Sunlight and vitamin D: have your cake and eat it too

Posted on Mon, 30 Jan 17

The development of lighter skin to improve vitamin D synthesis in northern latitudes was thought to come at a cost: increased risk of sun damage. But a new study suggests that protective mechanisms evolved as well.   

Exposure to ultraviolet radiation from the sun is essential for the production of vitamin D, but too much could causing alterations in DNA structure and increase risk for nonmelanoma skin cancer. 

“As our ancestors migrated north and south of the equator, the increase in the zenith angle of the sun reduced the amount of vitamin D₃-UVB-producing photons reaching the skin to produce an adequate amount of vitamin D₃,” says Dr. Michael Holick [1]. “To compensate for vitamin D deficiency, mutations occurred resulting in a reduction in skin melanin content for those who migrated to the farthest northern and southern regions including the Neanderthals who likely had a Celtic skin tone.^”

“However, the loss of skin pigment now permitted UVB-sensitive macromolecules, including DNA, to absorb the solar UVB radiation that penetrated the epidermis. This absorption caused thymidine dimerization and other alterations in the DNA structure, increasing the risk for the development of nonmelanoma skin cancer.”

Because of increased risk of DNA damage people have often been advised to stay out of the sun, advise now thought to have contributed to the widespread vitamin D deficiency epidemic. 

Turning our understanding on its head, a clever study in the British Journal of Dermatology suggests that people living in northern latitudes who developed lighter skin to maximize vitamin D synthesis also evolved new DNA protective mechanisms [2]. 

Comparing the effects of sunlight exposure (in June in England, equivalent to 13–17 min, six times weekly) on white (skin type 2) and six South Asian volunteers (skin type 5), it was found that the increase in vitamin D was much greater in those with skin type 2, which was perhaps to be expected. 

However, what was remarkable was that although there was initially a higher levels of DNA-damage in skin type 2, blood tests revealed rapid clearance in both skin types of damaged cells over 24-hours from a single sun exposure. And urinary tests for DNA damage during single and repeated exposures remained below the detection limit and did not increase from baseline in either skin type.

“Therefore, you can have your cake and eat it too,” commented Dr. Holick. “The World Health Organization recognizes that limited sun exposure is an important source of vitamin D₃ and at the same time warns against sunburning. The results of [this study] support the concept that sensible sun exposure that does not cause sunburning, and is appropriate for the person's skin type, can prevent vitamin D deficiency and its negative health consequences with little concern about this exposure increasing risk for skin cancer.”

References: 

1. Holick, M.F. (2016), Can you have your cake and eat it too? The sunlight D-lema. Br J Dermatol, 175: 1129–1131. doi:10.1111/bjd.15127
2. Felton SJ, Cooke MS, Kift R, et al. Concurrent beneficial (vitamin D production) and hazardous (cutaneous  DNA damage) impact of repeated low-level summer sunlight exposures. Br J Dermatol. 2016 Dec;175(6):1320-1328.