Twins can provide researchers with a unique opportunity to investigate how a trait might be influenced by genetics or by the environment, because identical twins carry the same genome at birth. For example, if a trait or condition is due to genetics, it will appear in both identical twins. A new study involving identical twin volunteers has shown that consistent exercise can alter how genes are expressed in people. In a set of identical twins, there were fewer signs of metabolic disease in the twin that performed more physical activity. While the researchers came to that conclusion by measuring waist sizes and body mass indexes, which were lower in the more active twin compared to their less active twin, there were also changes in the epigenomes.
Epigenetic factors can affect how genes are expressed, but they do not change the sequence of DNA. Those factors can be chemical tags that hang on the DNA molecule, for example, or physical changes that alter the accessibility of certain portions of the genome. A variety of environmental influences can change the epigenome, including stress, diet, pollution, or other toxins.
In this work, the researchers assessed methylation in the human genome in the study participants, in an epigenome-wide association study. There were 70 pairs of twins in the study, and the volunteers' DNA was obtained with cheek swabs. Fitness trackers and surveys were applied to get data about activity levels and neighborhood walkability.
The researchers found that over 150 minutes of exercise per week of physical activity, defined as a high level, was linked to changes in the methylation status of 50 different places of the human genome. These locations have been previously associated with metabolic disease and vigorous physical activity. They could help reveal the biochemical reasons why exercise is beneficial to health. The research has been published in Scientific Reports.
"The findings provide a molecular mechanism for the link between physical activity and metabolic disease," said corresponding study author Michael Skinner, a Washington State University biologist. "Physical exercise is known to reduce the susceptibility to obesity, but now it looks like exercise through epigenetics is affecting a lot of cell types, many of them involved in metabolic disease."
Most identical twins tend to develop different health problems as they age, even though their genome was the same at birth. Epigenetic factors may explain why that happens, Skinner suggested.
"If genetics and DNA sequence were the only driver for biology, then essentially twins should have the same diseases. But they don't." So there must be an environmental influence on twins that drives the development of disease, said Skinner.
It's also worth noting that so-called somatic mutations can arise in the genomes of different cells as people age for many reasons. And identical twins would probably not develop the same somatic mutations.
