25-hydroxyvitamin D accumulation during summer in elderly women at latitude 60∨N

Objectives. During half of the year, cutaneous synthesis of 25-hydroxyvitamin D (25(OH)D) is not detectable at northern latitudes, leaving the population dependent on other sources for optimal vitamin D status. During April to September, 25(OH)D status may be improved by solar exposure. In this study, we measured seasonal differences in serum 25(OH)D concentrations and identified the major predictors of summer 25(OH)D concentrations. Design. We assessd serum 25(OH)D concentrations during both winter and summer amongst 100 women, aged 61-83 years, randomly sampled from the Swedish Mammography Cohort. Participants completed two detailed questionnaires covering diet, use of dietary supplements and sun-related behaviour, the first in January through March and a second time in August through September. Results. The mean seasonal increase in serum 25(OH)D concentrations was 38% from mean 72 +/- 23 nmol L-1 during winter to 99 +/- 29 nmol L-1 in summer. High summer 25(OH)D concentrations were associated with higher winter concentrations, preference of staying in sun instead of shade, having a nonsensitive skin type and normal body mass index. Based on multiple linear regression modelling, preferring sun, having nonsensitive skin type and normal weight as compared with preferring shade, having sensitive skin type and being obese, was associated with a 64 nmol L-1 higher 25(OH)D concentrations during summer. Conclusions. Women with high winter 25(OH)D serum concentrations, with preference of staying in the sun instead of shade during summer, a skin type allowing for longer sun exposure and a normal weight had the highest summer 25(OH)D concentrations.