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Sun Savvy

Using sunscreen and avoiding the midday sun are mainstays of skin cancer prevention. However, many vitamin D researchers believe that by diligently shielding ourselves from sunlight we might be missing out on the many benefits of this powerhouse vitamin, which is made when our skin is exposed to ultraviolet light.

Ultraviolet B (UVB) rays cause sunburn and the cell damage that leads to skin cancer, although research implicates ultraviolet A (UVA) rays as well. But it's the energy from UVB that penetrates the skin and eventually becomes the biologically active form of vitamin D.

Active vitamin D helps regulate how cells grow and mature, so in some circumstances it may put the brakes on cancerous runaway cell growth. Vitamin D also helps keep the immune system in line, which may be the reason animal and epidemiological studies hint at a connection between low vitamin D levels and autoimmune disorders such as type 1 diabetes and multiple sclerosis.

Fortunately, a sizable patch of middle ground exists between the two camps that acknowledges the dangers of sun exposure while giving vitamin D its due.

The Winter Filter

During the winter in the upper latitudes (roughly 40°--the latitude of New York City--or higher) of the Northern and Southern hemispheres, sunlight hits the earth at an angle, so it travels a longer distance through the atmosphere. Much, if not all, of the UVB gets filtered out (although the longer waves of UVA still get through). Sunburn isn't a problem, but blood levels of vitamin D drop to about half of their summertime levels.

Apart from the risk of UVB, added sun exposure won't fix many people's vitamin D woes. With age, the skin's capacity for making vitamin D diminishes, so the effect of getting more sun is going to be limited in older folks. And the melanin that makes skin dark effectively filters out UVB, so African Americans are much more likely than whites to have low levels of vitamin D. Researchers in sun-drenched Arizona reported in 2008 that more than half of the African Americans in their colorectal cancer study had low vitamin D levels, defined as 20 nanograms per milliliter (ng/ml).

Source: Harvard Medical School, Cambridge, MA
 

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