Researchers at Washington University School of Medicine in St. Louis have found that diabetes patients with vitamin D deficiency are unable to process cholesterol normally, so it builds up in their blood vessels, thereby increasing the risk of heart attack and stroke.
They have also identified a mechanism linking low vitamin D levels to heart disease risk and may lead to ways to fix the problem, simply by increasing levels of vitamin D.
"Vitamin D inhibits the uptake of cholesterol by cells called macrophages," said principal investigator Dr Carlos Bernal-Mizrachi, a Washington University endocrinologist at Barnes-Jewish Hospital.
"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," Bernal-Mizrachi added.
The team reports that vitamin D regulates signaling pathways linked both to uptake and to clearance of cholesterol in macrophages.
"Cholesterol is transported through the blood attached to lipoproteins such as LDL, the 'bad' cholesterol," Bernal-Mizrachi explains.
"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," he added.
That process becomes accelerated when a person is deficient in vitamin D. And people with type 2 diabetes are very likely to have this deficiency.
The findings appear in the journal Circulation.