Background: Several lines of evidence indicate an important role for vitamin D in the prevention of a range of
diseases. Blood vitamin D levels show clear seasonal variation; however, data on the determinants of vitamin D status
for each season are limited. We investigated the association between lifestyle and serum vitamin D concentration by
season in Japanese workers.
Methods: Subjects were 312 men and 217 women aged 21 to 67 years who worked in municipal offices in Northern
Kyushu, Japan and participated in a periodic checkup in July or November. Multiple linear regression analysis was
used to examine the association between serum 25-hydroxivitamin D concentrations and lifestyle factors for each
season.
Results: Mean serum 25-hydroxyvitamin D concentration was 27.4 ng/ml (68.4 nmol/L) and 21.4 ng/ml (53.4
nmol/L) for workers surveyed in July and November, respectively (P < 0.001); the prevalence of vitamin D
deficiency (<20 ng/ml) was 9.3% and 46.7%, respectively (P < 0.001). In November, dietary vitamin D intake (in
both sexes) and nonsmoking and physical activity (in men) were significantly associated with higher concentrations
of serum 25-hydroxyvitamin D. In summer, fish/shellfish intake was associated with higher serum 25-
hydroxyvitamin D concentrations in women.
Conclusions: Vitamin D deficiency is common in Japanese workers during seasons with limited sunlight. The
lifestyle correlates of favorable vitamin D status in November were physical activity, dietary vitamin D intake, and nonsmoking
