Antioxidant supplementation, exercise and insulin resistance

There have been a number of negative reports on the health effects of vitamin/mineral supplements. Several studies concluded that supplements were either ineffective or made matters worse. The latest negative article on this topic claims that taking vitamins C and E suppresses the health benefits of exercise (1).

How could this be? After all, exercise does create free radials, and free radicals do cause damage. Since I too take supplements, I decided to have a closer look.

The title of the article, "Antioxidants prevent health-promoting effects of physical exercise in humans", is actually misleading. The investigators really set out to determine if and how antioxidant supplementation interferes with the beneficial effect of physical exercise on insulin resistance. Here is how they went about it.

Forty healthy non-diabetic young men were recruited for a two-part trial. Sixteen of the 40 completed the first part; the rest were used in the second stage. Each exercise session consisted of running or cycling (20 min), circuit training (45 min), and warm-up and cooling off periods (20 min). All 40 volunteers were randomly assigned to receive either placebo or 2×500 mg vitamin C and 1×400 IU vitamin E per day.

The first trial part was designed to test whether the chosen exercise regimen generated reactive oxygen species (ROS) — popularly known as free radicals — and whether the chosen antioxidant cocktail suppressed them. The sixteen participants, half getting placebo, exercised on three consecutive days. Muscle tissue samples were taken before and after the intervention and analyzed for ROS content. This stage confirmed that the chosen exercise protocol generated ROS, and that the antioxidants suppressed these ROS, i.e. that the experimental setup worked.

The second trial part was designed to answer the question if and how ROS formation improved insulin sensitivity, and if and how antioxidant supplementation interfered with that effect. In this trial part the remaining 24 recruits completed four weeks of exercise, five days per week. Blood and muscle tissue samples were taken before and after the intervention.

Analysis of the blood samples established that the 4-week exercise regimen improved insulin sensitivity, but only in the placebo group. No significant improvement was observed in the supplement group, i.e. the antioxidant cocktail suppressed the exercise effect on insulin sensitivity.

Analysis of the tissue samples revealed the reason why. In the placebo group, exercise strongly increased the gene expression of known molecular regulators of insulin sensitivity. In the supplement group this effect was much smaller. In other words, antioxidant supplementation largely eliminated the exercise-induced increase in insulin sensitivity.

Interestingly, ROS formation also increased the production of the antioxidant enzymes superoxide dismutase (SOD) and glutathione peroxidase (GPx). If the exercise-generated free radicals aren′t suppressed by antioxidant supplementation, the body will generate its own defenses against them — while simultaneously improving insulin sensitivity.

In summary, this study convincingly demonstrated that, and why, supplementation with antioxidant vitamins C and E suppresses a valuable exercise effect — improved insulin sensitivity.

Does all this mean that supplementation with vitamins C and E is useless? Not at all. These two antioxidants are after all vitamins, which are defined as substances that we need but cannot make at all or only in insufficient amounts. Nothing can take their place where they are needed, but they cannot be used like drugs. In this particular case the exercise-induced free radicals actually played a vital regulatory role — suppressing them backfired.

As the authors put it:

"… antioxidant supplements prevent the induction of molecular regulators of insulin sensitivity and endogenous antioxidant defense by physical exercise"

and

"… we propose that transiently increased levels of oxidative stress reflect a potentially health-promoting process at least in regards to prevention of insulin resistance and type 2 diabetes mellitus"

Sources:
  1. Ristow M, Zarse K, Oberbach A et al. Antioxidants prevent health-promoting effects of physical exercise in humans. Proc Nat Acad Sci, published online before print May 11, 2009;
    doi:/10.1073/pnas.0903485106
Posted by Helmut Beierbeck at 5/22/2009 12:53 PM
Categories: exercise, insulin, antioxidants, diabetes
 

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