Most appropriate levels of vitamin D and the value of sunlight are debated by experts

If there is no normal, what is abnormal? This existential question is one issue in the medical debate surrounding vitamin D and sunlight. It is also a question the Institute of Medicine (IOM) has set out to answer. The organization has formed a committee to assess the current relevant evidence and update its dietary reference intakes for vitamin D and calcium.

The most current IOM guidelines for the adequate daily intake of vitamin D were published in 1997. They recommend 200 IU for children and adults up to 50 years of age, 400 IU for adults 51 to 70 years of age and 600 IU for adults 71 years of age and older. The FDA suggests a slightly higher daily value of 400 IU for those older than four years. And vitamin D advocates believe children and adults will experience many benefits with an intake twice that, about 800 IU to 1000 IU daily.

If the IOM's results — expected May 2010 — suggest that the elevated levels are more accurate, then a significant portion of the population may be found vitamin D deficient, "at least during the winter," says Dr. Spencer. To counteract this deficiency and the subsequent negative effects, some vitamin D advocates recommend five to 30 minutes of daily unprotected sun exposure — no sunscreen, hats or umbrellas. This is another issue in the vitamin D controversy.

"On the other side of the coin is the skin cancer story. I think it's beyond debate that UV light causes skin cancer," says Dr. Spencer. The medical community has cautioned for years against unnecessary and/or extensive sun exposure to reduce the risk of melanoma.

So how can individuals incorporate these contradictory messages into their lifestyles? "There is no conflict. We can protect ourselves from sun and the development of skin cancer and wrinkles and still enjoy the benefits of vitamin D simply with a healthy diet and use of vitamin pills," says Dr. Spencer.

The Evidence

The exact benefits of vitamin D present a gap in available research. Much of the vitamin D literature focuses on the geographic variability of the incidence and/or mortality of disease. For instance, notes Dr. Spencer, the incidence of colon cancer is higher in the Northeast than the Southeast. "This latitudinal gradient is quite striking in the East and Midwest," says Dr. Spencer, adding it is not seen in the West.

Researchers have postulated the reason is linked to different vitamin D levels associated with varying climatic sun exposure. "There are 50 or so papers describing the geographic variability. They say multiple sclerosis is a rare disease near the equator, but does that prove anything?" asks Dr. Spencer.

Although the correlations have been well documented, vitamin D's role in disease incidence and progression remains unproven. "Vitamin D could be a stand-in for general nutritional status, for example," says Dr. Spencer.

Complicating the data are mixed results from prospective interventional trials examining vitamin D's impact on disease incidence. "Some studies have shown that vitamin D takers had the same incidence rate as those who didn't take vitamin pills, and some have shown vitamin D takers got less cancer," says Dr. Spencer, noting there is not a lot of literature of this type.

The lack of consistent evidence leads some to question if vitamin D is the new vitamin C. Dr. Spencer recalls the craze over ascorbic acid led by Linus Carl Pauling, a two-time Nobel winner, in the 1970s and 1980s. Research completed by Pauling and colleagues suggested very large doses of vitamin C could offer cold- and cancer-fighting benefits. This later turned out not to be true.

The IOM committee will identify where the gaps in research on vitamin D lie. Future studies will likely be designed to answer some of these questions.