Calcium bioavailability and absorption: a review.

The current United States Recommended Dietary Allowances (RDAs) for calcium are presented in Table 1 (1). The RDA for the adult has been set at 800 mg/day for many years. However, considerable debate persists about the optimal dietary calcium intake for man. The FAO/WHO Committee on Calcium Requirements suggested that 400 to 500 mg/day was a more practical estimate of adult calcium requirements (2). Others have claimed that the 800 mg/day recommendation is too low to maintain maximum calcium retention (3), expecially in the elderly (4). There are several reasons why this state of uncertainty exists. Lower recommendations are based on the observation that the efficiency of calcium absorption increases when intakes are low. It is often stated that the habitual intake of many people in the world is considerably less than 800/mg day without apparent adverse consequences (5). One difficulty in interpreting this observation is that the only methods available for measuring calcium nutritional status depend on detecting long-term changes in bone calcium, and a considerable amount of demineralization must occur before loss of bone calcium can be detected. More recent studies suggest that calcium intakes on the order of the FAO/ WHO recommendation may be inadequate to maintain positive balance and optimal bone mineralization (6, 7). Additional factors that make determination of the optimal RDA difficult are the marked effects that various food constituents and the nutritional and metabolic state of the host have on calcium absorption. The purpose of this paper is to summarize the current information on how dietary factors affect the bioavailability of calcium. However, this cannot be achieved without also considering host-related factors, such as calcium and vitamin D status, age. pregnancy, lactation, and disease, which affect the absorption of calcium once it is available in the intestinal lumen in a theoretically absorbable form. The RDAs for the adult are estimated from metabolic experiments, in which balance was achieved when the amount of dietary calcium was equal to calcium losses in feces, urine, and sweat. Fecal loss is composed of unabsorbed dietary calcium, plus an endogenous fecal calcium excretion of approximately 100 to 130 mg/day (4, 8). Urinary losses are 150 mg/day (4) and loss in sweat is 15 mg/day (1). Thus, obligatory endogenous losses total 250 to 280 mg/day. The losses in urine, endogenous intestinal secretions, and sweat are relatively unaffected by the level of dietary calcium (4, 9) so that in the absence of other factors which increase endogenous loss (some of which will be described herein), total body calcium content is largely regulated by the amount of calcium absorbed. To cover an obligatory loss of 250 mg/day, the 800 mg/day RDA for adults assumes an average absorption of 30%. In pregnancy, 30 g calcium is deposited in the fetus at the rate of 120 mg/day during the 20th and 30th wk, and 260 mg/day from the 30th wk until term ( 10). If the fetal calcium is obtained from the 1200 mg in the maternal diet, 30% must be absorbed between wk 20 and 30, assuming that 250 mg are needed to cover maternal endogenous losses, and that 120 mg are transferred to the fetus. Similarly, absorption must be 40% (5 10 mg) between the 30th wk and term. The calcium content