The investigators found that obese people, individuals who tended to gain weight around their abdomen, and those who did not respond normally to insulin had impaired blood flow compared with normal-weight people. When the obese participants were given vitamin C, their blood vessels widened, improving blood flow.

Previous research has found that antioxidant vitamins such as C and E may help arteries dilate and keep blood flowing smoothly. Antioxidants blunt the effects of free radicals.

The new study "reinforces the hypothesis" that damage caused by free radicals may be considered an important mechanism behind blood vessel problems in obesity, Perticone and colleagues explain.

The authors note, however, that there is no evidence that vitamin C actually lowers heart disease risk.