Researchers from Germany and the US report in the Proceedings of the National Academy of Sciences that a combination of 1,000 mg per day of vitamin C and 400 IU per day of vitamin E adversely affected insulin sensitivity, and thereby increase the risk of type 2 diabetes.
Participants, both trained and untrained, underwent 85 minutes of exercise five days per week for four weeks.
“Based on the evidence derived from the current study, we here propose an essential role for exercise-induced reactive oxygen species formation in promoting insulin sensitivity in humans,” wrote the researchers from the University of Jena, German Institute of Human Nutrition, University of Leipzig, and Harvard Medical School.
“Most importantly, […] changes in gene expression and the increase in insulin sensitivity following physical exercise are almost completely abrogated by daily ingestion of the commonly used antioxidants vitamin C and vitamin E.
“Thus, antioxidant supplementation blocks many of the beneficial effects of exercise on metabolism,” they added.
The study has been questioned by antioxidant expert Alexander Schauss, PhD, from AIBMR Life Sciences, a nutraceutical products consultancy (click here for Dr Schauss’ full reaction).
Dr Schauss told NutraIngredients.com: “I would never put an “untrained” person, no matter what their BMI or age, into a program that requires 85 minutes of exercise 5 days per week for 4-weeks, as required in this study. The body needs to accommodate an increase in physical activity to build up its endogenous antioxidant defences to the anticipated increase in ROS.
“Athletic clubs that sell memberships have long realized that individuals who sign up and begin at an intense level of exercise can feel defeated due to the side effects they experience in the first month and for this reason often stop their exercise program and revert to a more sedentary lifestyle.
“I have to question the design of the study in light of this and many similar known factors related to exercise,” he added.
Useful, useless, or harmful?
Lead author Michael Ristow told NutraIngredients.com that a number of studies have already shown that antioxidant supplements have no benefit, and his new study suggests that “such supplements may be harmful in regards to glucose metabolism and endogenous ROS defense capacity”.
“A significant number of studies have shown that fruits and vegetables promote health, while antioxidant supplements do not. This insinuates that fruits and vegetables are healthy despite their content in antioxidants, and that other compound contained within these are responsible for these health-promoting effects of fruits and vegetables,” he added.
Ristow and his co-workers note that the new study “applied comparably high doses of oral antioxidants, which have been tested in healthy young men only”.
Ristow and his co-workers recruited 19 untrained and 20 pre-trained healthy young men, and randomly assigned them to receive either a combination of vitamin C and vitamin E, or placebo, during four weeks of physical exercise (85 minutes of exercise five days per week).
At the end of the study, the researchers found that consumption of antioxidant supplements showed no changes in their ROS levels, while the control group experienced increases in the formation of the free radicals.
Insulin sensitivity was only restored in the control group, and not in the antioxidant group, said the researchers.
“If transient increases in oxidative stress are capable of counteracting insulin resistance in humans, it is possible that preventing the formation of ROS by, for example, antioxidants might actually increase, rather than decrease, the risk of type 2 diabetes,” wrote the researchers. “This remains to be determined,” they added.
Commenting on the science, Dr Schauss told NutraIngredients.com that he would be surprised to learn that an exercise trainer would advocate such an intense training programme, and limit the supplementation routine to daily intake of 1000 mg of Vitamin C and 400 IU of vitamin E.
“There are far more nutrients and phytochemicals found in the diet and available supplements and antioxidant-rich foods and juices that would be needed to anticipate the degree of oxidative stress an individual would experience with such an exercise program,” he said.
“This paper does NOT deserve media attention for many reasons,” said Dr Schauss. “[It certainly does not deserve] headline news that might discourage the use of vitamin C and/or E by individuals engaged in exercise routines, or those contemplating exercise.”
The study has already achieved headlines, however, with the BBC running with “Vitamins 'undo exercise efforts'”, The Daily Telegraph (UK) having the headline, “Vitamin supplements may block benefits of exercise”, and the New York Times running with "Vitamins Found to Curb Exercise Benefits”.
For Dr Schauss’ full reaction, and reaction from others, please click here.
Source: Proceedings of the National Academy of Sciences Published online ahead of print, doi: 10.1073/pnas.0903485106“Antioxidants prevent health-promoting effects of physical exercise in humans” Authors: M. Ristow, K. Zarse, A. Oberbach, N. Kloting, M. Birringer, M. Kiehntopf, M. Stumvoll, C.R. Kahn, M. Blüher