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Calcium is lost from the body in feces, urine, and sweat. The fecal calcium consists of unabsorbed dietary calcium, the amount of which depends on dietary intake and other factors, and a small portion of the endogenously secreted calcium (about 100 to 150 mg/day), which escapes reabsorption. Urinary calcium excretion of adults is about 100 to 250 mg/day, but varies widely among persons consuming selfselected diets (Nordin et al., 1967). Urinary excretion is influenced by hormonal and dietary factors. Among the latter are protein, sodium, and some carbohydrates, which increase calcium excretion, and phosphorus, which decreases it. Except under conditions of extreme sweating, loss of calcium from the skin is small (about 15 mg/ day).

Calcium Absorption

Intestinal absorption of calcium is variably influenced by several nutritional and physiological factors (Avioli, 1988). Studies of calcium absorption have frequently been flawed by failure to include the extended period (4 weeks or more) required for adaptation to changes in dietary intake and to control for intervening dietary variables. Nonetheless, the literature is consistent on several points. The efficiency of absorption is increased during periods of high physiologic requirement. Thus, children may absorb up to 75% of ingested calcium as compared to the 20 to 40% typically observed in young adults in the United States. Absorption is impaired in the aged (Heaney et al., 1982). A higher percentage of ingested calcium is absorbed at low intakes than at high intakes. Above an intake of about 800 mg/ day in normal adults, absorption is approximately 15% of the amount ingested (Heaney et al., 1975). Vitamin D is a recognized promoter of calcium absorption. The role of protein and phosphorus is less clear. Dietary protein enhances calcium absorption (McCance et al., 1942) in the protein intake range between inadequate and adequate levels, but has little additional effect beyond RDA levels of protein (Chu et al., 1975). The effect of phosphorus differs with the source, but, excluding phytate phosphorus (see below), this element appears to have little if any depressing effect on calcium absorption (Spencer et al., 1978, 1986).

It is uncertain if there are biologically important differences in the absorption of calcium from different foods or diets. In animal models, the presence of lactose tends to enhance paracellular calcium absorption, but this effect has not been consistently demonstrated (Scrimshaw and Murray, 1988). Phytate and oxalate bind calcium, rendering it insoluble, and certain fiber fractions may interfere with