The seasonal importance of serum 25-hydroxyvitamin D for bone mineral density in older women.
The impact of season when determining a serum 25-hydroxyvitamin D (S-25OHD) cut-off level for optimal bone health is unknown.



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



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).



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-1during summer, adjusted BMD was 11% lower [95% confidence interval (CI) 3-19] and in those with S-25OHD levels of 30-40 nmol L-1BMD 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.



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.
Department of Surgical Sciences, Uppsala University, Uppsala, Sweden.
K
K
Michaëlsson
Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden.
A
A
Wolk
Department of Surgical Sciences, Uppsala University, Uppsala, Sweden.
L
L
Byberg
Department of Surgical Sciences, Uppsala University, Uppsala, Sweden.
A
A
Mitchell
Department of Surgical Sciences, Uppsala University, Uppsala, Sweden.
H
H
Mallmin
Department of Medical Sciences, Uppsala University, Uppsala, Sweden.
H
H
Melhus
D000368:Aged; D000369:Aged, 80 and over; D015519:Bone Density / Q000502:physiology*; D002136:Calcium, Dietary / Q000008:administration & dosage; D019587:Dietary Supplements; D005260:Female; D006801:Humans; D008875:Middle Aged; D015663:Osteoporosis, Postmenopausal / Q000453:epidemiology; D012307:Risk Factors; D012621:Seasons; D013548:Sweden / Q000453:epidemiology; D014807:Vitamin D / Q000008:administration & dosage / Q000031:analogs & derivatives* / Q000097:blood
D002136:Calcium, Dietary; D014807:Vitamin D; C104450:25-hydroxyvitamin D
25-hydroxyvitamin D; bone mineral density; season; vitamin D; vitamin D intake