Benefit of exercise on diabetes risk may be undone by vitamin supplements, study

Benefit of exercise on diabetes risk may be undone by vitamin supplements, study

A team of scientists from Germany and the US found that vitamin C and E supplements may undo the benefits of exercise on diabetes risk and glucose metabolism. It would seem that exercise-induced oxidative stress helps the body defend itself against stress and metabolize carbodydrates more effectively, said the researchers.

The study was the work of scientists from Germany's Leipzig and Jena Universities and Harvard Medical School in Boston, US, and was published online in the Proceedings of the National Academy of Sciences, PNAS on 11 May.

Lead investigator, Dr Michael Ristow, a professor in the Department for Human Nutrition at the Institute for Nutrition in the University of Jena, said:

"Exercise causes repeated boosts of free radicals, which - according to our results - induce a health-promoting adaptive response in humans."

"Subsequently, our body activates molecular defense systems against stress, and metabolizes carbohydrates more efficiently, both of which prevents diabetes, and possibly other diseases," he added.

"Blocking these boosts of free radicals by antioxidants accordingly blocks the health promoting effects of exercise," explained Ristow, adding that short term doses of free radicals may behave like a vaccine, helping the body to boost defences against chronic stressors and building a long term adaptive response.

We already know that exercise promotes longer life and helps protect people against type 2 diabetes and insulin resistance, conditions that can lead to stroke, heart attacks, kidney failure and blindness.

However, we also know that exercise increases levels of reactive oxygen species (ROS) such as free radicals, because of increased metabolism and use of oxygen. ROS are thought to be harmful in excess or if allowed to persist for they can lead to oxidative stress and damage cells.

Many people, athletes and non-athletes, take antioxidants such as vitamin C and vitamin E in the belief that that they reduce the harmful effects of oxidative stress and promote health and long life.

However, we don't know whether taking antioxidants as supplements affects the health benefits of exercise, and its effect on reducing diabetes type 2 risk in particular, which is what the researchers in this study set out to discover.

For the study, Ristow and colleagues observed two groups of young men during four weeks of intensive exercise training. One group took a combination of vitamin C (1000 mg/day) and vitamin E (400 IU/day) while the other did not.

The men who took the supplements showed no changes in their levels of ROS, whereas those who did not showed increased levels of ROS and oxidative stress.

Insulin sensitivity and glucose metabolism only improved in the group that did not take the antioxidants; the major benefit of exercise on these two mechanisms was abolished in the group that took the supplements, said the researchers.

The researchers concluded that:

"Exercise-induced oxidative stress ameliorates insulin resistance and causes an adaptive response promoting endogenous antioxidant defense capacity. Supplementation with antioxidants may preclude these health-promoting effects of exercise in humans."

In other words, the positive effect of exercise on metabolism is undone by taking antioxidants.

Co-author Dr C Ronald Kahn, from the Joslin Diabetes Center and Harvard Medical School, said:

"This is a very important study for the millions of people at risk for type 2 diabetes."

"Exercise is a proven way to improve insulin action and reduce diabetes risk, but clearly this beneficial effect can be largely blocked by taking these very commonly used vitamin supplements."

Kahn said larger studies should now be done to fully assess this effect, but in the meantime people who are at risk for type 2 diabetes, or even some who already have it, need to think carefully about taking vitamin supplements, especially if they do a lot of exercise, he cautioned.

"Antioxidants prevent health-promoting effects of physical exercise in humans."

Michael Ristow, Kim Zarse, Andreas Oberbach, Nora Klöting, Marc Birringer, Michael Kiehntopf, Michael Stumvoll, C. Ronald Kahn and Matthias Blüher.

PNAS published online before print May 11, 2009.


Sources: University of Jena.

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