Dietary protein affects intestinal calcium absorption

Background: Changes in dietary protein in adults are associated with changes in urinary calcium excretion. The mechanisms underlying this effect are not completely understood, but alterations in intestinal absorption of calcium are not thought to be involved. Objective: We reexamined this mechanism by evaluating the effect of 2 amounts of dietary protein (low: 0.7 g/kg; and high: 2.1 g/kg) on fractional calcium absorption in 7 healthy, young women. Design: The experiment consisted of 2 wk of a well-balanced diet containing moderate amounts of calcium, sodium, and protein followed by 5 d of an experimental diet that contained 1 of 2 amounts of protein and constant amounts of other nutrients known to influence calcium metabolism. Seven subjects received both amounts of dietary protein in random order. Blood and urine were sampled at baseline and on day 4. Fractional calcium absorption was measured by dual-stable calcium isotopes on day 5. In a second study of 5 additional women, we evaluated the effects of dietary fiber on calcitropic hormones. Results: Subjects developed hypocalciuria and secondary hyperparathyroidism on day 4 of the low-protein diet. Urinary calcium excretion and the glomerular filtration rate were elevated significantly by day 4 of the high-protein compared with the low-protein diet. Fractional calcium absorption after the low-protein diet was 0.19 +/- 0.03, which was significantly lower than that after the high-protein diet (0.26 +/- 0.03, P = 0.05). Conclusion: These data provide evidence that depressed intestinal calcium absorption explains, in part, low-protein-induced secondary hyperparathyroidism.