Metabolic Constituents: Water, Minerals and Vitamins

Animals require water for metabolic reactions. Water is channeled while solutes are filtered and pumped across gills, salt glands, rectal glands, kidneys, urinary bladders and the digestive tract. The body water pool can be maintained by a wide range of complementary influxes (free water, preformed water in food, metabolic water produced from chemical reactions) and effluxes (excreta, respiratory and cutaneous water losses). Water requirements vary with age, body size and environmental conditions including temperature and drought. High water turnovers are associated with high energy demands during growth and reproduction. High energy demands increase respiratory water loss as well as intakes of food and solutes that enhance fecal and urinary water losses. The availability of minerals for animals depends on the geology and hydrology of their habitat. Mineral concentrations vary widely among the tissues of plants and animals, and each element can serve several functions. Body pools of macrominerals (Na, K, Ca, P, Mg, S) are large and widespread whereas pools of trace minerals (Mn, Cu, Fe, Zn, I, Se) are small and distributed among intracellular spaces. Trace minerals are mainly used as catalytic centers in enzymes and transport proteins. Both trace minerals and vitamins are required for metabolism of energy and for protecting cells against oxidative damage. Watersoluble vitamins include the B vitamins and ascorbic acid (vitamin C), which mainly are associated with the aqueous contents of cells distributed throughout the body. The fat-soluble vitamins (A, D, E and K) are found in lipids and membranes; accumulations in body lipids may be at high enough concentrations to have toxic effects. Requirements for trace minerals and vitamins are affected by interactions among nutrients and by stressors (such as disease), which may change with population density and animal movements.