Vitamin C and diabetes: risky mix?

Uncontrolled blood sugar can promote the production of unhealthy oxidation reactions in a person's blood. These reactions foster many diseases, such as atheroselerosis. That's why researchers have long suspected that supplementing the diet with antioxidants, such as vitamin C, would benefit people with diabetes. A study now finds that such supplements may actually promote the clogging of arteries.

David R. Jacobs Jr. of the University of Minnesota School of Public Health in Minneapolis and his colleagues evaluated cardiovascular disease in 1,923 postmenopausal women with diabetes. These individuals already had diabetes when they entered the long-running Iowa Women's Health Study, which collected data in 1986 about diets and vitamin C consumption in nearly 35,000 recruits.

After accounting for other heart risks, such as smoking, inactivity, high blood pressure, obesity, and fat intake, the researchers found that women with diabetes consuming at least 300 milligrams of vitamin C per day faced 2.3 times the risk of death from stroke and twice the risk of dying from coronary artery disease as did diabetic women who took in less of the vitamin. Such high doses are hard to achieve without taking vitamin pills.

Similar assessments for the study's 32,500 other participants, none of whom started the trial with diabetes, showed no link between heart disease and high intakes of vitamin C.

Jacobs and his coworkers report their findings in the November 2004 American Journal of Clinical Nutrition.

Many antioxidants can, at high doses, increase oxidation rather than reduce it, and that's what may be happening in the diabetic women, Jacobs says. "First and foremost," he concludes, "diabetics should maintain good [blood sugar] control because that's going to prevent a lot of oxidative stress."