Distribution of calcium absorption in middle-aged women.

Distribution of true fractional absorption of calcium in estrogen-deprived, nonosteoporotic, middle-aged women was examined in 273 person-studies, using a double-tracer method. Fractional absorption averaged 0.266 (+/- 0.096). Calcium intake averaged 0.802 g/day (+/- 0.419) and urine calcium, 0.144 g/day (+/- 0.062). Fractional absorption was inversely correlated with intake. Using parameters of the regression equation for the two variables, each absorption value was adjusted to the mean intake of the group so that absorption could be evaluated apart from the effect of intake. The distribution of such diet-adjusted values revealed that 55% of these estrogen-deprived women had insufficient absorption to maintain calcium balance at an intake equal to the 1980 RDA. Nearly one-fourth would still be in negative balance at an intake of 1.5 g/day. Since urine calcium was positively correlated with absorbed calcium (r = 0.521), we conclude that defective absorption and not bone breakdown is the basis for low absorptive performance in these women.