Dietary protein holds positive and negative effects on bone health in rats

Abstract

The effect of high protein intakes on bone health is controversial, with both positive and negative effects reported. Human studies are clouded by correlations of protein, calcium and other factors. To control these, 48 Sprague‐Dawley rats aged 52d were randomized to 10 wk diets: PRO: 35% energy as casein, 40% as carbohydrate; CHO: 15% casein and 60% carbohydrate. All other nutrients were matched. Measures included serum IGF‐1 by RIA, mechanical strength of the femur midshaft by 3‐point bend test, bone density by dual x‐ray absorptiometry and, in a random subsample (n=16), trabecular microarchitecture by micro‐computed tomography. Energy intakes did not differ. Respective PRO and CHO M±SD were: trabecular spacing, 232±6, 224±4 um; trabecular number, 2.97±0.16, 3.09±0.09 per mm (p=0.08); trabecular thickness, 120±4, 124±5 um (p=0.08); IGF‐1, 266±61, 204±62. No difference in mechanical strength was observed; however after covarying for IGF‐1 protein had both a negative effect (ß=‐0.37) on work to failure, and an opposing positive effect (ß=0.43) mediated by IGF‐1. Opposing effects were roughly equal in magnitude, producing no net effect on bone strength; however the PRO diet was detrimental to microarchitecture, increasing the average space between trabecular scaffolds. Results are consistent with the hypothesis that protein has both positive effects, mediated by IGF‐1, and negative effects on bone health.Grant Funding SourceUniversity of Illinois Division of Nutritional Science Margin of Excellence Award