The seasonal importance of serum 25-hydroxyvitamin D for bone mineral density in older women.

BACKGROUND: The impact of season when determining a serum 25-hydroxyvitamin D (S-25OHD) cut-off level for optimal bone health is unknown.

OBJECTIVE: To investigate the relative importance of S-25OHD for bone mineral density (BMD) by season.

METHODS: A subcohort of 5002 Swedish women (mean age 68 years), randomly selected from a large population-based longitudinal cohort study with repeat dietary and lifestyle information, was enrolled during 2003-2009 for a clinical examination, which included dual-energy X-ray absorptiometry and collection of fasting blood samples. Categories of vitamin D status were determined by S-25OHD (measured by HPLC-MS/MS).

RESULTS: In samples collected during summer, we found a gradual increase in BMD of the total hip up to a S-25OHD level of 40 nmol L-1 (6% of the cohort). In women with S-25OHD concentrations below 30 nmol L-1 during summer, adjusted BMD was 11% lower [95% confidence interval (CI) 3-19] and in those with S-25OHD levels of 30-40 nmol L-1 BMD was 6% lower (95% CI 1-11), compared with women with S-25OHD levels above 80 nmol L-1 . Low S-25OHD concentrations during summer (<30 nmol L-1 ) were also associated with higher adjusted relative risk of osteoporosis (4.9; 95% CI 2.9-8.4) compared with concentrations above 80 nmol L-1 . By contrast, no differences in mean BMD values between categories of S-25OHD were found during winter.

CONCLUSIONS: Summer concentrations of S-25OHD appear to be the most useful to predict BMD, whereas winter levels have limited value. To determine a S-25OHD cut-off level for vitamin D deficiency, it may be necessary to take into account the season of blood collection.