There’s no doubt that exercise is good for you. But vigorous physical activity—especially the strenuous kind that benefits your cardiovascular system—increases the production of cell-damaging free radicals in the body. That’s why, for more than three decades, many athletes and exercisers have followed the advice of some experts (notably Dr. Kenneth Cooper, who coined the term “aerobics”) and taken high doses of antioxidants, often vitamins C and E. These compounds help mop up a variety of free radicals and thus, in theory at least, may help improve exercise performance, reduce fatigue, and aid recovery. Some studies in athletes have found small benefits from antioxidants—either in performance or in markers of oxidative damage—but overall the results have been a mixed bag. And more recent research has raised red flags, suggesting that high-dose supplements are too much of a good thing.
In the early 2000s, findings from a few studies raised questions about high-dose antioxidants for athletes, but got little attention. That changed in 2009 when a German study in PNAS (Proceedings of the National Academy of Sciences) made headlines because it found that the supplements could actually blunt some of the benefits of exercise. Researchers had 39 young men (trained and untrained) run, cycle, or cross-train five days a week. Half took vitamin E (400 IU a day) and vitamin C (1,000 milligrams); those are common doses, equal to 10 to 20 times the Recommended Dietary Allowances (RDAs). After four weeks, only the men not taking the supplements had an improvement in insulin sensitivity and a boost in the body’s natural antioxidant system. (Improved insulin sensitivity, a known benefit of exercise, means the body is better able to control blood sugar, which could help reduce the risk of diabetes.) These benefits, however, were suppressed in the men taking the vitamins.
This was followed by several other studies with similar findings, including a 2014 Norwegian study in the Journal of Physiology. In it, 54 people in their twenties undertook an endurance training program; half took vitamin C (1,000 milligrams a day) and vitamin E (235 IU), while the other half took a placebo. After 11 weeks, testing revealed that some beneficial cellular adaptations to aerobic exercise were reduced in the antioxidant group. In particular, they had much smaller increases in markers for the production of new muscle mitochondria (essential for energy production in cells).
“There is growing evidence of the negative effects of antioxidant supplementation in exercise performance in both animal and human studies,” according to a 2012 letter by three leading researchers in the American Journal of Physiology, Endocrinology, and Metabolism. It concluded that "antioxidant supplements are, at the least, useless.”
A review in the Journal of Physiology in 2015, co-authored by one of the researchers who did the 2009 German study, reached similar conclusions. “While there is no convincing evidence to support antioxidant supplementation in regards to training adaptations,” it stated, “there is a growing body of literature suggesting it may hamper or prevent the signaling of important adaptations” such as the creation of new muscle mitochondria, muscle growth in response to training, and improved insulin sensitivity.
Questioning the rationale
It’s hard to generalize about the effect of antioxidants on exercise because it may depend on what types and doses of antioxidants are used, how long they are taken, the age and fitness of the exercisers, and what type of activity they do. Even if antioxidant pills turn out to be benign for exercisers, there are many reasons to question the rationale for taking them:
- It’s not clear how much oxidative damage exercise produces and what effects—bad or good—this has on health. There’s evidence that only high-intensity or exhaustive exercise, such as running a marathon, significantly increases oxidative damage.
- The body adapts to exercise over time by boosting its own antioxidant activity, thus decreasing oxidative damage.
- Free radicals are good in some ways. Some are involved in immunity, communication between cells, and other physiological functions, including favorable adaptations to exercise. Small amounts help stimulate the body’s antioxidant defenses.
- Overloading with individual antioxidants may be particularly undesirable, since these compounds work as a team. And the huge doses may disrupt the cells’ built-in mechanisms for dealing with free radicals and oxidative damage. Moreover, under some circumstances, antioxidants can act as pro-oxidants, thus inducing oxidative damage.
- No research has shown that athletes suffer long-term negative health effects from free radicals. Rather, it is clear that exercise makes people healthier, reducing the risk of heart disease, diabetes, obesity, and some cancers.
Bottom line: the Goldilocks effect
We don’t recommend antioxidant supplements for anyone—athlete or couch potato. It’s not known if free radicals generated during exercise are even harmful. The proven benefits of exercise—including the stimulation of the body’s own antioxidant defenses—far outweigh any theoretical risks. Get your antioxidants from fruits, vegetables, whole grains, and other plant foods. As the author of a 2012 review paper in Current Sports Medicine Reports concluded, “a small dose of vitamin C (200 milligrams a day), provided by five servings of fruit or vegetables daily, may be sufficient to reduce oxidative stress, but not past a threshold that will impair optimal training adaptation.” In other words, a healthy diet may provide just enough antioxidants and allow just enough oxidation to get a “Goldilocks effect”—not too little, not too much, just right.