Too much vitamin D can more than double a person's chance of developing a dangerous heart condition, a large-scale US study has shown.
Utah scientists studied 132,000 medical center patients and found that those with high vitamin D levels from taking supplements were two and a half times more likely to develop atrial fibrillation (AF), which causes the heart to beat too fast and out of rhythm.
The condition has been linked to an increased risk of stroke, heart failure, heart attack, dementia and even Alzheimer's disease.
The study's lead author, Dr. Jared Bunch, from the Intermountain Medical Center in Murray, Utah, presented the findings Wednesday at the American Heart Association Scientific Sessions in Florida.
"Patients don't think of vitamins and supplements as drugs," Bunch said. "But any vitamin or supplement that is touted as 'healing' or 'natural' is a drug and will have effects that are both beneficial and harmful."
He added, "Just like any therapy, vitamins need to be taken for the right reasons and at the right doses."
The Utah study found that patients with vitamin D levels above 100 nanograms per 100 milliliters were two and a half times more likely to have AF as those with normal levels of between 41and 80 nanograms per 100 milliliters.
Few foods are natural sources of vitamin D. The best sources are fatty fish such as salmon and tuna, but cheese, egg yolks, beef liver and mushrooms contain small amounts. Some foods, such as milk and soy milk, are fortified with vitamin D.