Contributions of vitamin D intake and seasonal sunlight exposure to plasma 25-hydroxyvitamin D concentration in elderly women.

We investigated the contributions (1.00-8.03 micrograms/d, or 40-321 IU/d) of vitamin D intake and seasonal sunlight exposure to plasma 25(OH)D in 59 healthy women aged 70-97 y with a mean total vitamin D intake of 8.58 micrograms/d (343 IU/d). In the summer and winter, each subject had a fasting blood measurement and assessments of vitamin D intake and sunlight exposure. Vitamin D intake was significantly correlated with plasma 25(OH)D in the summer (r = 0.52, P < 0.01) and winter (r = 0.63, P < 0.01). The influence of sunlight exposure measured in the summer on the 25(OH)D concentration was dependent on the vitamin D intake. In subjects with lower vitamin D intakes (1.00-8.03 micrograms/d, or 40-321 IU/d), the wintertime 25(OH)D concentrations of those with low and high sunlight exposure were comparable. In subjects with higher vitamin D intakes (11.15-28.68 micrograms/d, or 446-1147 IU/d), however, the wintertime 25(OH)D concentrations of those with high sunlight exposure were lower than those with lower exposure [63.8 +/- 3.9 and 80.6 +/- 6.7 nmol/L, respectively, P = 0.066; P (intake by exposure interaction) < 0.05]. This suggests that the contribution of vitamin D intake to plasma 25(OH)D concentration may be influenced by sunlight exposure.