Those of us who have diabetes and don't get enough vitamin D can't process cholesterol normally. It builds up in our blood vessels, increasing our risk of heart attack and stroke.

Now, however, new research has identified how low vitamin D levels link to heart disease risk and a good way to fix the problem. The solution is simply to increase our levels of vitamin D. Researchers already knew that low levels of vitamin D nearly double our risk of cardiovascular disease. Since this is the major complication of diabetes, this research takes on great importance for us.

The American Heart Association's professional journal Circulation published the study in its August 25 issue. Only the abstract is free online. But the principal investigator, Carlos Bernal-Mizrachi, M.D., kindly sent me the full text of the research report. He is an assistant professor of endocrinology, metabolism and lipid research at Washington University in St. Louis, Missouri.

"Vitamin D inhibits the uptake of cholesterol by cells called macrophages," Dr. Bernal-Mizrachi, M.D., says. "When people are deficient in vitamin D, the macrophage cells eat more cholesterol, and they can't get rid of it. The macrophages get clogged with cholesterol and become what scientists call foam cells, which are one of the earliest markers of atherosclerosis."

Diseases like diabetes activate macrophages, which the immune system sends in response to inflammation. Dr. Bernal-Mizrachi says that among people with diabetes who have too little vitamin D the macrophages become loaded with cholesterol and eventually stiffen blood vessels and block blood flow.

Under his lead, the researchers studied macrophage cells taken from people with and without diabetes and with and without vitamin D deficiency. The study included 76 adults with type 2 diabetes.

The researchers exposed the cells to cholesterol and to high or low vitamin D levels. When vitamin D levels were low in the culture dish, macrophages from people with diabetes were much more likely to become foam cells.

"Cholesterol is transported through the blood attached to lipoproteins such as LDL, the 'bad' cholesterol," Dr. Bernal-Mizrachi says. "As it is stimulated by oxygen radicals in the vessel wall, LDL becomes oxidated, and macrophages eat it uncontrollably. LDL cholesterol then clogs the macrophages -- and that's how atherosclerosis begins."

The good news is that an environment with plenty of vitamin D suppresses the uptake of cholesterol, so macrophages don't become foam cells. Dr. Bernal-Mizrachi says that we can probably slow or reverse the development of atherosclerosis when we get adequate vitamin D levels.

The skin manufactures vitamin D in response to ultraviolet light exposure. But in much of the United States, people don't make enough vitamin D during the winter when the sun's rays are weaker, and we spend more time indoors. "There is debate about whether any amount of sun exposure is safe, so oral vitamin D supplements may work best," he says.