|LETTERS TO THE EDITOR
|Year : 2018 | Volume
| Issue : 6 | Page : 854-855
Exposure to sunlight and Vitamin D synthesis
Srinivasa P Munigoti
Department of Diabetes and Endocrinology, Fortis Hospital, Bannerghatta Road, Bengaluru, Karnataka, India
|Date of Web Publication||4-Dec-2018|
Srinivasa P Munigoti
Consultant Endocrinologist, Department of Diabetes and Endocrinology, Fortis Hospital, 154/9, Bannerghatta Road, Opposite IIM-B, Bengaluru - 560 076, Karnataka
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Munigoti SP. Exposure to sunlight and Vitamin D synthesis. Indian J Endocr Metab 2018;22:854-5
I read with interest, an article by Patwardhan et al. published in your journal. It is commendable to note that the authors have put in effort to validate questionnaire to estimate sunlight exposure and then correlate the same with vitamin D status. But authors mention ‘Nevertheless, the association of UV exposure with serum 25(OH)D concentrations in Asian adults from tropical climates like India, where angle and latitude are favourable for receiving optimal sunlight, has to the best of our knowledge not been reported so far’ which is far from truth.
A study from south Indian city, Tirupati (latitude 13.40° N and longitude 77.2° E) using in vitro ampoule model with precursors of vitamin D (7-dehydrocholesterol) when exposed to sunlight demonstrated best conversion to active vitamin D between 11 A.M. to 2 P.M. clearly proving the efficacy of sunlight in vitamin D synthesis. The median percentage conversion of 7-dehydrocholesterol to pre-vitamin D3 and its photoproducts and percentage of pre-vitamin D3 and vitamin D3 was shown to be 11.5% and 10.2% respectively at a solar zenith angle of 36.8° and at 12:30 P.M.
Vitamin D surveys from rural south India (Tirupati) by Harinarayan et al., have similarly demonstrated higher vitamin D levels in agricultural workers who are exposed to long hours of sunlight as part of their work compared to urban dwellers (~24 ng/mL vs 19 ng/mL) establishing positive correlation between exposure to UV rays from sun and serum vitamin D levels.
In another study from north India, authors compared serum 25(OH) D and bioavailable 25(OH) D in males with varying durations of sun-exposure between indoor and outdoor environments in Delhi during August–September. They demonstrated that serum 25(OH)D increases by 2.03 ng/mL per hour of sun-exposure (95% confidence interval 1.77–2.28; P < 0.001). Also they found that outdoor workers with prolonged sun-exposure were vitamin D-sufficient, with higher serum bioavailable 25(OH) D than the indoor workers during summer.
Studies from Pune (latitude 18.31° N and longitude 73.55° E) involving current authors, themselves have shown that toddlers (of course not adults in this case) exposed to sunlight (playing outside) for more than 30 min a day exposing more than 40% of their body surface area have a normal vitamin D status (males: 91.6 nmol/L and females: 67.7 nmol/L) which was three times more compared with the toddlers who were indoors for most part of the day (males: 32 nmol/L and females: 21.1 nmol/L. A similar association is proven across India in adolescent school children aged 10–15 years by Marwaha et al.
In a dose–response study by Mark et al., healthy adults of South Asian ethnicity (n = 60; 20–60 years old) while wearing casual clothes that revealed a 35% skin area were exposed to ultraviolet rays equivalent to 15–90 min unshaded noontime summer sunlight at 53.5° N (Manchester, United Kingdom), three times/week for 6 weeks. They demonstrated that participants who received exposures equivalent to 45 min unshaded sunlight (n = 33) attained a mean (±SD) rise in 25(OH) D concentration of 8.7 ± 5.7 ng/mL (95% CI: 6.8, 10.6 ng/mL; P < 0.001), and 94% of subjects achieved concentrations >10 ng/mL proving an association between sunlight exposure and vitamin D status.
I think the association between sunlight exposure and vitamin D synthesis is well established in south Asians especially Indians. Hence, author's claim of novelty of data presented with regards to this association is not true and is ill-founded.
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. Duration of casual sunlight exposure necessary for adequate vitamin D status in Indian Men. Indian J Endocrinol Metab 2018;22:249-55.
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Harinarayan CV, Ramalakshmi T, Prasad UV, Sudhakar D. Vitamin D status in Andhra Pradesh: A population based study. Indian J Med Res 2008;127:211-8.
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