The best time of day to take in sunshine to produce vitamin D, while minimizing the risk of cancer, is noon, according to Norwegian researchers.
In a paper published online in International Journal of Dermatology (Nov. 6, 2015), Mantas Grigalavicius, Ph.D. and his colleagues, from Oslo University Hospital in Norway, looked at how changes in latitude and time of day changed the make up of the ultraviolet (UV) radiation from the sun that reached the ground. UVB light is needed by the skin for vitamin B production.
Investigators calculated the exposures of UVA and UVB at several selected locations, and also searched cancer registries and published medical articles to determine the incidence of squamous cell carcinomas (SCC) and cutaneous melanomas (CM).
They found that as they looked at higher latitudes, the amount of UVA radiation reaching the ground dropped more slowly than the UVB radiation did. Rates of CM decreased at more northern latitudes as well, but not as quickly as rates of SCC.
The ratio of UVA to UVB also changed over the course of the day, with the ratio most favouring UVB at noon, according to the findings.
The authors suggested their findings support the role of UVA in CM development.
“The best way to obtain a given dose of vitamin D with minimal carcinogenic risk is through a non-burning exposure in the middle of the day, rather than in the afternoon or morning,” the authors write.