Behind the Headlines

Friday May 15 2009

“Taking vitamins after exercise may undo some of the beneficial effects of the workout,” according to BBC News. The website says that while some people take antioxidant vitamins, such as vitamin C, to help protect their body from harmful chemical by-products (free radicals), a German study suggests that these substances may in fact be good for us and protect against diabetes.

The small study behind this article was set up to investigate whether free radical chemicals, an inevitable by-product of muscle use and exercise, play a positive role in the body’s processing of glucose blood sugar. The results suggest free radicals do improve glucose metabolism, and that taking antioxidants may prevent this exercise-related benefit.

This was a small study, and it should be interpreted in the context of other studies in this field, which found contradicting results. This is an area where clear conclusions cannot be drawn and the research base is ambiguous. Larger studies will be needed to investigate the long-term impact of exercise on insulin sensitivity and whether vitamin supplements might reduce any benefits. Further research is necessary before researchers can speculate on how these factors relate to serious complications of glucose metabolism, such as type 2 diabetes.

 

Where did the story come from?

This research was conducted by Drs Michael Ristow and colleagues from the University of Jena in Germany, the German Institute of Human Nutrition, the University of Leipzig and Harvard Medical School. The research was funded by Deutsche Forschungsgemeinschaft (German Research Association) and was published in the peer-reviewed medical journal Proceedings of the National Academy of Sciences.

 

What kind of scientific study was this?

This was a randomised controlled trial looking at how vitamin intake might alter exercise-related free radical levels and the body’s glucose metabolism (processing of blood glucose).

Exercise has many positive effects on the body, including the improvement of glucose metabolism. Impaired glucose metabolism and insulin resistance lead to health problems including type 2 diabetes, which is characterised by high blood glucose. During exercise, muscle makes reactive oxygen species (free radicals) as a by-product of metabolism. Antioxidant vitamin supplements, for example vitamins C and E, are often used on the basis that they might reduce the negative effects of free radicals in the body. In this study, researchers were investigating whether free radicals produced in exercise might actually have a positive role in improving insulin sensitivity.

To do this, the authors enrolled 20 previously exercise-trained individuals and 20 untrained individuals, randomly assigning half of each group to receive either antioxidant supplementation or no supplementation. All participants then underwent a four-week exercise training programme.

To assess whether the increase in free radical production from exercise is linked to the effects of exercise, researchers assessed previously untrained participants before and after three days of exercise. They took samples of muscle tissue (biopsy) and measured the concentration of substances that indicated oxidative stress (the damage caused by free radicals).

In their second set of experiments, all participants underwent a total of 20 sessions of physical exercise, five days a week for four weeks (for 85 minutes each). Throughout the period their insulin sensitivity was assessed. The researchers investigated the production of various chemical regulators in skeletal muscle taken through muscle biopsy and compared the concentration of these between the groups.

 

What were the results of the study?

The previously-untrained individuals taking antioxidant vitamins had evidence of less oxidative stress at the end of a three-day exercise programme.

Following the more intensive training programme, individuals who did not take antioxidant supplements had improved glucose processing (increased insulin sensitivity) after four weeks of exercise, regardless of whether they had previous training or not. Those who took antioxidants, by contrast, did not demonstrate improved insulin sensitivity following exercise.

Insulin regulators were less concentrated in the muscles of people who were receiving antioxidant supplementation.

 

What interpretations did the researchers draw from these results?

The researchers conclude that short-term physical exercise induces the production of free radicals by skeletal muscle and that (at least in the first three days) antioxidant supplements reduce this free radical production. They also state that antioxidant supplementation blocks exercise-induced improvement of glucose metabolism. Previous training had no impact on the effects of exercise.

 

What does the NHS Knowledge Service make of this study?

This small study suggests that exercise-induced free radicals play a role in promoting insulin sensitivity in humans. As the researchers note, the published evidence is ambiguous, with some research finding different results. The authors offer some explanations for the conflict in the evidence, including that some of the other studies assessed the effect of continuous exposure to free radicals, while this one assessed the effects of increases during limited periods of physical exercise.

The researchers have proposed a link between the effects they see here and risk of type 2 diabetes, saying that if antioxidants prevent the benefits of exercise on glucose metabolism then they may increase, rather than decrease, the risk of diabetes. However, this remains to be determined through other research, particularly since a meta-analysis of several related studies has suggested that dietary intake of antioxidants (including fruit and vegetables) can reduce the risk of type 2 diabetes.

There are some weaknesses in this study, including the small sample size and limitation of the study to only male test subjects. The researchers also highlight that their study used comparably “high doses of oral antioxidants”.

Clearly, this is an area where conclusions are indefinite and the research base is ambiguous. Larger studies are needed to investigate the long-term impact of exercise on insulin sensitivity and any effects vitamin supplements may have in reducing the benefits of exercise. This further research is particularly important if researchers are to make extrapolations to serious complications of glucose metabolism, such as type 2 diabetes.

Analysis by Bazian

Edited by NHS Choices

Links to the headlines

Vitamins 'undo exercise efforts'. BBC News, May 13 2009

Vitamin supplements may block benefits of exercise. The Daily Telegraph, May 13 2009

Links to the science

Ristow M, Zarse K, Oberbach A et al. Antioxidants prevent health-promoting effects of physical exercise in humans. PNAS [Published online before print May 11, 2009]

Further reading

Orozco LJ, Buchleitner AM, Gimenez-Perez G, Roqué i Figuls M, Richter B, Mauricio D. Exercise or exercise and diet for preventing type 2 diabetes mellitus. Cochrane Database of Systematic Reviews 2008, Issue 3.

Thomas D, Elliott EJ, Naughton GA. Exercise for type 2 diabetes mellitus. Cochrane Database of Systematic Reviews 2006, Issue 3.

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