Exercise usually leads to an increase in insulin sensitivity. This is thought to happen through a process mediated by reactive oxygen species (ROS), a type of free radical.
These particles have been linked to cell damage and ageing and it has been suggested that vitamins may react with them to reduce their effects.
Vitamins may be undoing the health benefits of exercise by blocking exercise-induced increases in insulin sensitivity, researchers suggested.
What is the research?
Findings are based on a study of 20 physically trained and 19 untrained young men. Half of each group were randomly assigned vitamin C and E supplementation. All participants then undertook four weeks of intensive training.
Measures of insulin sensitivity, including glucose infusion rate, were assessed at baseline and seven days after the final training session.
Exercise increased insulin sensitivity on these measures, but only in those who had not taken vitamins. This applied whether participants had previously trained or not.
Vitamin supplementation led to a two-fold increase in levels of thiobarbituric acid-reactive substances (TBARS), a marker of oxidative stress and ROS formation. There was no increase in TBARS levels for those taking vitamin supplements.
What do the researchers say?
Lead researcher, Dr Michael Ristow from the University of Jena, Germany, said the find-ings provided support for 'an essential role for exercise-induced ROS formation in promoting insulin sensitivity'.
'Importantly, these 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,' he said.
What do other researchers say?
Claire Williamson, nutrition scientist at the British Nutrition Foundation said: 'Just because something is good for you, it does not mean that more of it is better.
'By following a healthy and varied diet, you generally get enough of the nutrients you need and don't run the risk of consuming high doses of vitamins that may be harmful.'
- Vitamins may reduce an exercise-induced increase in insulin sensitivity.
- Findings are from a study of men in intensive training, half of whom took vitamin supplements.
- A healthy, varied diet should provide an adequate source of vitamins.