Seasonal changes in vitamin D status and bone turnover in healthy Irish postmenopausal women

Objectives: To examine the effect of season on biochemical markers of bone turnover in 51 -to 75-year-old Irish women and to investigate whether such changes are related to vitamin D status. Design: Longitudinal observational study. Setting: Cork, Ireland (52°N). Subjects: 76 apparently healthy, free-living postmenopausal women (aged 51-75 years), not taking any medication and free from any condition likely to affect vitamin D status or calcium/bone metabolism. Results: Serum 25-hydroxy vitamin D [S-25(OH)D] showed a clear seasonal variation with significantly higher (p < 0.001) values during late summer than late winter. Both urinary pyridinoline (Ur-Pyr) (p < 0.01) and deoxypyridinoline (Ur-Dpyr) (p < 0.001), but not serum parathyroid hormone or osteocalcin, showed a marked seasonal variation with lowest values during late-summer. Stratifying women into those with S-25(OH)D levels ≥ or < 50 nmol/L (vitamin D adequate and inadequate, respectively) during late-winter, showed that Ur-Pyr and Ur-Dpyr in both groups were significantly (p < 0.05) lower during late summer, and the magnitude of the reduction from winter to summer was similar in both groups. Conclusion: Seasonal changes in bone resorption markers appeared to be linked to seasonal changes in vitamin D status. Further research is needed to investigate the impact of these changes on risk of bone loss and fracture.