Calcium absorption as a function of calcium intake

The relationship between the amount of calcium absorbed and the quantity ingested was evaluated in 180 adult humans. Absorption was measured from the concentration ratio of concurrently administered oral and intravenous calcium isotopes. Intake ranged from 0.163 to 7.48 Gm. Ca per day. In 14 subjects, intakes were artificially elevated for purposes of this study. All others were studied at their usual intake levels. Absorption (Ca Abs) was found to follow a curvilinear relationship with intake (Ca_D), and was characterized by the following equation: Ca Abs = 0.1541 · Ca_D + 0.3127[exp(-1.0539 · Ca_D] · Ca_D. The exponential term of this equation provided the major component of total absorption at intakes below 0.8 Gm. per day, but fell to negligible values when intake reached 2 to 3 Gm. per day, above which absorption was characterized by a simple linear function of intake. We found that there was no detectable upper limit to absorption capacity, which, at the 7.48 Gm. intake level, averaged more than 1.0 Gm. per day. The observed mathematical description is consistent with the generally recognized inverse relationship between absorption efficiency and intake. At the same time it indicates that a component of absorption is independent of control mechanisms and is related solely to intake. A more general form of the foregoing equation, suggesting provision for other physiological variables such as growth hormone and cortisol, is proposed and discussed.