The exercise benefit in question is better sensitivity to insulin, a hormone that regulates blood sugar.
The new study shows that when healthy men took vitamin C and vitamin E supplements daily during a month of exercise, their insulin sensitivity didn't improve, though other men following the same exercise plan and taking placebo pills did improve their insulin sensitivity.
The antioxidant supplements shut down the brief, normal spike in oxidation that follows exercise; that temporary bout of oxidation is needed to improve insulin sensitivity, according to the researchers, who included Michael Ristow, MD, chair of the department of human nutrition at Germany's University of Jena.
"The data confirm that exercise exerts positive effects on glucose metabolism, and thus may prevent type 2 diabetes," Ristow tells WebMD via email. He adds that the results also "suggest, for the first time, that these effects are mediated by free radicals or oxidative stress that are caused by exercise, and that antioxidants interfere with this health-promoting effect of exercise."
But Andrew Shao, PhD, vice president for scientific and regulatory affairs at the Council for Responsible Nutrition (a trade group for the supplements industry), says people shouldn't jump to conclusions based on one report.
Supplements and Exercise Study
Ristow's study included 40 healthy men in Germany. When the study started, half of the men were sedentary and half got regular exercise.
The researchers put all of the men on a monthlong exercise plan. For four weeks, the men got 85 minutes of exercise on five consecutive days per week.
During each session, the men wore heart rate monitors while they biked or ran for 20 minutes, did circuit training for 45 minutes, and spent a total of 20 minutes warming up and cooling down.
Throughout the study, half of the men took 500 milligrams of vitamin C twice daily and 400 international units of vitamin E daily. For comparison, the other men took placebo pills.
Lab tests done at the beginning and end of the study show that insulin sensitivity improved for all of the men taking the placebo pills -- but not for any of the men taking the antioxidants.
"Physical activity induced an increase in insulin sensitivity only in the absence of antioxidants," Ristow and colleagues write in the advance online edition of Proceedings of the National Academy of Sciences.
But don’t take this study to mean that if you’re active you should avoid antioxidant-rich fruits and vegetables. Fruits and vegetables are still beneficial because they "contain hundreds of other substances and compounds that are health-promoting," Ristow tells WebMD.
Supplements Industry Responds
Ristow's study was "well designed" and "interesting," says Shao.
But Shao says the study was small and "is really an incomplete picture" of the relationship between antioxidants and exercise.
Shao notes that antioxidants may help mend muscle damage, that not all antioxidants may act like vitamins C and E, and that different doses or the timing of when the doses were taken may matter.
Ristow says it's possible that the timing of when the supplements were taken might matter, "but we have not studied this."