The Possible Adventures
"We now know that there are receptors for vitamin D in almost all body tissues. Those receptors would not be there if they didn't serve some purpose."
— Susan Harris, D.Sc., N87
"We now know that there are receptors for vitamin D in almost all body tissues," Harris says. "Those receptors would not be there if they didn't serve some purpose."
Researchers are still not clear about the exact mechanisms that might make vitamin D an effective cancer-fighter, or why it seems to be associated with lower incidence of other diseases, but the quest to answer those questions has made it "the hot vitamin nowadays," as cardiologist Thomas Wang, M.D., puts it. Wang and his colleagues -- including Friedman School professors and HNRCA scientists Sarah Booth, Ph.D., and Paul Jacques, D.Sc. -- analyzed work from the famous Framingham Heart Study, and concluded that people with low levels of vitamin D were at increased risk of developing heart disease. These results held even for those who had few other cardiovascular risk factors, such as obesity or high blood pressure.
"That's obviously one of the intriguing questions," said Wang, an assistant professor of medicine at Massachusetts General Hospital. "Why do some people who develop cardiovascular disease do so inexplicably, in the absence of any obvious risk factors? Could vitamin D deficiency be a novel risk factor?"
Wang stresses that the connection between vitamin D and heart disease is far from certain, and much more work is needed. "It's easy to understand why there is this interest in vitamin D," he says. "There is a longstanding interest in vitamins and any kind of disease; vitamins are very easy to take ... in cardiology, there have been other reported links between vitamin deficiencies and heart disease, but they have been hard to prove."
Call for more
When it comes to heart health or cancer prevention, "we don't have enough information yet to know what D level in the blood you need to achieve the maximum benefit," Dawson-Hughes says. But her research and that of others has given plenty of direction on D levels and bone health. "We now have a reasonable estimate of the minimal amount needed for bones; less is known about the amount needed for the other areas," she says.
And the ideal for bones, by most accounts, is a higher one than what is currently recommended. With that in mind, in an editorial in the American Journal of Clinical Nutrition in March 2007, 15 vitamin D experts from around the world, including Dawson-Hughes, criticized the "perpetuation of outdated intake recommendations."
"The balance of evidence leads to the conclusion that the public health is best served by a recommendation of higher daily intakes of vitamin D," the editorial said. "Relatively simple and low-cost changes, such as increased food fortification or increasing the amount of vitamin D in vitamin supplement products, may very well bring about rapid and important reductions in the morbidity associated with low vitamin D status."
The editorial also questioned the current tolerable upper intake level (UL) for vitamin D -- the safe ceiling, so to speak, for how much of any nutrient should be consumed in one day. At present, it is 2,000 IU for adults. The researchers recommended a limit five times that, or 10,000 IU.
The government is not unaware of the situation. But for the sake of safety, before the federal government can recommend a change in any of the Daily Required Intakes (DRI), there is a years-long review process.
"The government is very much interested in the whole issue of vitamin D," says Dwyer, who, in addition to her work at Tufts, is also a senior scientist in the federal Office of Dietary Supplements (ODS), part of the National Institutes of Health.
In August 2007, the federal Agency for Healthcare Research and Quality completed an ODS-funded, evidence-based review of the literature on vitamin D and bone health. This year, it began a second review, examining broader research on D. "Those reviews generally take about a year," Dwyer said. "I don't know where it will go, but I know there is great interest in seeing a good, solid review done."
In the meantime, Dawson-Hughes says an increased daily intake is especially vital for maintaining bone and muscle strength in the elderly. The current recommendation for those over age 70 "is simply not enough," she says. "To get to what appears to be a reasonable threshold, the average older person needs 800 to 1,000 IU" -- at least 200 IU more than the current recommended intake. Those ages 51 to 70 would also benefit from consuming 800 to 1000 IU, she says, which is more than twice the current recommendation. (continued)
Feature by Helene Ragovin, senior writer, Office of Publications
Homepage image by istockphoto
This story ran online on Jan. 12, 2008. It originally appeared in the Fall 2008 issue of Tufts Nutrition.