There is still a lot of debate about how much a person's genes influence their behavior. Twin studies are one way to help decipher the links between genetics and some trait, because if two people with the same DNA have a trait in common, it suggests the trait is likely to have a genetic link. Population studies can also reveal genetic links; when many people carry the same sequence variation in a particular gene, and also share the same trait, it indicates there may be a connection between that variant and trait. Scientists were interested in whether genes have an impact on how active or sedentary people might be, especially since we know there are links between activity levels and health. Social trends have also shown that people are becoming more sedentary in high income countries.
Twin studies have already shown that there are associations between genes and a person's activity habits. Researchers have now pinpointed regions of DNA that are linked to physical activity or sedentary screen time. The research has confirmed that physical activity improves health, and suggested that lifestyles that are more sedentary could be explained by the response of muscle to exercise. The findings have been reported in Nature Genetics.
There are variations in the sequences of genes, and the investigators used those variants as variables; this indicated that less screen time lowers obesity risk. Reduced screen time and more time exercising moderately to vigorously also lowered the risk of attention deficit hyperactivity disorder, diabetes, depression, and younger age of mortality.
"We confirmed that physical activity has beneficial effects on health outcomes. We also found that all outcomes that we examined are driven by physical activity's beneficial effect on body mass," noted first study author Zhe Wang of Mount Sinai School of Medicine.
In this work, the researchers analyzed genetic data collected in 51 different studies and encompassing over 700,000 people. This revealed 99 regions of DNA that are connected to how much time people say they spend watching a screen in leisure time, or performing moderate-to-vigorous physical activity; there are 46 genes in these regions that could be related to physical activity.
"We cannot currently claim that these 46 genes cause someone to be more or less physically active in daily life, but they provide great leads for further studies," noted lead study author Marcel den Hoed of Uppsala University.
"We know that people tend to overreport how much time they spend on physical activity, but around half of the DNA regions we identified also show robust associations with physical activity as measured using devices that people wore during daily life. This adds further credibility to our findings," noted study co-author Ruth Loos of the Novo Nordisk Foundation Center for Basic Metabolic Research at the University of Copenhagen.
The DNA variants that were associated with leisure screen time were found to be close to genes that are expressed in skeletal muscle, and whose expression levels change during strength training. The researchers suggested that these genes might influence how people respond to training, and thus, how likely they are to adopt an active lifestyle. Biochemical pathways may be involved that are related to locomotion, and weakness in muscle due to fiber dysfunction.
The investigators also focused on one gene in particular, and found that a variant in this gene alters a protein building block that is only present in fast-twitch skeletal muscle fibers.
"Our results show that this change [causes] more elastic muscle fibers that can deliver less force, but are likely less susceptible to exercise-induced muscle damage. We think that this reduced risk of muscle damage after exercise makes it easier for people to have a more active lifestyle," explained study co-author Andrew Emmerich of Uppsala University.