provides more time for absorption of nutrients. Diets with sufficient fiber produce softer, bulkier stools and help to promote bowel regularity and avoid constipation and other disorders, such as \Tdiverticulosis\t. Fruits, vegetables, whole-grain breads, and products made from nuts and legumes are all sources of dietary fiber (see \Tbran\t). A diet overly abundant in dietary fiber, however, can cut down on the absorption of important trace minerals during digestion. FOOD GROUPS Meat and Meat Substitutes Foods can be classified into several groups: milk, vegetable-fruit, meat and meat substitutes, bread-cereal, and other foods. The meat-and-meat-substitutes group includes beef; veal; lamb; pork; variety meats, such as liver, heart, and kidney; poultry and eggs; fish and shellfish; and dried peas and beans and nuts. This group contains many valuable nutrients. Because meat contains 9 to 19 percent protein, it is a very useful protein source. The protein quality is minimally affected by normal heating. Severe protein heating, however, alters amino acids and bonding forces within the protein and thus reduces protein quality. The amount of fat in a serving of meat depends on the kind of meat as well as on the amount of trimming and the method of cooking. The minerals copper, iron, and phosphorus occur in meats in significant amounts. The quantity of iron and copper found in liver is of particular interest. Pork liver contains considerably more iron than beef liver, but beef liver is also an excellent source of iron. Different meats vary in their vitamin content. Thiamine, riboflavin, and niacin, all B vitamins, occur in significant amounts in all meats. Pork is particularly high in thiamine. Liver usually contains a useful amount of vitamin A. Fruits and Vegetables Most vegetables are an important source of minerals, vitamins, and cellulose, and certain vegetables, such as potatoes, contribute appreciable quantities of starch. Large amounts of the minerals calcium and iron are in vegetables, in particular, beans, peas, and broccoli. Vegetables also help to meet the body's need for sodium, chloride, cobalt, copper, magnesium, manganese, phosphorus, and potassium. Carotenes (the precursor of vitamin A) and ascorbic acid (vitamin C) are abundant in many vegetables. Vegetables are useful in the diet for their cellulose content. Very little, if any, cellulose is digested in the body, but cellulose provides the roughage required to promote motility of food through the intestines. The \Tcitrus fruits\t are a valuable source of vitamin C, and the yellow fruits, such as peaches, contain carotene. Dried fruits contain an ample amount of iron, and figs and oranges are an excellent source of calcium. Like vegetables, fruits have a high cellulose content. Milk The milk group includes milk, cheese, and ice cream. The importance of milk in the diet has long been recognized. Whereas milk contains important amounts of most nutrients, it is very low in iron and ascorbic acid, and it is low in niacin. Calcium and phosphorus levels in milk are very high. Vitamin A levels are high in whole milk, but in the production of skim milk this fat-soluble vitamin is removed. Riboflavin is present in significant quantities in milk unless the milk has been exposed to light. The composition of whole milk is approximately 4.9% carbohydrate, 3.5% fat, 3.5% protein, and about 87% water. The main carbohydrate in milk is lactose, or milk sugar. The most abundant fatty acids in milk are oleic, palmitic, and stearic acids. Milk is a complete protein food containing several protein complexes; the chief protein fraction in cow's milk is casein. The whey protein complex includes lactalbumin, lactoglobulin, and lactomucin. Bread-Cereal The bread-cereal group includes all breads and cereals that are whole grain, enriched, or restored. Protein content is not high in cereals, but the large intake of cereals in some diets makes these products a significant source of protein. All cereals are very high in starch and hence are good, generally inexpensive sources of energy. The fat content of cereal products generally is very low unless the germ is included in the food. The whole-grain products contribute significant quantities of fiber and such trace vitamins and minerals as pantothenic acid, vitamin E, zinc, copper, manganese, and molybdenum. Other Foods To round out meals and to satisfy the appetite, other foods included in the diet are butter, margarine, other fats, oils, sugars, or unenriched refined-grain products. These are often ingredients in baked goods and mixed dishes. Fats, oils, and sugars are also added to foods during preparation or at the table. The other foods supply calories and can add to total nutrients in meals. DIETARY STANDARDS The dietary standards used in the United States are the Recommended Dietary Allowances (RDA), which were developed by the Food and Nutrition Board of the National Research Council, National Academy of Sciences. The allowances are described as the levels of intake of essential nutrients considered--in the judgment of the Food and Nutrition Board on the basis of available scientific knowledge--to be adequate to meet normal nutritional needs. Energy As Table 1 shows, energy allowances are determined according to age and average heights and weights. Individual activity level is also used to determine the level of ideal calorie consumption. For example, a decrease in recommended energy allowance with increasing age is consistent with the known decrease in basal metabolic rate that occurs with aging and with a possible decrease in physical activity. The recommended daily allowance for protein for adults is no more than 1.6 g/kg desirable weight. This assumes 70 percent efficiency in utilization of dietary protein. FEDERAL GUIDELINES In 1980 the U.S. Department of Health and Human Services outlined certain guidelines for nutrition, which recommended that people eat a variety of foods daily, including fruits; vegetables; whole and enriched grain products; dairy products; meats, poultry, fish, and eggs; and dry peas and beans. The greater the variety of foods eaten, the less likely is a deficiency or excess of any single nutrient to develop. Women who are pregnant or of childbearing age, elderly people, and infants have special nutritional needs, but on the whole most people can derive essential nutrients by eating the type of foods mentioned in the guidelines. The report emphasized that people should increase their consumption of complex carbohydrates--fruits, vegetables, and other unrefined foods--and naturally occurring sugars. The guidelines also recommended reducing the consumption of refined and processed sugars. They encouraged a reduction in fat consumption by decreasing the amount of fatty meats and replacing foods in the diet that have saturated fats with those having unsaturated fats. A reduction in the sodium intake by decreasing the amount of salt added to food was also recommended. Controversy The dietary goals suggested reducing \Tcholesterol\t intake by decreasing the amount of butterfat, eggs, and other sources in the diet. Controversy arose concerning cholesterol intake even though abundant evidence indicates that dietary fat and cholesterol are factors that increase the risk of atherosclerosis and coronary heart disease, which leads to heart attacks. In a separate action, the Food and Nutrition Board concluded that the evidence warrants no specific recommendations about dietary cholesterol for the healthy person, who does not need to be concerned about fat intake. Scientists and industry personnel are debating whether the diet of the U.S. population should be modified on the basis of these federal guidelines. The proportion of the population that would benefit from such changes is unknown, although early in 1984 the U.S. National Heart, Lung, and Blood Institute reported the results of a long-term study showing that persons with high cholesterol levels are clearly under greater risk of heart attacks and developing heart diseases than are persons with normal cholesterol levels. Additives The use of \Tfood additives\t is an unresolved dilemma. Although essential to the modern food industry, these additives are being associated with toxic hazards and health risks. A whole system of federal food-safety laws and regulations, as well as international food-safety agencies and guidelines, have been evolved in an attempt to keep foodstuffs free of toxic elements. FOOD LABELING Nutritional labeling seems to be a solution to the problem of consumer ignorance of nutrients in foods. For a meaningful labeling program, the nutritional constituency in terms of percentage of the RDA for each nutrient and a system of freshness dating should be included on each label. This development would bring major changes in food labeling on the packages and would give information about serving size, number of servings, calories, proteins, carbohydrates, fats, vitamins, and minerals per serving. Since 1973 the U.S. Food and Drug Administration has required complete nutrition labeling, but only if a nutrient is added to a food or a nutrient claim is made by the manufacturer. Some manufacturers have also voluntarily supplied such information on packages. Education concerning food and its value is necessary for individual action. The diets of many people, including those in the affluent world, are deficient in important nutrients--unnecessarily so sometimes because the food is available in abundance or at least in adequate supply. Even when people possess the purchasing power to buy the food, it is often not consumed in favor of either more traditional dishes or more sensually appealing foods. Nutritionists, physicians, and dietitians are faced with the problems of acquainting people with nutritious foods available to them, supplying them with the knowledge to make good nutritional decisions, and educating them to the dangers of poor food habits. Merely introducing people to wholesome food is not enough. People must also come to enjoy and actually prefer nutritious food if nutritional health is to become a reality. D. K. SALUNKHE Bibliography: Bourne, G. H., ed., International Nutrition in Health and Disease (1987); Burton, B. T., and Foster, W. R., Human Nutrition, 4th ed. (1988); Coleman, Ellen, Eating for Endurance (1988); Cook-Fuller, Charlotte, ed., Nutrition, 1990-1991, 3rd rev. ed. (1989); Ensminger, A. H. and M. E., Foods and Nutrition Encyclopedia (1983); Gibney, Michael J., Nutrition, Diet, and Health (1986); Gibson, Rosalind S., ed., Principles of Nutritional Assessment (1989); Guthrie, Helen A., Introductory Nutrition, 7th ed. (1988); Hamilton, Eva M., et al., Nutrition: Concepts and Controversies (1988); Lappe, Frances M., Diet for a Small Planet, 10th anniv. ed. (1982); McLaren, Donald S., ed., Nutrition in the Community, 2d ed. (1982); Scarpa, Ioannis S., and Kiefer, Helen C., Sourcebook on Food and Nutrition, 2d ed. (1980); Spallholz, Julian E., Nutrition: Chemistry and Biology (1988); Stanfield, Peggy, Nutrition (1987); Whitney, A. N., and Hamilton, E. M., Understanding Nutrition, 4th ed. (1987).