Older women with diabetes who take high doses of vitamin C for the sake of their hearts may be doing more harm than good, new research suggests.
The study, which followed nearly 2,000 postmenopausal women with diabetes for 15 years, found that those who took heavy doses of vitamin C supplements -- 300 milligrams (mg) a day or more -- were roughly twice as likely to die of heart disease or stroke compared with women who took no supplemental C.
The researchers did find statistically weak evidence that lower supplement doses -- up to 99 mg per day -- curbed the risk of cardiovascular death, and high intakes of vitamin C from food were not related to a greater risk of death from cardiovascular causes.
The findings appear in the November issue of the American Journal of Clinical Nutrition.
According to the researchers, their results suggest that taking supplements to correct the lower blood levels of vitamin C commonly seen in diabetes is not necessarily the right choice. And though the research focused on older women, the findings may apply to men as well, according to the study's senior author.
"Our results, if confirmed by other research, would suggest that diabetics should be more cautious than others about taking supplements," Dr. David R. Jacobs Jr., of the University of Minnesota in Minneapolis, told Reuters Health.
That additional research is important, Jacobs cautioned, because public health recommendations cannot be made on the basis of one study.
Vitamin C is an antioxidant, meaning it helps neutralize potentially cell-damaging substances known as oxygen free radicals, which are a normal byproduct of metabolism. While the vitamin is clearly necessary for good health, studies have garnered conflicting results on whether supplements help lower the risk of heart disease and stroke.
In addition, Jacobs and his colleagues note in the report, although people with diabetes often show subnormal vitamin C levels in their blood, it remains unclear whether using supplements to raise those levels has health benefits.
For their study, the researchers recorded deaths from cardiovascular disease among 1,923 postmenopausal women who had diabetes at the study's start. At the outset, the women completed surveys on factors that sway cardiovascular disease risk, including diet, exercise levels, high blood pressure and smoking habits.
Over the next 15 years, women who reported taking 300 mg or more of vitamin C each day had just over a two-fold higher risk of dying from coronary artery disease or stroke than women who did not take vitamin C pills.
According to Jacobs, past lab research offers some potential explanations for the findings. For example, vitamin C has been shown, in the test tube, to damage cell proteins in the same manner that high blood sugar harms diabetics' body cells. Jacobs also pointed to the complexity of the "antioxidant defense system." When antioxidants interact with free radicals, he explained, they become "pro-oxidants" that must be detoxified by other antioxidants. It's possible, Jacobs speculated, that this detox process happens more slowly in diabetics -- both women and men -- and that under certain circumstances, the altered vitamin C molecules are able to harm body cells.
Jacobs said he and his colleagues favor getting vitamin C, along with the full complement of nutrients, from food rather than supplements.
Whereas the antioxidants in the food may be "balanced biochemically," the researchers write, any vitamin pill would lack such balance. Taking high doses of a single antioxidant, they speculate, may "perturb" the body's balance of antioxidants and pro-oxidants.
The findings follow study results reported last week showing that high daily doses of another antioxidant, vitamin E, may not extend life, and instead may slightly raise the risk of dying earlier.
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