"Man sitting under beach umbrella.JPG" by Johntex User:Johntex (of the English Wikipedia)derivative work. Licensed under CC BY-SA 3.0 via Commons
Presentation at World Congress of Dermatology in Vancouver addresses controversial topic
The seeming evolutionary paradox of ultraviolet light both producing vital vitamin D while also
causing potentially fatal damage to the skin can be straightforwardly unravelled by looking at the literature on vitamin D production, according to a presentation by Dr. John Hawk at the 23rd World Congress of Dermatology in Vancouver.
“We need to consider the relationship between sun exposure leading to both genetic damage and vitamin D production as it does seem rather strange that an evolutionary advantage and a disadvantage would both be created from the same source,” said Dr. Hawk, Emeritus Professor and recent head of the photobiology unit at at the St. John’s Institute of Dermatology, St. Thomas Hospital and King’s College in London, U.K.. He was speaking at the British Association of Dermatologists’ ‘Best of British’ member society meeting held during the WCD.
“We need to think about, particularly, the amount of ultraviolet exposure needed to maintain human vitamin D sufficiency,” Dr. Hawk said. “We also need to consider the effects of various forms of photoprotection such as a life indoors, or clothing, or sunscreen use, have on vitamin D production. We need too to think about the effect of constitutional pigmentation on vitamin D production, as dark skinned races now regularly live in relatively weak solar ultraviolet areas.
“There have been concerns about increases in rickets and other more doubtful but very much publicized major health issues claimed from sun avoidance to prevent skin cancer, which of course is caused by the same source, ultraviolet, the same as is involved in vitamin D production,” Dr. Hawk noted.
However, research data does not support extensive ultraviolet exposure as being necessary— or even more effective—to achieve sufficient levels of vitamin D in the body, Dr. Hawk said.
UVB spectrum source of vitamin D
“Ultraviolet hits the ski n and transforms 7-Dhydrocholesterol into pre-vitamin D3. If you have continuing exposure, [pre-vitamin D3] gets converted then to tachysterol and lumisterol, both of which are metabolically useless,” he added. So beyond a certain level of exposure the body is gaining no additional raw material for the production of active vitamin D.
The ultraviolet radiation the body utilizes to produce vitamin D is primarily in the UVB spectrum,
while the range of wavelengths that can produce sunburn erythema extends into the UVA band, Dr. Hawk said. This means that natural sunlight and sunlight simulating artificial light is less efficient for the production of vitamin D than artificial UVB.
So how much UVB do humans actually need? Dr. Hawk cited a Danish study that found that half of a dose of artificial UVB sufficient to cause sunburn on white skin, applied four times over two to three day intervals on an area just equal to the forearms and face, produced a significant response in vitamin D production.
“That’s not significantly increased by larger doses or larger skin areas. So if we sunbathe for four hours it wouldn’t make very little difference,” Dr. Hawk reported.
The same study found that one-fifth of a sunburning dose on an area the size of the backs of the hands was still sufficient to produce some response in vitamin D.
There is notably less UVB radiation in high latitudes, peaking in August, said Dr.Hawk. Yet Dr.Lesley Rhodes, professor of Experimental Dermatology at the University of Manchester, has shown that a small ultraviolet B dose, one well underneath the sunburning dose, administered every two weeks is sufficient to maintain some vitamin D levels throughout the winter.
“And it has been shown in Denmark that low vitamin D goes up with mild exposures, and high [vitamin D level] goes up only a little with strong exposures,” said Dr. Hawk.
Finally, it has been shown that average summer and winter vitamin D levels in populations
living at a series of latitudes between the tropics and the arctic circle are virtually the same.
Sun protection to avoid premature aging
As for barriers to healthy UVB exposure, the prevalence of vitamin D deficiency is greatest in housebound and disabled subjects, said Dr. Hawk, adding the risk is increased in individuals with constant, extensive clothing cover.
“Sunscreens of SPFs around 20 do not much reduce—even carefully applied—the amount of vitamin D produced in holiday situations,” Dr. Hawk reported. “However, very careful, very high SPF sunscreen usage is indeed likely to prevent solar vitamin D production, but non-obsessional use, as most people are likely to do, is not at all likely to do that.”
All this suggests that the best strategy for adequate vitamin D status is to live normally using such sun protection as is suggested to avoid premature skin aging and cancer, Dr. Hawk added. “But if you are in an at-risk group, such as people always confined indoors, take vitamin D supplementation.”
This article first appeared in print in The Chronicle of Skin & Alllergy (Sept. 2015; 21(6):1,10)