Determinants of endogenous fecal calcium in healthy women

Endogenous fecal calcium (EFC) excretion was measured in 518 studies in 191 normal perimenopausal women, most studied two to three times over a 15 year period. EFC averaged 102 ± 25 mg/day. Absorption fraction was simultaneously determined by both double-isotope and balance methods. EFC was found to vary inversely with absorption fraction, and the observed relationship was used to calculate the total amount of calcium (TIC) entering the gut from endogenous sources. TIC averaged 140 ± 34 mg/day and was found to be correlated with a number of intake and body size variables. Phosphorus intake was the most strongly correlated of all the variables (r = 0.404; P <0.0001), each increment of 0.1 g phosphorus intake being associated with an increase in TIC of 6 mg. Lean body mass was the best correlated of the body size variables, with TIC rising by 1.6 mg/day for every kg lean mass. There were also small but significant correlations with protein and energy intakes, the latter suggesting that some of the variation of TIC is related to the amount of food consumed. Caffeine, previously reported as elevating TIC, did not exhibit a significant relationship in this study.