DIETARY-PROTEIN AFFECTS INTESTINAL CALCIUM-ABSORPTION

Background: Changes in dietary protein in adults are associated with c hanges in urinary calcium excretion. The mechanisms underlying this ef fect are not completely understood, but alterations in intestinal abso rption of calcium are not thought to be involved. Objective: We reexam ined this mechanism by evaluating the effect of 2 amounts of dietary p rotein (low: 0.7 g/kg; and high: 2.1 g/kg) on fractional calcium absor ption in 7 healthy, young women. Design: The experiment consisted of 2 wk of a well-balanced diet containing moderate amounts of calcium, so dium, and protein followed by 5 d of an experimental diet that contain ed 1 of 2 amounts of protein and constant amounts of other nutrients k nown to influence calcium metabolism. Seven subjects received both amo unts of dietary protein in random order. Blood and urine were sampled at baseline and on day 4. Fractional calcium absorption was measured b y dual-stable calcium isotopes on day 5. In a second study of 5 additi onal women, we evaluated the effects of dietary fiber on calcitropic h ormones. Results: Subjects developed hypocalciuria and secondary hyper parathyroidism on day 4 of the low-protein diet. Urinary calcium excre tion and the glomerular filtration rate were elevated significantly by day 4 of the high-protein compared with the low-protein diet. Fractio nal calcium absorption after the low-protein diet was 0.19 +/- 0.03, w hich was significantly lower than that after the high-protein diet (0. 26 +/- 0.03, P = 0.05). Conclusion: These data provide evidence that d epressed intestinal calcium absorption explains, in part, low-protein- induced secondary hyperparathyroidism.