Scientists and healthcare professionals have known for some time that low levels of vitamin D almost double the risk of cardiovascular disease in people with diabetes. But until now, they haven't known why.

According to researchers at Washington University School of Medicine in St. Louis, vitamin D deficiency increases cardiovascular risk by preventing people with diabetes from processing cholesterol normally. As a result, the sterol builds up in arteries, often leading to blockage that can bring on a heart attack.

Vitamin D works by inhibiting the uptake of cholesterol by macrophages, cells that the immune system sends in response to inflammation such as that found in diabetes. With no vitamin D to stop them from consuming cholesterol, the macrophages become clogged with it and turn into "foam cells."

Foam cells are early markers of the build-up of fatty deposits on arterial walls, known as atherosclerosis.  But in an environment with adequate amounts of vitamin D, macrophages do not become foam cells. The researchers believe that simply increasing intake of vitamin D not only will lessen the risk of cardiovascular disease in people with diabetes, but also may even reverse atherosclerosis.

So the question is, how to get proper amounts of the vitamin to people who need it. Sunlight is the main generator of vitamin D, which is produced by the skin in response to the sun's ultraviolet rays. In months and at latitudes where there is abundant sunlight, most people generate a sufficient amount of the vitamin to prevent macrophage clogging. In places where there is little winter sunlight, however, or among people with diabetes (who generally have lower levels of vitamin D), getting sufficient vitamin D can be a problem. In any case, given concerns about overexposure to sunlight, the best approach is probably oral vitamin D supplements taken on a routine basis.