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Improved Estimation of the Calcium Content of Total Digestive Secretions

Osteoporosis Research Center (R.P.H.), Creighton University, Omaha, Nebraska 68131; and U.S. Department of Agriculture/Agricultural Research Service (S.A.A.), Children’s Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine and Texas Children’s Hospital, Houston, Texas 77030

Address all correspondence and requests for reprints to: Robert P. Heaney, M.D., Creighton University, 601 North 30th Street, Suite 4841, Omaha, Nebraska 68131. E-mail: rheaney{at}creighton.edu.

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)].

This work was supported by National Institute of Arthritis and Musculoskeletal and Skin Diseases Grant 079112 and the U.S. Department of Agriculture/Agricultural Research Service under Cooperative Agreement no. 58-6250-6-001, the National Institutes of Health (NIH), National Center for Research Resources General Clinical Research for Children Grant no. R00188, and NIH Grant HD 36591.

Abbreviations: AbsFx, Fractional calcium absorption; DIC, distal component of intestinal calcium entry; EFC, endogenous fecal calcium; GI, gastrointestinal; PIC, proximal component of intestinal calcium; TIC, total intestinal calcium.

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