You may have heard about recent scuffles between prominent dermatologists and experts on vitamin D, a crucial vitamin obtained in part by exposure to UV light. In a front-page story this April titled "BU Advocate of Sunlight Draws Ire," the Boston Globe reported on the brouhaha surrounding Dr. Michael Holick, whose recent book, The UV Advantage, touts the health benefits of moderate sun exposure and downplays the threat of skin cancer. (Dr. Holick, perhaps the most prominent vitamin D researcher in the country, was forced to resign from the dermatology department at Boston University's medical school in February, though he retains his appointment in medicine.) The Globe piece—and a recent article in the New York Times—also noted that Dr. Holick's work is partly funded, and actively promoted, by the Indoor Tanning Association, an industry group with obvious financial interests. It's easy to see why the conflict has escalated and why many in the public are confused about the basic issues underlying this debate.
What exactly are the health benefits of vitamin D? How much does a person need? And why is the issue so often framed in terms of sun exposure—can't you just drink fortified milk or take a multivitamin?
Let's start with the basics: Vitamin D is a fat-soluble substance (that is, it dissolves in and can be stored by fat deposits in the body). It is present in salmon, mackerel, sardines, and cod liver oil; fortified foods including milk, breakfast cereals, and some juices; and vitamin supplements. It can also be synthesized in the skin during sun exposure. Vitamin D (in its active form, which is created after several modifications by the body) functions as a classic steroid hormone, which means it binds to nuclear receptors in various tissues to influence the expression of genes, thereby affecting a range of processes, especially the regulation of calcium.
It has long been known that vitamin D is crucial for healthy bones. The presence of vitamin D in the small intestine aids in the absorption of dietary calcium—people with vitamin D deficiency are able to absorb only a third to half as much calcium as those with sufficient levels—and calcium is vital to the hardness of bone. The two diseases traditionally associated with severe vitamin D deficiency—rickets in children and osteomalacia in adults—are characterized by deformation or softening of bone. And chronic vitamin D deficiency is strongly linked to osteoporosis, a disease defined by loss of bone density and associated with increased risk of fractures.
The common assumption has been that with the fortification of milk, instituted in the United States in the 1930s, and casual exposure to sunshine, most people get all the vitamin D they need. But a small resurgence of rickets in the last few years, particularly among African-American children, has caught the health-care community off guard. As studies have probed the issue, it has become clear that vitamin D deficiency (usually defined as blood levels of less than 15 ng/mL [or nanograms/milliliter]) and insufficiency (less than 20 ng/mL,) are far more widespread than researchers had expected. The elderly, who often receive little sun, are at particular risk, as are African Americans and other dark-skinned people, since skin pigmentation, which protects against damage by UV rays, also interferes with vitamin D production. (Those with dark skin need to spend more time in the sun to produce the same amount of vitamin D.) Infants who are exclusively breast-fed are also at high risk since breast milk, for all its virtues, contains almost none of this vitamin.
Perhaps the biggest surprise, though, has been the prevalence of vitamin D deficiency among women of child-bearing age—particularly African-American women—and among healthy children and adolescents. Dr. Catherine Gordon, an adolescent-health specialist at Children's Hospital in Boston and an expert on vitamin D, told me that calling it a "hidden epidemic" would not be an overstatement. While severe cases early in life result in rickets, less-pronounced deficiencies may slip under the radar because they do not cause noticeable symptoms. Gordon and other doctors worry that for children and adolescents, insufficient vitamin D can prevent proper bone development and increase the risk of disorders such as osteoporosis later in life.
Vitamin D deficiency can easily go undetected in adults as well. In one study, published in 2003 in the Mayo Clinic Proceedings, researchers in Minneapolis tested vitamin D levels in patients suffering from chronic, non-specific, musculoskeletal pain: 93 percent of them turned out to be vitamin D deficient—a condition very likely (though not conclusively) related to their symptoms. And of the East Africans, African Americans, and Hispanics in the study, 100 percent were vitamin D deficient. As a result, the authors argue, all patients with such pain should have their vitamin D levels tested because osteomalacia may turn out to be the underlying cause.
But vitamin D's benefits may go beyond the protection of bone and muscle. There is new research to suggest that vitamin D may also guard against an array of diseases, including colon, breast, and prostate cancers. Much of the evidence here is merely suggestive: Epidemiologic studies in the United States and Europe have observed an inverse relationship between the risk of certain cancers—breast cancer, for instance—and the intensity of sunlight in a given area. More specific studies have examined levels of vitamin D in the blood directly, measuring them over time. Here, too, an inverse relationship has been found: lower vitamin D levels, higher risk of cancers. A prospective study of colon cancer published in the Lancet, for example, found that the "risk of getting colon cancer decreased three-fold" in people with a blood vitamin D concentration of at least 20 ng/mL, the level currently defined as sufficient.
Though the role of vitamin D in protecting against cancer has not been conclusively proved, the research is promising—in part because a plausible mechanism has been identified. The active form of vitamin D can initiate a range of responses, one of which is to inhibit cell growth and proliferation—processes known to spiral out of control in cancer. For this reason, it seems likely that vitamin D can either prevent normal cells from becoming cancerous, or can slow the growth of tumors, or both. Dr. Holick is studying the progression of colon cancer in two groups of mice—one vitamin D deficient and one not. Preliminary results suggest that tumors grow substantially larger in mice without sufficient vitamin D.
The potential role of vitamin D in forestalling other diseases, particularly autoimmune conditions such as multiple sclerosis, Type 1 diabetes, and rheumatoid arthritis, has generated widespread interest as well. The strongest studies have tested the role of vitamin D directly, with some fairly suggestive results: A 2004 study in the European Journal of Clinical Nutrition found that mice and rats that received activated vitamin D were less likely to develop multiple sclerosis. Another study, published in the Lancet in 2001, concluded that children who were given vitamin D supplementation were less likely to develop Type 1 diabetes. Although individual mechanisms are not well-established, it is known that active vitamin D interacts with cells of the immune system and very likely plays an anti-inflammatory role in the body.