Sun exposure is a tricky thing. No one wants too much, of course, but too little can also be a problem. That's because sunshine encourages skin cells to produce vitamin D, a substance that keeps bones strong and may play an as yet undetermined role in warding off a variety of other ailments.
There was a time when lack of vitamin D was a problem only for people living in the far north, where months could go by with no more than a few hours of weak light a day. For the rest of the world, simply spending time outside was sufficient to keep the blood's D levels healthfully high. "In the right location, less than half an hour outside can give you all the vitamin D you need for a week," says Susan Harris, DSc, a researcher at Tufts University.
Susan Harris, DSc
Epidemiologist, Jean Mayer USDA
Human Nutrition Research Center on Aging at Tufts University
But these days—between office jobs, television, long commutes, and, of course, sunblock—regular sun exposure is hard to come by. "It wasn't till industrialization that we started spending all of our time inside," says Harris. "Now … we depend more on dietary sources [of vitamin D], of which there are few—and most of us don't get enough." Fortified foods like milk and cereal, in addition to fatty fish like salmon and mackerel, can make up for some of what we’re missing, but most of us also need either appropriate sun exposure or vitamin D supplements.
In addition, some people have a harder time producing vitamin D than others. "Skin pigment acts as a natural sunscreen—it protects the skin, but also reduces vitamin D production," Harris says. The darker your skin, in other words, the harder it is to get enough vitamin D, one reason African Americans seem to have lower levels of D than other Americans. In fact, in a 2004 study published in the journal Diabetes Care, University of Auckland researcher Robert Scragg showed that non-Hispanic blacks in the United States had significantly lower levels of vitamin D than whites and Hispanics. Scragg showed that high vitamin D levels were associated with low diabetes risk—for some. "In that study, the protective effect of vitamin D is seen in Caucasians, but not in non-Hispanic blacks," Harris says.
Harris is one of a growing number of scientists examining the role of vitamin D in the body. While it has long been known to influence skeletal health, over the past two decades more and more researchers have begun to appreciate the many other roles vitamin D may have. "People have gone from looking at deficiency diseases like scurvy and rickets"—afflictions caused by the lack of vitamins C and D, respectively—"to looking at more subtle deficiency effects," Harris says.
It turns out that there are vitamin D receptors almost everywhere in the human body—even in the islets of Langerhans, the structures in the pancreas that produce insulin. For people with type 2 diabetes or pre-diabetes, it seems that vitamin D allows the body to secrete more insulin. At the same time, there's evidence that vitamin D may increase insulin sensitivity.
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With the help of a grant from the American Diabetes Association, Harris is taking a closer look at whether vitamin D supplements might in fact help African Americans avoid developing diabetes. She has set up a study examining the links between vitamin D and diabetes in about 100 African Americans age 40 and older. To research people at high risk for developing diabetes, she specifically recruited subjects who were overweight or obese but not diabetic. Half of them are given a placebo, half a vitamin D supplement; over the course of 12 weeks, she measures things like insulin secretion, glucose control, and insulin resistance in the two groups to see if taking vitamin D supplements makes any difference.
Pinning down exactly what makes vitamin D work is made even tougher by related factors: "confounders," in academic jargon. Take, for example, the links between vitamin D levels and diabetes. Are people with high levels of vitamin D less likely to develop diabetes because of the vitamin D, or because their lifestyle gets them out of the house or office? "It's hard to know if it was the vitamin D. The same things that affect vitamin D, like being outside, can affect health in other ways," Harris says. "Maybe it wasn't because of vitamin D; it was because they were more active." To avoid confounding her results, Harris didn't ask participants to change their lifestyle in any way.
If it turns out that vitamin D supplements can help prevent type 2 diabetes in African Americans, it wouldn't necessarily mean it's a universal panacea. "But if we see an association here, I would absolutely want to repeat the study with other groups," Harris says. The ways in which vitamin D works remain a mystery, but that doesn't bother Harris. "Vitamin D is big," she says. For now, she just wants to know whether vitamin D can help people; figuring out why can come later.