• Expert opinion
  • 2009

Vitamins E and C Supposedly Increase Risk of Diabetes in Athletes ─ A Critical Examination of Study Results

Published on

01 May 2009

“ Antioxidantsvitamin C and vitamin E in particular, are suspected of increasing the risk of diabetes in athletes” – this message is a reaction by the press to a study by Ristow et al. (1). The influence of taking antioxidants on insulin sensitivity after a demanding four-week training phase was investigated in 40 healthy young men, 20 of whom were classed as exercising regularly and 20 as exercising less regularly. The participants in the study received either 1 g vitamin C and 400 IU vitamin E per day or a placebo. The conclusion was that the strenuous training ultimately resulted in a rise in insulin sensitivity. This reaction was only observed in the non-supplemented athletes, however, and was absent in the athletes who received vitamins C and E. High sensitivity to insulin is seen as a health-sustaining physical requirement in protecting against diabetes.

Athletic strain leads to oxidative stress in the organism through oxidative metabolic products, i.e., aggressive oxygen particles or so-called free radicals. Their biochemical opponents are antioxidative substances such as vitamins C and E. Based on the lower glucose uptake observed only in the cells of the vitamin-treated athletes, the authors conclude that supplementation with the two vitamins C and E enhances the risk of contracting diabetes. The observed lower glucose uptake is to be understood as an insulin resistance. Where this is a lasting condition it can be seen as a risk factor for the development of diabetes. Since according to the present observations free radicals seem to stimulate the uptake of glucose in the cell, the authors conclude that the suppression of the formation of free radicals by antioxidants is more a negative factor, as this impedes the health benefits of sport.

In response to this conclusion, it should be said that the formation of free radicals in heavy physical strain on the organism is not necessarily desirable. Evidence for this has been provided by numerous other studies, which have shown that sports activities lead to an adaptation (upward adjustment) of antioxidative systems which prevent the formation of free radicals or deactivate existing radicals. This means that people who exercise regularly have better antioxidative defences. This reduces the formation of free radicals. Supplementation with vitamins C and E with their antioxidative effects has a result that in principle is comparable to this adaptation and thus a reduction in free radicals. In particular people who do not exercise can improve their lower antioxidative defences, caused by less adaptation, by taking vitamins C and E, which is also shown in the study by Ristow et al. (2009).

The assumption that a transient insulin resistance increases the risk of diabetes by inhibiting the formation of free radicals is also more than speculative. So far no data exist at all regarding the emergence – as feared by the authors – of type 2 diabetes through a transient reduction of insulin sensitivity in healthy men of normal weight. Rather, the results of Ristow et al. (2009) might be interpreted among other things to show that while the increase of free radicals in the non-supplemented athletes has a positive influence on glucose uptake in the cell, it can definitely contribute in the long term to other damage such as atherosclerosis. This is suggested by numerous studies on the induction of oxidative stress (formation of free radicals). The far more important question thus arises of what antioxidant levels are required in athletes and to what extent these can be provided through the diet.

The authors argue that the development of diabetes can be inhibited through a diet rich in fruit and vegetables but not through supplementation. It is problematic that the authors quite evidently give an erroneous account of the Hamer and Chida meta-analysis (2009) quoted as evidence of this. This meta-analysis summarises five large nutritional studies and nine studies on the application of antioxidants. It comes to the surprising conclusion that even the consumption of five or more portions of fruit and vegetables per day only leads to a risk reduction of barely 3%, while supplementation of antioxidants results in a risk reduction of 13%. The latter was particularly observable in supplements with vitamin E, which occurs in fruit and vegetables in only very small amounts.

It would be much welcomed if the authors were to apply their outstanding methodology in a comparative study of people with insulin resistance, as these individuals do in fact have an increased risk of illness. Should this show that the administration of vitamins E and C further aggravates insulin resistance in the long term, an application of such antioxidants would indeed have to be warned against. On the basis of the group chosen by Ristow et al. (2009) in the present study, consisting of healthy people and offering no grounds to suppose a pre-diabetic or even diabetic metabolic condition, an attempt to discuss the status of antioxidants in the prevention of diabetes has little meaning.”

Hohenheim, May 2009


  1. Ristow M. et al. Antioxidants prevent health-promoting effects of physical exercise in humans. Proc Natl Acad Sci, 2009; 106(21):8665–70.

This site uses cookies to store information on your computer.

Learn more