Looking at muscle samples taken from healthy volunteers before and after exercise, the team found altered patterns of methylation, a process where certain DNA bases are altered to 'mask' them from the machinery in the cell that produces proteins. This is quite unusual in itself, because it was thought that methylation in adult cells, like muscle cells, is irreversible.
'Our muscles are really plastic', says study leader Professor Julian Zierath, of the Karolinska Institute in Stockholm, Sweden. 'We often say, "you are what you eat". Well, muscle adapts to what you do. If you don't use it, you lose it, and this is one of the mechanisms that allows that to happen'.
Methlylation, which is used to slow down or turn off gene production at specific times, is most commonly found in genes that produce proteins known as transcription factors. These proteins control the expression of a number of other genes in the cell.
Using tissue biopsies taken from the thigh muscles of 14 volunteers, who did not exercise frequently, before and 20 minutes after intense exercise, the researchers found a partial demethylation of three genes. These genes are involved in regulating other genes linked to energy generation from sugar.
Further studies with a sub-group of volunteers established a link between levels of demethylation and exercise - volunteers who exercised the hardest had the greatest levels of demethylation.
'It's one of the first studies that really proves that DNA methylation can affect things in a very short timeframe', says Dr Marloes Dekker Nitert, who studies these kinds of genetic changes at Lund University in Sweden.
Though very distinct, the effects of exercise on methylation were temporary. A similar analysis 48 hours after volunteers had finished a three-week exercise program found that DNA methylation had returned to pre-exercise levels.
The exact mechanism of DNA demethylation remains unclear. However, a parallel study exposing rat muscles cells to caffeine showed a similar pattern of demethylation.
Professor Zierath suggests there is a link between calcium concentrations and demethylation triggers: 'Caffeine releases calcium into the muscle tissue and it sort of mimics a contracting muscle'.
The study offers new evidence that external factors, such as exercise, can alter gene expression, indicating it is more flexible than once thought. In the long run it is hoped this will help us better adapt to changes in our environment.