The Impact of Dietary Protein on Calcium Absorption and Kinetic Measures of Bone Turnover in Women

Abstract

Although high-protein diets induce hypercalciuria in humans, the source of the additional urinary calcium remains unclear. One hypothesis is that the high endogenous acid load of a high-protein diet is partially buffered by bone, leading to increased skeletal resorption and hypercalciuria. We used dual stable calcium isotopes to quantify the effect of a high-protein diet on calcium kinetics in women. The study consisted of 2 wk of a lead-in, well-balanced diet followed by 10 d of an experimental diet containing either moderate (1.0 g/kg) or high (2.1 g/kg) protein. Thirteen healthy women received both levels of protein in random order. Intestinal calcium absorption increased during the high-protein diet in comparison with the moderate (26.2 +/- 1.9% vs. 18.5 +/- 1.6%, P < 0.0001, mean +/- sem) as did urinary calcium (5.23 +/- 0.37 vs. 3.57 +/- 0.35 mmol/d, P < 0.0001, mean +/- sem). The high-protein diet caused a significant reduction in the fraction of urinary calcium of bone origin and a nonsignificant trend toward a reduction in the rate of bone turnover. There were no protein-induced effects on net bone balance. These data directly demonstrate that, at least in the short term, high-protein diets are not detrimental to bone.

Nutritional aspects of the prevention and treatment of osteoporosis

“…In fact, previous calcium balance studies did not show an improvement in calcium absorption (even with high protein intake), the increase in urinary calcium was assumed to be of bone origin, which leads to the hypothesis that high protein diets are detrimental to the skeleton (37). However, a recent study demonstrated that the increase in urinary calcium excretion is due to improved intestinal calcium absorption, and high protein diets do not result in negative skeletal calcium balance (37,38).…”

“…Furthermore, many proteinrich foods such as meat and dairy foods are also rich in phosphorus and potassium, both of which have an opposing effect in that they tend to prevent urinary calcium loss (39). Finally, some aminoacids promote calcium absorption which would also offset urinary losses, and as long as calcium intake is adequate, higher protein intakes have been shown to improve bone density (37).…”

“…Reassessments of the nitrogen balance studies show that a protein intake of 1.0 to 1.3 g/kg per day is needed to offset the typically lower energy intake and impaired insulin response in elderly individuals (40). Moderate protein diets of 1.0 to 1.5 g/kg per day are shown to be associated with normal calcium homeostasis without altering bone metabolism (37). These data seem to indicate that doubling the RDA from 0.8 g/kg per day to 1.5 to 1.6 g/kg per day may result in better muscle and bone health in elderly individuals.…”

Inhibiting gastric acid production does not affect intestinal calcium absorption in young, healthy individuals: A randomized, crossover, controlled clinical trial

“…We also estimated sample size based on the assumption that the observed 7.7% AE 3.5% difference in mean calcium absorption in normal subjects ingesting a diet containing 1.0 g/kg of protein versus 2.1 g/kg is due to an attendant increase in gastric acid production. (13) Using an effect size of 7.7% AE 3.5% (mean AE SD), a sample size of 12 provides power of 90% at a ¼ 0.05. We therefore chose a sample size of 12.…”

“…The study employed a crossover design in which each study subject served as his or her own control, thereby eliminating the confounding effect of interindividual variation on calcium absorption. We chose to use a high-protein diet for all interventions in our study because available data indicate that this would augment gastric acid production and intestinal calcium absorption, (10)(11)(12)(13) allowing us the best opportunity to quantify the contribution of gastric acid to calcium absorption.…”