Dr Christian Aldridge – Dermatologist

By Dr Christian Aldridge

I recently came across this article published in the British Journal Dermatology

(BJD) this January 2017. This is an interesting experimental study which set out to demonstrate the dual impact of repeated low-level sunlight exposures on vitamin D status and DNA damage in healthy adults with light and dark skin types.

The trial involved exposing 10 white and 6 South Asian volunteers to a simulated ( using established phototherapy cabinets within a hospital setting ) 6-week course of ultraviolet radiation ( UVR) exposures, concordant with the length of the U.K. school summer holiday , when the population is most exposed to sunlight. This trial was conducted in winter, when ambient UVB is negligible at U.K. latitudes and people are at trough ( low ) vitamin D status. The team measured vitamin D levels but also urinary cyclobutane pyrimidine dimers ( CPD), a surrogate marker for DNA damage caused by UVR.

As expected , the research team observed a nearly 50% increase in circulating levels of vitamin D, particularly in the fair-skinned individuals. However, as UVB is penetrating into the epidermis to form vitamin D, it is also absorbed by DNA resulting in the formation of CPD, that if unrepaired have been associated with increased risk for non-melanoma skin cancer. These serious changes are seen to much less of an extent in Asian skin as increased skin-protecting pigmentation efficiently absorbs UVB radiation. Asian skin is thus less equipped to produce vitamin D.

Throughout evolution many organisms have depended on sun exposure for their vitamin D requirements. As our ancestors migrated north and south of the equator this reduced the amount of vitamin D-UVB-producing photons of sunlight reaching the skin. To compensate for vitamin D deficiency, mutations occurred resulting in a reduction in skin melanin ( pigment ) content for those who migrated. As a result of the lack of natural melanin sunscreen , other mechanisms were developed to repair UVB-induced DNA damage. Several enzymes have been identified that specifically recognise UVB damage and then excise the damaged portion.

Thus the study shows that although vitamin D production is important, the DNA damage that occurs shows that care needs to be taken with excessive sun exposure and thus the judicious use of sunscreen should be employed as per national recommendations.