A threshold for low-protein-diet-induced elevations in parathyroid hormone

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 amoun ts of dietary protein (1.0 g/kg) did not appear to perturb calcium homeosta sis. Objective: The purpose of this study was to evaluate the effect of graded i ntakes of dietary protein (0.7, 0.8, 0.9, and 1.0 g/kg) on calcium homeosta sis. 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 exp erimental diet containing 1 of 4 amounts of protein. Eight young healthy wo men received the 4 amounts of protein in random order. The average age of t he subjects was 23.1 +/- 2.3 y, their weight was 64 +/- 3 kg, and their bod y 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 a nd 0.8 g protein/kg but not during the diets containing 0.9 or 1.0 g protei n/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 t han 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 c alcium 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.