Regional and seasonal variations in ultraviolet B irradiation and vitamin D synthesis in India

Evaluation of ultraviolet B index (UVBI) and its impact on vitamin D synthesis is important. We observed the maximum UVBI between 11 am and 1 pm. There was no increase in serum 25(OH)D levels following sun exposure during winter as the UVBI was significantly low, emphasizing the need for vitamin D supplementation during these months.

The amount of vitamin D3 synthesizing UVB irradiation (290-320 nm) reaching the earth's surface at different altitudes and seasons in different parts of India and it's impact on vitamin D synthesis has not been well studied.

The hourly UVB index (UVBI) from 10 am to 3 pm everyday for 12 months was measured by a solar meter in 4 different zones (North, Northeast, West and South) of the country. To study the impact of sun light exposure on vitamin D synthesis during winter, healthy school children aged 10-15 years were exposed to sunlight for a period of 30 min per day, between 11 am to 12 noon with 10 % body surface area, for 4 weeks. The main outcome measures were serum 25(OH)D, PTH, calcium, phosphate, and alkaline phosphatase levels before and after sun exposure.

The mean UVBI was highest between 11 am and 1 pm throughout the year in all locations. The highest UVBI was recorded from the North zone (4.5 ± 2.7 μW/Cm(2)), while the least was recorded in the Northeast zone (2.1 ± 1.2 μW/Cm(2)). UVBI readings in the Northeast zone were consistently low throughout the year, while all the other three zones showed significant seasonal fluctuations. Surprisingly, we observed a significant decrease in serum 25(OH)D levels from baseline (6.3 ± 4.6 to 5.1 ± 2.7 ng/mL; p < 0.001) despite sun exposure.

The mean UVBI was highest between 11 am and 1 pm throughout the year in all locations. No increase in the serum 25(OH)D levels was observed following sun exposure in winter, emphasizing the need for vitamin D supplementation during these months.