In May 2009, the Proceedings of the National Academy of Sciences  published a small study involving 40 participants conducted by German and American researchers on the effects of antioxidant supplementation with 1000 mg/day of vitamin C and 400 IU per day of vitamin E on the impact of exercise. In particular, the study examined the impact of a combination of exercise and vitamins C and E on insulin sensitivity and other parameters of diabetic illness and as well as endogenous (body-produced) antioxidant defenses include superoxide dismutase (SOD)  and glutathione peroxidase.
Exercise Increases Insulin Sensitivity and SOD and Glutathione Levels
The study’s authors conclude that exercise improves measurements of insulin sensitivity and blood plasma adiponectin  concentrations. Adiponectin is a hormone made by fat cells which affects fat cell functioning and also influences appetite. Higher levels signal fat cells to stop storing more fat and may also signal the brain to reduce appetite. They study also found that antioxidants produced by the body such as superoxide dismutase (SOD) and glutathione peroxidase were increased by exercise.
Vitamin C and E Supplements Reduce Exercise-Induced Improvements
However, in participants who took vitamins C and E supplements, these changes in blood plasma composition did not occur to the same degree. The authors thereby conclude that supplementation with vitamins C and E counteracts the benefits of exercise, possibly by reducing oxidative stress on the body that causes beneficial adaptations.
Other Studies Show Vitamin C and E Benefits
The authors’ conclusions are certainly worthy of attention, but we would caution against changing dietary and exercise plans simply on the basis of this one study. Even if the authors’ conclusions are exactly right that vitamin C and E supplements reduce the benefits of exercise, vitamin C and E appear to have many other benefits.
There are many large and long duration studies of vitamin C and E supplementation which show positive health effects in areas such as reducing atherosclerosis and other chronic health problems. For instance:
Additional Health Benefits of Vitamin C
Exciting new research suggests that vitamin C may confer an array of unexpected health benefits, from enhancing cancer survival to promoting bone health.
Averting kidney damage. Kidney impairment is exceedingly common and often undetected, and may lead to permanent damage of the kidneys’ filtering apparatus. Nutritional scientists report that vitamin C may help prevent or halt the progression of kidney impairment related to scarring of the kidneys’ blood vessels. This finding may be particularly important for diabetics, who are especially prone to this type of kidney
damage. [emphasis added]48
Vitamin C Reduces Dangers of Blood Lipids
Scientists long ago discovered that high levels of low-density lipoprotein (LDL) are important predictors of coronary heart disease risk. The chemical state of these lipids, however, also affects risk. For example, a recent report showed that LDL oxidation increased risk for coronary heart disease by an astonishing 14-fold, especially in patients 60 years of age or younger.62 Chemical testing indicates that high doses of vitamin C can block dangerous blood lipid oxidation by about 75%.63
Research demonstrates that daily supplementation with a mixture of vitamin C and other antioxidant nutrients can reduce lipid peroxidation following a heart attack, suggesting that antioxidant supplements may be valuable for patients with a history of cardiovascular disease.64 One analysis suggests that vitamin C may in fact be even more beneficial than other commonly used antioxidants in protecting against lipid oxidation.65
Studies of foods high in antioxidants have reached similar conclusions, but as the authors rightly point out, those could be due to other substances in antioxidant rich foods and perhaps even in spite of the presence of vitamins and E. One piece of advice we believe should be taken to heart from this study is that eating antioxidant rich fruits and vegetables is a good idea, especially if they are substituted to fill in for calories from less healthy foods. For instance, most fruits and vegetables tend to be rich in dietary fibers that help reduce after-meal blood glucose spikes and thereby help improve insulin sensitivity.
Question Raised by Limitations of This Study
Notable limitations of the study are its small size with only 40 participants, one whom dropped out and all of whom were male. Its short duration of only 4 weeks is likely not enough to see long-term effects of exercise.
For instance, there is no particular reason to believe that if the study had been continued for 52 weeks that the participants taking vitamin C and E in addition to exercise would not have also seen a rise in insulin sensitivity, adiponectin serum concentration, and production of SOD and glutathione.
It could be that the oxidative stresses without vitamin C and E supplementation more quickly raise a reaction from the body, but with the supplementation since the body is already better protected from oxidative stresses that the exercise effects take longer to show up but that they would have shown up at some point. Although such as argument could be used to justify ever-longer study durations, certainly 4 weeks is not a long duration for any study such as this.
Further, that same short duration is likely not enough to “wash out” the pre-study supplementation effects on the participants. How the participants were selected to reduce or eliminate the effect of whatever dietary supplements they were taking prior to the study is not even discussed.
The study raises many interesting questions that may not be entirely obvious. Does the relatively bursty nature of exercise-induced oxidative stress tend to produce effects different than more continuous oxidative stress? Can the results of the exercise-induced blood serum improvements be extended to the group taking vitamin C and E supplements by other changes? If vitamin C and E were taken every second or third day and/or in larger amounts, would the results have changed?
It would also be interesting to study if adding supplements to further improve endogeneous antioxidant production would help. For instance, GliSODin  has been established as a means to raise the body’s levels of superoxide dismutase and supplementing with NAC (N-acetylcysteine) raises production of glutathione peroxidase.
Although it is not prominently called out, the vitamin E supplement used was d-alpha tocopherol. We’re pleased to see the authors cited this detail as it is an amazingly common and confusing oversight for published studies to fail to state the exact form of vitamin E used.
The reasons why this matters are many. For instance, the d-alpha tocopherol is believed to be about twice as active in the human body as the possibly more common synthetic dl-alpha tocopherol. There are 8 types of vitamin E (alpha, beta, gamma, and delta tocopherols and tocotrienols) and 16 types if you count the D and DL variants of each of them separately, all having different properties.
Some studies show that alpha tocopherol tends to concentrate outside of cells and can displace gamma tocopherol inside of cells and thereby increase oxidative damage inside of cells. This clearly raises the question of whether supplementation with both alpha and gamma tocopherol would have yielded different results than seen in this study.