Effect of dietary protein supplements on calcium excretion in healthy older men and women

Currently there is no consensus on the impact of dietary protein on calcium and bone metabolism. This study was conducted to examine the effect of increasing protein intake on urinary calcium excretion and to compare circulating levels of IGF-I and biochemical markers of bone turnover in healthy older men and women who consumed either a high or a low protein food supplement for 9 wk. Thirty-two subjects with usual protein intakes of less than 0.85 g/kg.d were randomly assigned to daily high (0.75 g/kg) or low (0.04 g/kg) protein supplement groups. Isocaloric diets were maintained by advising subjects to reduce their intake of carbohydrates. Selected biochemical measurements were made at baseline and on d 35 and either d 49 or 63. Changes in urinary calcium excretion in the two groups did not differ significantly over the course of the study. The high protein group had significantly higher levels of serum IGF-I (P = 0.008) and lower levels of urinary N-telopeptide (P = 0.038) over the period of d 35-49 or 63. We conclude that increasing protein intake from 0.78 to 1.55 g/kg.d with meat supplements in combination with reducing carbohydrate intake did not alter urine calcium excretion, but was associated with higher circulating levels of IGF-I, a bone growth factor, and lowered levels of urinary N-telopeptide, a marker of bone resorption. In contrast to the widely held belief that increased protein intake results in calcium wasting, meat supplements, when exchanged isocalorically for carbohydrates, may have a favorable impact on the skeleton in healthy older men and women.