Improved estimation of the calcium content of total digestive secretions

Because endogenous fecal calcium (EFC) excretion plays a larger role in the calcium economy of adults than does actual calcium intake, and because at typical absorption efficiencies EFC is largely determined by endogenous entry of calcium into the gut [total intestinal calcium (TIC)], it is important to be able to quantify TIC when studying the effect of nutrients, drugs, and disease on body calcium handling. Measured values for EFC, derived from 553 balance studies in middle-aged women (mean age, 48.8 yr), were pooled with 76 values obtained from premenarcheal girls (mean age, 11.2 yr) after adjusting for differences in body size. The aggregate sample provided values for calcium absorption efficiencies spanning a range from under 10% to over 90%. EFC is known to be inversely related to fractional calcium absorption (AbsFx), and the broad range of absorption efficiencies provided by the composite sample allowed derivation of reasonably precise estimates of EFC at 0% and 100% absorption. These were 0.8067 (+/- 0.0143) mg/cm height.d and 0.1799 (+/- 0.0310) mg/cm height.d, respectively. At 0% absorption, EFC is identical to the total entry of calcium into the intestine from endogenous sources (secretions plus sloughed mucosa), whereas at 100% absorption, EFC measures only endogenous entry effectively distal to the absorptive surface. The partition described facilitated derivation of a formula to calculate TIC for any paired values of EFC and AbsFx, i.e. TIC = EFC/[0.223 + 0.777 (1 - AbsFx)].