The finding, interesting on its own, is also the latest drop in what's become a steady stream of news about the health effects of vitamin D -- the sunlight-produced vitamin once known mainly for helping to prevent the bone disease rickets in children.
A study released in May, for example, found that women with breast cancer who had low vitamin D levels at the time of their diagnosis were 73 percent more likely to die from the disease, and nearly twice as likely to have it recur.
And over the past few years, researchers have linked low vitamin D levels to prostate cancer, colon cancer, multiple sclerosis, influenza and chronic muscle pain, among other maladies.
How can one vitamin be linked to so many disparate diseases?
"Activated vitamin D is probably the oldest hormone on earth, phytoplankton that have existed for 750 million years [contain] it," said vitamin D research Michael Holick, of Boston University. "Every cell and tissue in our body has a vitamin D receptor, and all use it for different purposes."
Vitamin D is found in small amounts in a few foods, including fatty fish like salmon, as well as in milk and eggs. But mainly the human body produces its own vitamin D, triggered when the UVB rays in sunlight hit the skin. Enzymes in the liver, kidney and in other cells then convert the vitamin D into an activated form that the rest of the body can use.
In the most recent study, published this week in the Archives of Internal Medicine, Harvard University School of Public Health researcher Edward Giovannucci and his colleagues looked at blood levels of vitamin D in 454 men who had had heart attacks between 1993 and 2004. The men were part of a study of 18,255 men, between ages 40 and 75, that investigated links between diet and chronic disease. Researchers collected blood samples from the men between 1993 and 1995.
Over the next decade, 454 of the men suffered heart attacks. The researchers compared the level of vitamin D in their blood samples to the levels of vitamin D in the samples of 900 men who didn't have heart attacks or heart disease, but who were demographically similar to the heart disease victims -- matched for age, smoking status and other heart disease risk factors.
They found that men with insufficient levels of vitamin D (less than 15 nanograms per milliletre of blood) were 2.4 times more likely to develop cardiovascular disease than those with enough vitamin D (at least 30 nanograms per millilitre.)
Researchers don't know exactly how vitamin D is linked to heart disease, Giovannucci says, but there's evidence from animal studies that vitamin D levels can affect both blood pressure -- a major heart disease risk factor -- and the heart muscle itself. In fact, another study released this week, this one in the Journal of Cardiovascular Pharmacology, found that giving rats vitamin D helped prevent hypertrophy, an ailment in which the heart muscle becomes enlarged and overworked.
Cancer researchers, meanwhile, are exploring the mechanisms linking vitamin D and cancer. They've found that many cancer cells have vitamin D receptors, as well as an enzyme that allows them to activate vitamin D, according to University of Toronto researcher Pamela Goodwin, the lead author of last month's study on breast cancer and vitamin D. When the cells receive the vitamin D, it can have many cancer-fighting properties, including curbing the formation of the blood vessels that feed the tumor.
Given all of these benefits, vitamin D researchers say, it's especially worrisome that so many people are vitamin D-deficient. In the breast cancer study, only 24 percent of the women had sufficient blood levels of vitamin D. In the heart study, 23 percent of the men were found to have sufficient levels. People who live in Northern latitudes and people with darker skin tones, who don't absorb UVB rays as quickly, are particularly at risk.
Given the emerging knowledge of the benefits of vitamin D, researchers like Michael Holick say that the current standard recommendations for how much vitamin D the average person needs are out of date. For example, the Institute of Medicine recommends that adults take between 200 and 600 international units per day, depending on age. Holick says that he himself takes 1400.
"The recommendations have not kept pace with the science," said Giovannucci.
Holick also spends between five to 15 minutes outside, without sunblock, a few times each week, and says that most people should do the same, depending on their skin type and where they live.
But that's a controversial recommendation, given that years research show that sun exposure can lead to melanoma -- and the vitamin D researchers have yet to convince the dermatologists.
"Any individual or organization advocating intentional UV exposure as the preferred means of obtaining vitamin D is doing a tremendous disservice to the public," the American Academy of Dermatology says in its public position statement on the topic. "The safe way to obtain it is through a healthy, balanced diet, vitamin D-fortified foods or oral supplements."
But Holick says that that's unrealistic. "Supplements are fine," he says, but "it is unrealistic to have the entire U.S. and for that matter world population take a 1000 IU vitamin D supplement every day [...] also vitamin D from the sun is free."