The studies build on the researchers' previous work linking low levels of vitamin D to an increased risk for heart disease.
The researchers, from the Intermountain Medical Center Heart Institute in Murray, Utah, presented the new studies at the American College of Cardiology's 59th annual scientific session.
Vitamin D vs. Heart Disease: Study Details
The first study involved more than 9,400 patients whose blood tests revealed low vitamin D levels during a routine trip to the doctor. Their average vitamin D level was 19.3 nanograms per milliliter; levels of 30 are generally considered "normal," according to J. Brent Muhlestein, MD, the Institute's director of cardiovascular research.
At their next follow-up visit, about half had raised their vitamin D levels to above 30 nanograms per milliliter.
Compared with patients whose vitamin D levels were still low, patients who raised their vitamin D levels were 33% less likely to have a heart attack, 20% less likely to develop heart failure, and 30% less likely to die over an average follow-up period of one year.
In the second study, the researchers placed more than 41,000 patients into three categories based on their levels of vitamin D -- normal, moderate deficiency, and severe deficiency. Then they combed their medical records to see who had been diagnosed with heart disease or stroke.
Then the researchers put all the information into a computer algorithm to see if there is an optimal level of vitamin D when it comes to heart disease prevention.
"While normal has generally been considered to be 30, some people have suggested 40 or 50 is better.
"What we found is that people who increased their vitamin D blood level to 43 nanograms per milliliter had the lowest rates of heart disease and stroke. But increasing it beyond that, say to 60 or 70, offered no greater benefit," he says.
Vitamin D Findings May Change Some Doctors' Practices
Although doctors say better-designed studies showing that vitamin D supplements help are needed, Muhlestein says the results of this research will already change the way he treats his patients.
"There is enough information here for me to start treatment based on these findings," he says.
Treatment options in this case are simple, starting with a blood test to determine a patient's vitamin D level. If low levels are detected, Muhlestein recommends that men and women boost their vitamin D levels by taking vitamin supplements and briefly exposing skin to the sun's vitamin D-producing ultraviolet light.
The Institute of Medicine suggests that an adequate daily intake of vitamin D is between 200 and 400 international units (IU) for children and adults up to age 70. But increasing vitamin D intake by 1,000 to 5,000 IU a day may be appropriate, depending on a patient's health and genetic risk, Muhlestein says.
Gina Lundberg, MD, medical director of St. Joseph's Heart Center for Women in Atlanta, says more and more patients are reading about the link between vitamin D and heart disease and asking to be tested.
"Sometimes if a 28 or 29-year-old turns out to have slightly low levels, we just recommend a little more time in the sun. These new results may make us think about supplementation to bring levels higher," Lundberg tells WebMD.
American Heart Association spokeswoman Alice Lichtenstein, DSc, a nutritionist at Tufts University, says the research "is interesting," adding to growing evidence suggesting a link between vitamin D insufficiency and cardiovascular disease.
But people should refrain from taking supplements on their own, Lichtenstein says. "This should always be done under a doctor's supervision.”