NUTRITION

the act or process by which living organisms absorb and utilize food substances. Food serves three major functions: it generates energy for growth, maintenance, and activity; supplies the reducing agents that help to make enzymes, which carry out cellular processes; and provides the materials for cell building. Some organisms (such as green plants) are autotrophic, requiring only inorganic nutrients, whereas others (such as fungi) are heterotrophic, requiring both inorganic and organic substances. Green plants are lithotrophic (deriving reducing agents from such inorganic sources as water), whereas animals are organotrophic (deriving reducing agents from ingested organic compounds). In addition, green plants are phototrophic (they get their energy for life processes from light), whereas animals are chemotrophic (they get their energy from ingested chemicals). The earliest organisms were probably heterotrophic, and, as the Earth's supply of organic nutrients began to be depleted, certain life forms evolved an ability to synthesize food from inorganic components by photosynthesis. Because these forms were able to replenish the supply of organic nutrients, heterotrophic life forms were able to proliferate. The essential human nutrients are proteins, minerals, and vitamins; a dietary deficiency or a defect in the body's absorption of any of these will result in malnutrition and disease. Proteins are broken down during digestion into amino acids, which are used for tissue building, as transmitters of genetic information, and for energy. Protein requirements are greater for growing children and adolescents than for adults. In some poor countries, kwashiorkor, a deficiency of amino acids, is a common cause of death among children. Minerals are important in a number of body processes. Iron is used in the blood to make hemoglobin; iron-deficiency anemia is more common in women than in men, as it may be a consequence of heavy menstrual bleeding or childbirth. Calcium is crucial to the growth of bones and teeth; its deficiency can cause rickets in children and osteomalacia (bone softening) in adults. Both sodium and potassium are primary contributors of ions to the electrolytic extracellular fluid; deficiency of either may upset the homeostatic equilibrium. Loss of potassium through diuresis or diarrhea may result in loss of tissue excitability and muscle paralysis. Excessive sodium levels may bring on high blood pressure, and sodium deficiency may cause low blood pressure and epileptic seizures. Iodine is used to synthesize thyroid hormones. Traces of other minerals are also found in the human body, but the functions that they serve are not all known. Vitamins are organic substances required by the body but either not synthesized by it or not synthesized in amounts sufficient to meet the body's requirements. Vitamins often serve to help enzymes with cellular reactions. Major nutritional disorders resulting from vitamin deficiencies include beriberi (lack of vitamin B1, or thiamine), pellagra (lack of niacin), scurvy (lack of vitamin C), xerophthalmia (lack of vitamin A), and rickets and osteomalacia (lack of vitamin D). All these disorders were prevalent in the 19th century, when they were associated with the abandonment of agriculture for urban and industrial occupations. Only xerophthalmia, which leads to keratomalacia and blindness in children, has not been controlled. Human foods are divided into as many as 17 groups, including cereals, starchy roots, legumes, other vegetables, fruits, sugars, meat, fish, eggs, milk and cheese, fats and oils, alcoholic beverages, nonalcoholic beverages, nuts and seeds, herbs and spices, dietetic preparations, and miscellaneous (salt, vinegar). A basic nutritional goal is to have a balanced diet, with items from all of the major food groups, although certain diets (e.g., for weight loss or for other therapeutic purposes) may require more or less from one group than another. Several national and international organizations have been founded to assess the nutritional needs of populations and to discover ways in which these needs can be met. These agencies have devised nutritional requirements and recommended standards as well as programs of food inspection and distribution. The provision of an adequate food supply and nutritional education to all people, however, remains a crucial problem. the process by which an organism absorbs and utilizes food substances. The study of nutrition involves the identification of individual nutrients essential for growth and for the maintenance of individual organisms; it includes the determination of interrelationships among nutrients within individual organisms, as well as the evaluation of the quantitative requirements of organisms for specific nutrients under various environmental conditions. Treated in this article are the identification and determination of the requirements of nutrients, the ways in which they vary from one organism to another, and the ways in which requirements for certain of them arose. Additional reading Major journals covering general human and animal nutrition include The Journal of Nutrition (monthly); The British Journal of Nutrition (bimonthly); Nutrition Abstracts and Reviews: Series A. Human and Experimental (monthly); Nutrition Abstracts and Reviews: Series B. Livestock Feeds and Feeding (monthly); The American Journal of Clinical Nutrition (monthly); European Journal of Clinical Nutrition (monthly); Journal of the American Dietetic Association (monthly); Nutrition Reviews (monthly); and Annual Review of Nutrition. The nutritional requirements of microorganisms are detailed in Beverly M. Guirard and Esmond E. Snell, Biochemical Factors in Growth, ch. 7, pp. 79111, in Philipp Gerhardt (ed.), Manual of Methods for General Bacteriology (1981). Good sources on animal nutrition in general with information on particular animals, such as ruminants, include P. McDonald, R.A. Edwards, and J.F.D. Greenhalgh, Animal Nutrition, 4th ed. (1988); and D.C. Church, Digestive Physiology and Nutrition of Ruminants, 2nd ed., vol. 2, Nutrition (1979). Most common (and some uncommon) animals are covered in Nutrient Requirements of Domestic Animals, a multivolume series by the National Research Council (U.S.); as well as in The Nutrient Requirements of Farm Livestock, a series by the British Agricultural Research Council. See also E.J. Underwood, The Mineral Nutrition of Livestock, 2nd ed. (1981); and Walter Mertz (ed.), Trace Elements in Human and Animal Nutrition, 5th ed., 2 vol. (198687), a classic text on inorganic micronutrients. A. Stewart Truswell