Influence of ultraviolet radiation on the production of 25 hydroxyvitamin D in the elderly population in the city of São Paulo (23 degrees 34'S), Brazil.

The lack of vitamin D is a major changeable factor involved in the pathophysiology of osteoporosis. Since the major source for this hormone is its cutaneous synthesis via ultraviolet radiation (UVR), we studied the serum level of 25-hydroxyvitamin D (25OHD) in 250 free-living elderly people (79.1 years old) from a subtropical region according to the UVR incidence and its correlations with parathormone (PTH) and ionized calcium. UVR and 25OHD differed according to the season of the year (P<0.001), with greater radiation in the summer and less in the winter, whereas the 25OHD peak and nadir occurred in autumn and spring, respectively. The highest 25OHD mean was 67.2 nmol/l, and the lowest was 29.1 nmol/l corresponding, respectively, to the measure of the month subsequent to the one of most and least sunlight incidence. Clustered by season, the correlation between UVR and 25OHD for the following seasons was r=0.98 and between the PTH and 25OHD of corresponding seasons, r=-0.95. Vitamin D deficiency occurred in 15.4% of patients, insufficiency in 41.9% and secondary hyperparathyroidism in 55%. In conclusion, we found a seasonal variation in 25OHD levels that strongly correlated with the PTH levels when separated by the seasons of the year. The 25OHD levels correlated with the UVR of the previous quarter, requiring no less than 30 days for serum changes arising from exposure to or deprivation of UVR to be observed. The prevalence of vitamin D deficiency/insufficiency found was greater than expected, even when compared to countries exposed to less solar irradiation. Thus, measures to encourage greater sun exposure and food enrichment policies should also be considered.