Low vitamin D levels have long been associated with heart problems, but researchers said Monday they have identified a specific danger level for increased risk of heart attack and stroke.

Patients are at the highest risk of cardiac woes if their vitamin D levels dip below 15 nanograms per milliliter, according to researchers at the Intermountain Medical Center Heart Institute in Salt Lake City, Utah.

A simple blood test is the best way to determine one's vitamin D level. "Although vitamin D levels above 30 were traditionally considered to be normal, more recently, some researchers have proposed that anything above 15 was a safe level," said J. Brent Muhlestein, co-director of cardiovascular research at the Intermountain Medical Center Heart Institute.

"But the numbers hadn't been backed up with research until now," he said in a statement.

The findings, presented at the American Heart Association Scientific Session in Orlando, Florida, were based on a database of more than 230,000 patients who were followed for three years.

Researchers tracked major adverse cardiac events, including death, coronary artery disease, heart attacks, stroke, heart and kidney failure. In the highest risk group, the chance of cardiovascular events increased by 35 percent compared to the others in the study whose vitamin D levels were above 15 nanograms per milliliter.

About one in 10 people are estimated to have low vitamin D levels. Vitamin D is naturally produced by the body when a person is exposed to sunlight, and it is also found in fish, egg yolks and some dairy products. Next, Muhlestein said he hopes to do a randomized trial with low vitamin D patients to study whether supplements help combat heart problems over the long term.

"As we continue to study vitamin D and the heart, we hope to ultimately gain enough information so we can inform all patients specifically what they should do to reduce their cardiac risk as much as possible," he said.