Vitamin D deficiency among Black people may be a risk factor for heart disease, a new, small study suggests.

Black adults are more prone to heart disease than whites, and lack of vitamin D — the so-called sunshine vitamin — might be one reason. People with darker skin make less vitamin D than those with lighter skin, especially when they live in areas where exposure to sunlight is low, the researchers said in background notes.

So taking vitamin D supplements might help to reduce the risk of heart disease, they propose.

"Higher amounts of melanin in darker skin absorb more of the UV rays from the sun, which protects against the damaging effects of the sun but also reduces UV-B-induced vitamin D production upon sun exposure," said lead researcher S. Tony Wolf. He's a postdoctoral fellow in the kinesiology department at Penn State University.

Wolf's team measured skin pigmentation, vitamin D, and the activity of nitric oxide in the small blood vessels under the skin in 18 adults with varying skin tones.

The researchers found that people with darker skin had lower levels of vitamin D and nitric oxide. They also found that lower levels of vitamin D were tied to lower nitric oxide and poorer blood vessel function.

Nitric oxide is essential for blood vessels to function optimally, and less of it might predispose someone to develop high blood pressure or heart disease. Vitamin D helps keep nitric oxide levels normal.

So, according to Wolf, ensuring adequate vitamin D levels in more darkly pigmented people may help, at least in part, to improve blood vessel function and lower the risk of heart disease.

The possible solution? Vitamin D supplements can help raise levels of the vitamin in those who are at the greatest risk of being deficient, Wolf said.

"We recently published a study showing that four weeks of vitamin D supplementation improved blood vitamin D concentrations and nitric oxide-mediated-blood vessel function in young, darkly pigmented African-American adults," Wolf said.

The precise amount of supplementation a person needs, however, likely varies based on skin pigmentation, age and amount of time spent outdoors. "So the best way to know how much vitamin D to take is to get tested," he said.

Wolf cautioned that this study only showed an association between vitamin D levels and heart disease, not a direct cause-and-effect relationship.

"There are many other factors that contribute to cardiovascular disease, so we are certainly not suggesting that vitamin D deficiency alone causes cardiovascular disease. But there appears to be a role for vitamin D in promoting blood vessel health," he said.

The findings were released Monday as part of the virtual annual meeting of the American Physiological Society.

Findings presented at medical meetings are usually considered preliminary until published in a peer-reviewed journal.

The idea of vitamin D supplements associated with heart health may be controversial, however.

At least one expert not part of the study said the theory that vitamin D pills will improve Black adults' heart health is in question.

Dr. Gregg Fonarow is director of the Ahmanson-University of California, Los Angeles Cardiomyopathy Center.

"A variety of observational studies have found that lower vitamin D levels are associated with higher blood pressure and an increased risk of cardiovascular events. These observations resulted in hypotheses that low vitamin D levels may negatively impact cardiovascular health and that vitamin D supplementation could be beneficial," Fonarow said.

However, the VITAL trial, a nationwide trial sponsored by the U.S. National Institutes of Health, disputed this theory. It randomly assigned nearly 26,000 men and women, including more than 5,100 Black participants, to vitamin D supplements or placebo.

"Through more than five years of follow-up, vitamin D supplementation did not result in lower risk of cardiovascular events overall or in the participants who were Black compared to placebo," Fonarow said. "The study concluded that vitamin D supplementation was not effective in reducing cardiovascular event risks."