Sun exposure, UV irradiance and serum 25-hydroxycholecalciferol in pregnant women in rural north India

Objective: To document the effect of season and environmental pollution on UVB irradiance; and to estimate cutaneous vitamin D synthesis in village women in different seasons. Design: Radiant UVB energy was measured by a spectroradiometer in different seasons and, in April and May, on successive days in open areas at the city outskirts, at a crowded inner-city area and the villages of our participants. Clothing, outdoor activity pattern and serum 25-hydroxycholecalciferol (25(OH) D) levels were documented. Setting: Rural north India, latitude 26.8 degrees N. Subjects: Pregnant women (n 139, aged 20-40 years). Results: UVB irradiance ranged from 56 mu W/cm(2) in January to 470 mu W/cm(2) in June. Proportion of skin exposed was 18.5% in summer and 9.5% in winter. Mean (SD) daily duration of sun exposure was 3.2 (0.2) h during winter and 2.1 (0.4) h during summer. Cutaneous vitamin D synthesis was estimated to be 19.25 mu g (770 IU) during winter and 37.5 mu g (1490 IU) during summer. Mean (SD) serum 25(OH) D was 28 (15) nmol/l during winter (92% of participants with <50 nmol/l) and reached 56 (20) nmol/l during late summer (60% with >50 nmol/l). Mean (SD) UVB irradiance at peak summer was significantly higher at the open areas and in the villages than at the inner-city location (340 (45) and 310 (60) v. 250 (50) mu W/cm(2), P=0.03). Conclusions: In our population, at latitude 26.8 degrees N, poor skin exposure is a limiting factor in all seasons. During winter, low UVB radiation energy also contributes. Particulate pollution limits UVB irradiance. Vitamin D supplementation during winter may be necessary.