Dark Tan? Don't Assume Vitamin D is at a Healthy Level

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.