Supplementation with antioxidant vitamins C and E may preclude the health-promoting effects of exercise, according to a study published in The Proceedings of the National Academy of Sciences.

Researchers evaluated the effects of a combination of vitamin C (1000 mg/day) and vitamin E (400 IU/day) on insulin sensitivity in about 40 healthy young men, as measured by glucose infusion rates (GIR) over a period of four weeks.

GIR was determined, and muscle biopsies for gene expression analyses as well as plasma samples were obtained to compare changes over baseline and potential influences of vitamins on exercise effects.

According to research results, exercise increased parameters of insulin sensitivity (GIR and plasma adiponectin) only in the absence of antioxidants. This was paralleled by increased expression of ROS-sensitive transcriptional regulators of insulin sensitivity and ROS defense capacity, peroxisome-proliferator-activated receptor gamma (PPARγ), and PPARγ coactivators PGC1α and PGC1β only in the absence of antioxidants. Molecular mediators of endogenous ROS defense (superoxide dismutases 1 and 2; glutathione peroxidase) were also induced by exercise, and this effect too was blocked by antioxidant supplementation.

The study concludes that while exercise-induced oxidative stress ameliorates insulin resistance and promotes endogenous antioxidant defense, supplementation with antioxidant vitamins may preclude these benefits.

The New York Times quoted Dr. Andrew Shao, vice president of scientific and regulatory affairs with the Council for Responsible Nutrition, Washington, D.C., as saying that the majority of evidence indicates the opposite of this study’s conclusions. “I wouldn’t change recommendations for anyone based on one study,” he said. “This is one small piece of the puzzle.”