To examine those questions, scientists at the University of Missouri in Columbia recently interbred rats to create two very distinct groups of animals, one of which loves to run. Those in the other group turn up their collective little noses at exercise, slouching idly in their cages instead.
Then the scientists closely scrutinized and compared the animals’ bodies, brains and DNA.
For some time, exercise scientists have suspected that the motivation to exercise — or not — must have a genetic component. When researchers have compared physical activity patterns among family members, and particularly among twins, they have found that close relations tend to work out similarly, exercising about as much or as little as their parents or siblings do, even if they grew up in different environments.
These findings suggest that the desire to be active or indolent is, to some extent, inherited.
But to what extent someone’s motivation to exercise is affected by genes — and what specific genes may be involved — has been hard to determine. There are only so many human twins around for study purposes, after all. And even more daunting, it’s difficult to separate the role of upbringing from that of genetics in determining whether and why some people want to exercise and others don’t.
So the University of Missouri researchers decided to create their own innately avid runners or couch potatoes, provide them with similar upbringings, and see what happened next.
They began with ordinary adult male and female lab rats. These rats generally embrace the opportunity to run, although individual mileage can differ substantially among rats.
The scientists put running wheels in the animals’ cages and, for six days, tracked how much they ran. Afterward, the males and females that had logged the most miles were bred to each other, while those who’d run the least were likewise paired. Then the pups from each group were bred in a similar way, through 10 generations.
At that point, the running rats tended to spontaneously exercise 10 times as much as the physically lazier animals.
Now, the researchers set out to determine why.
In very broad terms, two elements are especially likely to influence whether we, as individuals, habitually exercise or not. One is physique. Animals or people that are overweight or ill, or who have poor muscle quality or tone or other physiological impediments to activity, tend to be sedentary. If moving is difficult, you don’t do it.
So, the researchers now compared their two sets of animals’ bodies. You might expect that after 10 generations of running frequently or running almost not at all, the animals’ builds would be substantially different. But they weren’t. The non-runners were slightly heavier, but the two groups’ average body compositions, or percentage of muscle versus fat, were very similar. Both groups also had similarly healthy muscles and good appetites.
Differences in physique were not driving differences in exercise behavior.
So the researchers began to examine the other primary determinant of exercise behavior: psychology. How closely rats’ emotions echo our own, if at all, is hard to know. But the runners in this experiment did seem to enjoy running, while rats in the other group appeared to want to avoid it.
And it was here that genetics entered. The scientists compared the activity of thousands of genes in a specific portion of the brain that controls reward behavior, or the motivation to do things because they’re enjoyable.
They found dozens of genes that differed between the two groups.
The rats’ decision to run or not to run, in other words, was being driven, at least in part, by the genetics of motivation.
What this study means for those of us with two legs and many excuses for not making it to the gym is not yet clear. “It does seem likely that there is a genetic element to the motivation to exercise,” in people as well as in rats, says Frank Booth, a professor of physiology at the University of Missouri who oversaw the study.
But whether the same genes are involved in people as in lab rats isn’t known, although part of the rationale for developing these strains of rats is to isolate genes that can be tested for in people.
At some point, Dr. Booth says, scientists conceivably could develop a test that would reliably inform someone whether he or she is genetically predisposed to being physically lazy, or the reverse.
But genetic profiles will never be destiny, Dr. Booth adds. His study’s findings “are not meant to be an excuse not to exercise.” Behavior, he says, remains a mix of innate tendencies and personal choice. So, even if it is in your nature to enjoy long hours on the couch, you can choose to get up and move.