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 (