Mean intakes below the RDA do not necessarily mean that individuals in the group are malnourished. In USDA surveys, calcium intakes have been reported in terms of the 1980 Recommended Dietary Allowance (RDA), which is highest (1,200 mg) at ages 11 to 18 years and is only 800 mg for ages 1 to 10 and 18 and above (NRC, 1980). Information on current intakes of calcium, phosphorus, and magnesium has been collected in national surveys, including the 1977-1978 Nationwide Food Consumption Survey (USDA, 1984), the second National Health and Nutrition Examination Survey (Carroll et al., 1983), the Continuing Survey of Food Intakes of Individuals (USDA, 1986, 1987), and the Total Diet Study (Pennington et al., 1986) (see Chapter 3). The decline resulted primarily from the decreased use of grains and flour and increased practice of low-extraction milling. The per-capita availability of phosphorus in the food supply has remained fairly steady at 1,500 mg/day since 1909-1913, and that of magnesium has declined from 380 mg/day during 1909-1913 to 320 mg/day in 1985 (see Table 3-3). The change resulted primarily from an increased supply of dairy products during this period. Per-capita calcium availability in the food supply increased 23% from 750 mg/day during 1909-1913 to 920 mg/day in 1985 (see Table 3-3). These data do not represent actual consumption, however, since they fail to document how much food was wasted. Department of Agriculture (USDA) since 1909 (see Table 3-3). Historical trends in the amounts of various minerals present in the food supply have been reported by the U.S. Magnesium is second only to potassium as the most predominant cation within cells and is essential both for the functions of many enzyme systems and for neuromuscular transmission. Of total body magnesium, 60 to 65% is found in bone and 27% is located in muscles (Shils, 1988). The phosphate ion also plays an important role in acid/base balance. In the form of high-energy phosphate compounds, phosphorus contributes to the metabolic potential. The remainder of body phosphorus is needed in soft tissues as a cofactor in myriad enzyme systems essential in the metabolism of carbohydrates, lipids, and proteins. Phosphorus, along with calcium, is essential for calcification of bones (85% of body phosphorus is located in the skeleton). If it is insufficient when intestinal absorption is impaired or when there are inordinate losses, the serum concentration of calcium ion (Ca 2+) can be maintained at normal levels only at the expense of bone calcium (Arnaud, 1988). If there is insufficient dietary calcium during bone formation, linear growth will be impeded and peak bone mass may not be achieved. People need more calcium in their diets when they are forming bone, when intestinal absorption of calcium is impaired, and when there are inordinate losses of calcium to the environment (e.g., through increased renal excretion or lactation). Thus, bone acts as a vital physiological tissue providing a readily available source of calcium for maintenance of normal plasma calcium levels, 50% of which is ionized and physiologically active (Arnaud, 1988). These functions are so vital to survival that during severe dietary deficiency or abnormal losses of calcium from the body, they can demineralize bone to prevent even minor degrees of hypocalcemia (i.e., low plasma calcium). All living animals possess powerful mechanisms both to conserve calcium and to maintain constant cellular and extracellular concentrations (Arnaud, 1978, 1988 Exton, 1986). Approximately 1,200 g of calcium are present in the body of an adult human more than 99% of that amount is found in bones. Minerals needed in smaller amounts are called trace elements these are discussed in Chapter 14.Ĭalcium is the most abundant mineral in the human body, making up 1.5 to 2% of the total body weight. Calcium, phosphorus, and magnesium are required in relatively large amounts and are designated as macrominerals. Mineral salts are responsible for structural functions involving the skeleton and soft tissues and for regulatory functions including neuromuscular transmission, blood clotting, oxygen transport, and enzymatic activity.
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