Taking high doses of vitamin D once a month won't lower your risk for heart disease, new research indicates.
But, even though the vitamin came up short in this study of more than 5,000 adults, the researchers aren't ready to scrap hopes for vitamin D supplementation altogether.
"Our study only rules out monthly dosing," said study lead author Dr. Robert Scragg. His team did not explore whether daily vitamin D supplementation might prove more protective of heart health.
Scragg is a professor of epidemiology at the University of Auckland in New Zealand.
Previous studies have reported higher chances of heart disease among people with low vitamin D intake.
Natural sources of vitamin D include ultraviolet radiation from the sun, as well as foods such as fatty fish, fortified dairy products, orange juice and egg yolks.
The idea that vitamin D -- particularly from sun exposure -- might offer protection against heart disease has been around since the early 1980s, the study authors said.
The theory gained traction as doctors observed that "rates of cardiovascular disease are much higher in winter, when body vitamin D levels are low, than summer," said Scragg.
But, "there is very limited research on this topic," he added.
To explore the potential benefit, the researchers behind the new study tracked the heart health of roughly 5,100 adults.
All participants were between 50 and 84 years old. About one-quarter were vitamin D deficient at the start of the trial -- registering vitamin D levels of less than 20 nanograms per milliliter via blood tests, Scragg said.
Half were assigned to receive a high-dose vitamin D supplement once a month, with an initial dose of 200,000 International Units (IUs). That was followed by a regular monthly dose of 100,000 IUs. The other half received a monthly regimen of placebo supplements.
The participants continued this regimen for more than three years, on average.
In the end, nearly 12 percent of both groups had developed some form of heart disease, the researchers found.
And the risk for developing high blood pressure and/or experiencing a heart attack, stroke, heart failure or angina was more or less the same whether or not a participant had begun the study deficient in vitamin D.
The researchers concluded that monthly high-dose vitamin D supplementation doesn't alter heart disease risk one way or another.
The findings were published in the April 5 issue of JAMA Cardiology.
Dr. Adrian Hernandez is co-author of an editorial accompanying the study. He expressed little surprise with the findings.
"We often find that great ideas do not always show the benefits when tested formally," he said.
"The dosing may not be optimal, either [in terms of] frequency or amount," said Hernandez, a professor of medicine at Duke University School of Medicine in Durham, N.C.
"There may be other components/supplements that are needed to make it effective. Or it simply is not pivotal for improving heart disease risk," he said.
These points were seconded by Lona Sandon, an assistant professor of clinical nutrition at the University of Texas Southwestern Medical Center at Dallas.
"There are several possibilities for these findings," she said. "One being vitamin D supplementation may just not work. Two, [the study period] may not have been long enough."
A representative of a supplement trade group agreed with the conclusion that monthly high-dose vitamin D does not prevent heart disease.
"However, the study's conclusion should not deter consumers from taking vitamin D at levels recommended by their doctor or other health care practitioner," said Duffy MacKay, senior vice president for scientific and regulatory affairs with the Council for Responsible Nutrition.
"We also agree that further research on the effects of vitamin D in other doses is in order," he added.