Frequent casual exposure to sunlight has long been considered the most important source of vitamin D.  However, a recent study published in The Journal of Nutrition demonstrated that although dietary requirements for vitamin D may vary by geographical latitudes, a moderate dose of vitamin D supplementation is effective in raising and maintaining circulating vitamin D concentrations in adults over the winter months.

Current dietary vitamin D recommendations do not consider differences in country-specific sunlight availability or spontaneous individual exposure. To address this critical issue, researcher Marcela Mendes (University of Surrey) and colleagues conducted a study to investigate the effects of vitamin D supplementation and sunlight exposure on vitamin D status in Brazilian women living in high compared with low latitudes.

The study trials were conducted in Surrey, Southern England, which was considered high latitude, and in Goias, mid-Western Brazil, which was considered low latitude. Participants were randomized to receive a daily supplement of 15 µg of vitamin D or placebo for 12 weeks during winter. The first trial was conducted in England from December 2016 to March 2017 (when there is almost no UVB radiation), followed by the second trial conducted in Brazil from June 2017 to September 2017 (when there is an abundance of UVB radiation). There were four intervention groups: 1) those receiving placebo and living in England; 2) those receiving 15 µg of vitamin D and living in England; 3) those receiving a placebo and living in Brazil; and 4) those receiving 15 µg of vitamin D and living in Brazil. Dietary intake was determined by 4 consecutive days of estimated diet diaries.  To measure ambient UVB radiation exposure, individuals wore UV exposure dosimeter film badges on their outer clothing. 

A total of 120 women completed the 12-week intervention study. Vitamin D supplementation of 15 µg, an intake amount in accordance with recommendations in many countries, was found to be effective in raising and maintaining adequate vitamin D concentrations, regardless of latitude, during winter. It is worth noting that postintervention vitamin D concentrations were affected by baseline concentrations. Those with the lowest baseline vitamin D concentrations had the greatest increases in response to vitamin D supplementation over the 12-week wintertime trial, and those receiving no supplement and no UVB exposure had the lowest vitamin concentrations.

These results suggest that the effect of vitamin D supplementation is independent of latitude residency. This study also corroborates the importance of dietary or supplement vitamin D intakes during winter for those living in higher latitudes.  Considering these findings, the researchers hypothesized that if vitamin D supplementation provides enough vitamin D, the endogenous production via the skin may be downregulated, which would influence the required sunlight exposure and therefore related vitamin D supplementation recommendations.


Mendes MM, Hart KH, Williams EL, Mendis J, Lanham-New SA, Botelho PB. Vitamin D supplementation and sunlight exposure on serum vitamin D concentrations in 2 parallel, double-blind, randomized, placebo-controlled trials. J Nutr., Volume 151, Issue 10, October 2021, Pages 3137–3150,

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