Determinants of endogenous calcium entry into the gut.

BACKGROUND: In addition to food sources, calcium enters the gut by way of digestive secretions and shed mucosa. In health, such entry is as large as or larger than urinary calcium excretion. Because calcium absorption is inefficient, most of this endogenous intestinal calcium is excreted.
OBJECTIVE: Our aim was to determine the dietary, anthropometric, and physiologic determinants of calcium entering the digestive stream from endogenous sources.
DESIGN: Multiple regression modeling of intake and excretion data was used with 553 metabolic balance and kinetics studies performed in 190 midlife, white women.
RESULTS: Endogenous intestinal calcium averaged 3.29 +/- 0.83 mmol/d. Multiple regression models explaining variation in this endogenous intestinal calcium were developed with use of dietary intake, anthropometric, and serum mineral variables. All 3 groups of predictor variables individually explained up to 22% of the variation in measured values for endogenous intestinal calcium. A composite model, incorporating all 3 groups explained 29% of the variation, with phosphorus and meat protein intakes, height, weight, and serum calcium and phosphorus concentrations all independently entering the model. Phosphorus intake dominated over all the other predictors, explaining 20% of the variance all by itself, with endogenous intestinal calcium rising by 0.037 mmol for every 1 mmol of phosphorus ingested. Meat protein (but not nonmeat protein) was the only other significant dietary contributor, exhibiting a negative coefficient.
CONCLUSION: As a first approximation, the amount of endogenous calcium entering the digestive stream rises with body size and with the amount of phosphorus-rich food consumed.