Factors influencing serum 25-hydroxivitamin D levels and other bone metabolism parameters in healthy older women

Older women have frequently low serum 25-hydroxivitamin D (25[OH]D) concentrations, high parathormone (PTH) levels and low bone mineral density (BMD) values. Endogenous synthesis, dietary habits, sunlight exposure and fat-mass-mediated storage may influence 25(OH)D levels and bone metabolism, but the relevance of these factors in the elderly has yet to be fully elucidated. We aimed to investigate the influence of dietary vitamin D intake and fat mass on serum 25(OH)D levels and bone metabolism in older women. Design: Cross-sectional. Setting: Community. Participants: 218 fit older women attending a biweekly mild fitness program. Measurements: Dietary habits was investigated through a 3-day record questionnaire. Serum 25(OH)D and intact parathormone (PTH) concentrations were measured by radioimmunoassay and by a 2-step immunoradiometric assay, respectively. BMD and body composition were estimated using dualenergy X-ray absorptiometry with fan-beam technology. Results: Only fat mass showed a significant negative association with 25(OH)D (β=-3.76, p<0.001), and positive associations with whole body, lumbar, femoral neck and total hip BMD. Binary logistic analysis revealed a protective effect of adiposity on secondary hyperparathyroidism (OR=0.42, 95%CI:0.19-0.92, p=0.03). Dietary vitamin D intake was not associated to any of these outcomes. Conclusion: Fat mass has a greater influence on serum 25(OH)D than dietary vitamin D intake