There have been few clinical trials focusing exclusively on vitamin D, and the few that have been done have limitations, such as evaluating a specific dose rather than a range of doses. Other studies, as Chung pointed out, looked at both calcium and vitamin D, making it impossible to separate out the effects of vitamin D.
The Institute of Medicine issued a report last November that included dietary intake guidelines for vitamin D and calcium based on a review of available research.
For adults ages 19 to 50, the IOM report says average recommended intake is 400 IU per day, and the maximum recommended intake is 4,000 IU per day. Older adults may require even more vitamin D.
But the IOM also explained that appropriate levels are difficult to determine, since one of the sources of vitamin D is sun exposure, which can vary from person to person. The IOM based its recommendations on low sun exposure, but because it's such a variable factor, people may need more or less than what guidelines indicate.
And while experts say the body does need vitamin D, too much of it can be risky.
"We would not want any American to be deficient in vitamin D, but the real controversy relates to taking more than the requirement. There really aren't any good data to support what the right amount is," Clinton said. "Once you get up in the higher ranges, there actually could be negative outcomes."
A study presented at this year's American Heart Association meeting found that people with very high vitamin D levels were at higher risk for irregular heartbeat than people with normal levels.
Although there were also research findings presented at the meeting that linked low vitamin D levels to cardiovascular disease, much of the other available data are insufficient, according to another research review published in Annals of Internal Medicine.
Vitamin D deficiency has been associated with high blood pressure, heart attacks, stroke and other related diseases, wrote the authors, led by Cora McGreevy of Ireland's Royal College of Surgeons. But, they added, "few randomized, controlled trials have evaluated the effect of vitamin D replacement on cardiovascular outcomes, and the results have been inconclusive or contradictory."
Cardiologists say despite the lack of solid data, they still recommend vitamin D to some of their patients whose levels are low.
"My feeling is if I replace vitamin D levels, the least I'm doing is improving their bone health," said Dr. Carl Orringer, associate professor of medicine at UH Case Medical Center in Cleveland. "If I'm also reducing the tendency of other things to develop, like diabetes, heart attacks or strokes, that's a bonus. Those studies are ongoing and will be available over the next few years."
"I do feel that in individuals with a vitamin D deficiency, it's reasonable to supplement and bring them up to normal levels," said Dr. Philip Ragno, director of cardiovascular health and wellness at Winthrop University Hospital in Mineola, N.Y. "I also stress that if you do supplement vitamin D, a primary care physician must monitor vitamin D levels so they don't get too high."
Orringer also said another observed effect of vitamin D is it enables some patients to tolerate statins by diminishing the muscle pain often associated with the cholesterol-lowering drugs.
This relationship hasn't been studied extensively and no one knows why it happens, but it does help some people take drugs they need for their heart health.
As a whole, people's vitamin D levels seem to be declining, making the need for concrete data on vitamin D even more vital.
"Because of the emphasis on skin cancer and the need for sun protection, sun exposure has done down and so have vitamin D levels," Ragno said. "Also, people who are debilitated can't get out as much, which also contributes to lower levels."
One of the biggest challenges in studying the relationship between cardiovascular disease and vitamin D is its complexity. There are many factors, such as weight, nutrition and physical activity that can play a role.
"One other item on that list may be vitamin D," said Clinton. "That warrants study in clinical trials so we know what dose and at what period of life and what infdividuals will benefit."