For decades, fitness gurus have admonished sofa spuds to adopt a can-do attitude toward exercise, as if the only thing keeping them from the gym or walking path was the right attitude.
Yet a growing body of evidence suggests that it's not merely motivation but also genetics that separate slouches from fitness fanatics, and at least some of these genes appear to act on the brain's pleasure and reward center.
Though the science doesn't imply that people disinclined to exercise can't get moving, it helps explain why some people find it more difficult than others to "just do it."
"We all know people who can't sit still and we all know people who can't get off the couch," says J. Timothy Lightfoot, an exercise physiologist at the University of North Carolina in Charlotte.
Studies of twins suggest that some of the differences between these types of people come down to genetics. A 2006 Swedish investigation looked at leisure-time physical activity in 5,334 identical and 8,028 fraternal twins. The findings revealed that the exercise habits of identical twins were twice as closely matched as those of fraternal twins.
Fraternal twins share half their genes on average, whereas identical twins are genetic duplicates, so the finding implies that genes account for much of the variability in physical activity levels between people.
Likewise, a 2006 study that pooled data on exercise participation in more than 37,000 twin pairs from seven European countries calculated the genetic influence on physical activity at somewhere between 48% and 71%.
And these are not isolated findings.
"We now have more than 20 twin studies showing almost unanimously that [identical] twins are more alike in their physical activity than [fraternal] twins," says geneticist Claude Bouchard, executive director of the Pennington Biomedical Research Center in Baton Rouge, La. The studies make a compelling case that the inclination to exercise runs in families, he says.
Studying mice
In an effort to find the genes involved, physiologist Theodore Garland at UC Riverside turned to rodents. He placed exercise wheels in the cages of ordinary mice and measured how often they scurried around in the wheels.
"This was voluntary exercise," Garland says. "It's sort of like how some people jog and others don't."
Researchers then selected the mice who ran the most and bred them with other so-called "high-runners" and repeated the experiment for more than 50 generations.
