In a study published this month, scientists at the Karolinska Institute in Stockholm found that exercise can change the shape and function of genes.

Through a process called methylation, clusters of atoms, called methyl groups, attach to the outside of a gene and make the gene more or less able to receive and respond to biochemical signals from the body. Methylation causes epigenetic changes on the outside of the gene, a process where the operation of genes is affected, but DNA stays the same. 

Depending on which genes are involved, methylation may affect health and risk for disease. In the past, it has been difficult to identify when activities such as exercise caused certain methylation changes.

During the Karolinska study, 23 young and healthy men and women bicycled using only one leg, leaving the other unexercised, allowing each person to become his or her own control group. “…Both legs would undergo methylation patterns influenced by his or her entire life; but only the pedaling leg would show changes related to exercise,” according to an article in The New York Times.

Using sophisticated genomic analysis, the researchers determined that more than 5,000 sites on the genome of muscle cells from the exercised leg now featured new methylation patterns. Some showed more methyl groups; some fewer. But the changes were significant and not found in the unexercised leg.

 Most of the genes in question are known to play a role in energy metabolism, insulin response and inflammation within muscles. In other words, they affect how healthy and fit muscles — and bodies — become.