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.

Subject headings

Medical and Health Sciences  (hsv)

Clinical Medicine  (hsv)

Orthopedics  (hsv)

Medicin och hälsovetenskap  (hsv)

Klinisk medicin  (hsv)

Ortopedi  (hsv)

Keyword

25-hydroxyvitamin D

bone mineral density

season

vitamin D

vitamin D intake