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Background: We reported previously that lowering dietary protein intake in young healthy women to 0.7 g/kg depressed intestinal calcium absorption and was accompanied by elevations in parathyroid hormone (PTH). Moderate amounts of dietary protein (1.0 g/kg) did not appear to perturb calcium homeostasis., Objective: The purpose of this study was to evaluate the effect of graded intakes of dietary protein (0.7, 0.8, 0.9, and 1.0 g/kg) on calcium homeostasis., Design: The experiment consisted of 2 wk of a well-balanced diet containing moderate amounts of calcium, sodium, and protein followed by 4 d of an experimental diet containing 1 of 4 amounts of protein. Eight young healthy women received the 4 amounts of protein in random order. The average age of the subjects was 23.1 +/- 2.3 y, their weight was 64 +/- 3 kg, and their body mass index (in kg/m(2)) was 24.3 +/- 0.9., Results: Elevations in PTH developed by day 4 of the diets containing 0.7 and 0.8 g protein/kg but not during the diets containing 0.9 or 1.0 g protein/kg. By day 4 of the 0.7- and 0. 8-g/kg diets, midmolecule PTH, calcitriol, and nephrogenous cyclic adenosine monophosphate were 1.5-3.5-fold higher than on day 0. Calcitropic hormones on day 4 of the diets containing 0.8 and 0.9 g protein/kg were within the normal range and 23-57% lower than values observed with the 0.7- and 0.8-g/kg diets (P < 0.005). Mean 24-h urinary calcium was 3.29 +/- 0.35 mmol with the diet containing 0.7 g protein/kg and 3.54 +/- 0.46 mmol with the diet containing 1.0 g protein/kg., Conclusions: Our data suggest that in young healthy women consuming a well-balanced diet, the current recommended dietary allowance for protein (0.8 g/kg) results in short-term perturbations in calcium homeostasis.

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.