Is protein intake associated with bone mineral density in young women?

Background: The range of protein intakes for optimizing bone health among premenopausal women is unclear. Protein is a major constituent of bone, but acidic amino acids may promote bone resorption. Objective: The objective was to examine cross-sectional and longitudinal associations between baseline dietary protein and bone mineral density (BMD) among 560 females aged 14-40 y at base-line enrolled in a Pacific Northwest managed-care organization. The role of protein source (animal or vegetable) and participant characteristics were considered. Design: Dietary protein intake was assessed by using a semiquantitative food-frequency questionnaire in participants enrolled in a study investigating associations between hormonal contraceptive use and bone health. Annual changes in hip, spine, and whole-body BMD were measured by using dual-energy X-ray absorptiometry. Cross-sectional and longitudinal associations between baseline protein intake (% of energy) and BMD were examined by using linear regression analysis and generalized estimating equations adjusted for confounders. Results: The mean (±SD) protein intake at baseline was 15.5 ± 3.2%. After multivariable adjustment, the mean BMD was similar across each tertile of protein intake. In cross-sectional analyses, low vegetable protein intake was associated with a lower BMD (P = 0.03 for hip, P = 0.10 for spine, and P = 0.04 for whole body). For every percentage increase in the percentage of energy from protein, no significant longitudinal changes in BMD were observed at any anatomic site over the follow-up period. Conclusions: Data from this longitudinal study suggest that a higher protein intake does not have an adverse effect on bone in premenopausal women. Cross-sectional analyses suggest that low vegetable protein intake is associated with lower BMD.