Why vitamin D deficiency may be a hidden epidemic.
So, one of the biggest questions facing vitamin D researchers now is: How much vitamin D should people actually be getting? And are dietary sources sufficient, or should supplementation and/or moderate sun exposure be encouraged as well?
The guidelines established by the National Academy of Sciences currently recommend daily values of 200 IU (or International Units)for children and adults up to age 50, 400 IU for adults ages 51-70, and 600 IU for adults over 70. But the threshold required for healthy bones may be lower than that needed for protection from cancer or from autoimmune diseases. While no one yet knows precisely what these levels are, most vitamin D researchers believe that a daily value closer to 800-1000 IU would be beneficial. Dr. Mona Calvo, an expert regulatory review scientist at the FDA, said recently that an increase in suggested daily intake for vitamin D was likely to be considered—"we hope very soon."
Most vitamin D experts also argue that moderate UVB exposure—without sunscreen—is a key part of achieving adequate blood levels (except for people with a history of skin cancer or with medical conditions that make them abnormally sensitive to sun). The main reason for this is simply a pragmatic one: It is difficult to eat enough salmon and drink enough milk to attain the amount of vitamin D recommended. (Click here to see how much vitamin D different foods contain.) Children and adolescents drink much less milk than they used to, and, between lactose intolerance and calorie counting, most adults don't drink milk at all. In addition, the vitamin D content in fortified milk has been found to be erratic, often differing from the amount promised on the label. Multivitamins too generally contain only 400 IU of vitamin D each, and it is dangerous to take extras, given the high toxicity of vitamin A. While it is certainly possible to buy separate vitamin D supplements and to take them regularly, for many, this is impractical or simply inconvenient—one more health mandate that is easy, in the long run, to let slide. (While vitamin D toxicity from supplements is rare—the upper limit for adults is 2,000 IU per day—it can occur, especially since vitamin D is stored in fat; symptoms of overdose may include vomiting, weakness, weight loss, and calcium deposits in the kidney.)
Only a small amount of casual sun exposure is needed to trigger enormous vitamin D production. Exact amounts are difficult to pinpoint since they depend on a person's skin type and age, as well as on latitude, season, time of day, and amount of skin exposed. But Dr. Gordon (as well as Dr. Holick himself, along with other vitamin D experts with no ties to the Indoor Tanning Association) argue that 10 minutes of sun a few times a week is all that's needed to produce thousands of units of vitamin D. (This is for Caucasians living at mid-latitudes—say, New York City or Boston. Slightly more time is required for people with dark skin.) In addition, when vitamin D is obtained through sun exposure, there is no risk of toxicity, since UV light breaks down any excess vitamin formed. (This is why lifeguards, for instance, do not suffer from overdoses of vitamin D.)
Of course, any favorable mention of UV light is likely to cause some skin-cancer experts to balk. In interviews, several noted that UV light is a known carcinogen and that any unprotected exposure necessarily increases a person's risk. What also worries dermatologists is that a change in public health dogma may cause confusion, leading people to believe that if some is good, more sun is better.
In the end, however, it doesn't seem terribly hard to find a middle ground. It is true that UV exposure can cause skin cancer; it is true that this reality has been distorted, perhaps deliberately, by commercial interests; and it is true that vitamin D is available in the form of oral supplements—for those determined to seek it out. But it is also true that many people simply aren't getting enough of this crucial vitamin. And for most people, given its myriad benefits, both proven and potential, the advantages of a little sunshine very likely outweigh the risks.
Amanda Schaffer is a science and medical columnist for Slate.
Illustration by Robert Neubecker.