As the days are getting longer and the days getting sunnier, many of us are enjoying increasing the melanin in our skin, otherwise known as a tan. Melanin is the pigment that provides the brownish coloring that grows in intensity as we tan.
You'd think that by sporting a nice “healthy” tan, having a normal vitamin D blood level is guaranteed. Right?
Unfortunately, no. In fact, it is possible to remain severely deficient in vitamin D even with a nice tropical tan.
People coming to my office with nice tans obtained by sunning themselves for
several hours, several days, or several weeks may have normal vitamin D blood
levels, moderately low, even severely low. You simply cannot tell just by
having a tan.
Several people in my office, in fact, were so confident that sunning themselves
provided sufficient vitamin D that they reduced their usual dose. Some even
stopped their vitamin D altogether.
But, when blood levels of 25(OH) vitamin D were checked, they were virtually
all low, sometimes as low as <20 ng/ml. Yet
all had nice tans.
Why does this happen? Why would people with dark tans remain deficient in
vitamin D?
One big factor is age: Anyone over
40 years old has lost much of the potential to convert vitamin D in the skin to
its active form. So even a dark tan might raise blood vitamin D levels a little
or none at all; a tan does not ensure a vitamin D level in a desirable
range. Also, curiously, the more you tan, the more melanin skin pigment accumulates,
and the more vitamin D activation in the skin is blocked. That's because
melanin acts as a natural sunscreen. For this reason, people with naturally
dark skin, such as people from southeast Asia or African-Americans, tend to be
much more deficient in vitamin D, particularly when they migrate to northern
climates. They also require several times longer in the sun to obtain the same
quantity of vitamin D activation in the skin as a fair-skinned person.
Why does aging result in inefficient skin activation of vitamin D? It seems
that, once we are beyond our reproductively useful years, this ticking clock of
aging gets triggered. The older we get, the less activation of vitamin D occurs
in our skin, the less of the youth-maintaining, disease-preventing benefits of
vitamin D we obtain with sun exposure.
Weight is another factor: Heavier people need more vitamin D, sometimes three- or
four-fold more than slender people. It's not clear whether vitamin D is
stored in the fatty tissues, or whether vitamin D metabolism is somehow
disrupted by excess weight, but it is a clear-cut effect.
The message: Don't rely on a tan to gauge the adequacy of vitamin D. Maybe that
works when you're 16 years old, but not at age 50 or 60. I am a firm believer
that there is only one way to know your vitamin D status: a blood level of
25(OH) vitamin D.
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