Duration of Casual Sunlight Exposure Necessary for Adequate Vitamin D Status in Indian Men

Abstract
Objectives:
To investigate the duration of casual sunlight ultraviolet-B (UVB) exposure required to maintain optimal Vitamin D status (25-hydroxyvitamin-D [25(OH)D]) >50 nmol/L in urban Indian men, using polysulfone (PSU) dosimeters and a sunlight exposure questionnaire.

Methods:
In healthy men (aged 40–60 years) from Pune (18.52° N, 73.86° E), India, serum 25(OH)D was measured using enzyme-linked immunosorbent assay. Sunlight exposure was assessed using PSU dosimeter and by questionnaire.

Results:
Of 160 men (48.3 ± 5.6 years), 26.8% were deficient and 40.6% had insufficient Vitamin D concentrations. A hyperbolic function for the relationship between PSU measured sunlight exposure in standard erythema dose (SED) and serum 25(OH)D concentrations (r = 0.87, P < 0.01) revealed that daily exposure of 1 SED was sufficient to maintain serum 25(OH)D concentrations over 50 nmol/L. The curve plateaued around 5 SED (80 nmol/L) and extrapolation of the curve (>5 SED) did not increase 25(OH)D concentrations above 90 nmol/L. Receiver operating curve analysis confirmed that 1 SED-UV exposure was sufficient to maintain 25(OH)D concentrations over 50 nmol/L. Based on the questionnaire data, >1 h of midday casual sunlight exposure was required to maintain serum 25(OH)D concentrations above 50 nmol/L. Duration of sunlight exposure assessed by questionnaire and PSU dosimeter showed a significant correlation (r = 0.517, P < 0.01).

Conclusion:
In urban Indian men, >1 h of casual midday sunlight exposure daily was required to maintain serum 25(OH)D concentrations above 50 nmol/L, and >2 h of casual sunlight exposure was needed to maintain 25(OH)D concentrations above 75 nmol/L. Excess sunlight did not increase 25(OH)D linearly. The sunlight exposure questionnaire was validated for use in clinical studies and surveys.

One important point worth reading -

Our study indicates that in western Indian men living in an urban setting at 18.5° N, with a dark skin (Fitzpatrick Type 5), over 1 h of casual sunlight exposure of the face, forearms, and hands (15% skin surface area) was required (between 11 AM and 3 PM, or scaled equivalent time), to get a minimum 1 SED-UVB dose and to maintain serum 25(OH)D concentrations above 50 nmol/L and >2 h of casual sunlight exposure needed to maintain 25(OH)D concentrations above 75 nmol/L. Moreover, extrapolation of the curve showed that even with sunlight exposure >4.5 SED 25(OH)D concentrations do not exceed 90 nmol/L. This reaffirms that in the skin, concentrations of pre-Vitamin D reach equilibrium at moderate UV doses and prolonged exposures do not result in hypervitaminosis D in humans. Thus, nature has provided a regulatory mechanism which protects us from Vitamin D toxicity in case of continued prolonged exposure to sunlight and probably also indicates optimum upper limit for Vitamin D status. Several other studies have also observed that prolonged sunlight exposures either due to outdoor work or sunbathing result in mean 25(OH)D concentration always below 100 nmol/L. Therefore, this natural upper limit needs to be considered while deciding targets of 25(OH)D during intervention with Vitamin D supplements.

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What is meant by casual sunlight exposure here?

There days most people, especially females, have their body covered with clothes. Only the face is exposed to sunlight. And in case of Burka-clad women, none of their skin is exposed to sunlight.