Why you may not want to get off the couch to run: The skinny on genes and exercise

It’s pretty obvious that the ability to exercise is a product of our genes, the genes that build our bodies—muscles, bones, heart and lungs, all the rest. But genes underlie more than just our ability to exercise. They seem also to be part of the desire to get out of that recliner and on to the bike or off to the gym. Or contrariwise; for some, genes appear to contribute to the irresistible urge to stay in place, to remain immovable objects.

A recent demonstration of a role for genes in exercise motivation is a fairly large study of middle-aged twins published last fall. It compared levels of physical activity in 420 pairs of identical twins with 352 pairs of same-sex fraternal twins. You will not be surprised to learn that the identical twins were more alike in their exercise patterns. The researchers concluded that while individual variations in physical activity and sedentary behavior were largely the result of environmental factors, genes are also important, “suggesting that innate biological processes may be driving some of our daily physical activity.”

Studies showing genetic influences on physical activity in humans have varied in estimating how much contribution genes make, ranging from lowish (under 30 percent) to high (78 percent). The international team of researchers that reviewed what is known about correlates of physical activity for The Lancet in 2012 speculated that lack of agreement over the strength of an impact for genes might be due to big differences in age ranges between studies, differences in study design, or inaccurate data collection. This last is surely a huge barrier to being more certain about the degree of genetic contribution, but does it matter? Genes are a part of getting us on the move, and maybe that’s all we need to know.

The Lancet review notes several factors that could be involved in the urge to move. The brain’s reward system, for instance, probably figures in the attraction to exercise by athletes and other people who are good at it. Doing well at something is gratifying. It may even be a factor in getting non-athletes to the gym when they don’t much feel like it. A case of virtue being its own reward, or at least part of the reward. The neurotransmitter most identified with the brain reward system is dopamine, and the dopamine pathway involves several genes. More about the dopamine system and exercise in a moment.

Genes and the desire to exercise

Please note the qualifiers in these declarations: genes are a part of the desire to move, but only a part. How we live and who we live with matter enormously. But genes are becoming more evident in discussions of how to get people moving and take advantage of the health and other benefits of regular exercise.

The role of genes is likely to become even more widely recognized still thanks to Gretchen Reynolds’s piece in the New York Times last week. Reynolds mentioned the twin study cited above, but she focused chiefly on ongoing research with lab rats specially bred to adore exercising. Many studies over several decades have demonstrated that both wild and laboratory rodents love to run on wheels and will voluntarily run long distances. Rats prefer places where they have had previous access to a wheel and also prefer running a maze that allows access to a wheel. Both rats and mice can be trained to perform certain tasks that get them on a wheel, including unpleasant (to a rodent) tasks like crossing water.

Rodents differ in their enthusiasm for running, just as people do. But researchers are able to do with rodents something they can’t do with people: breed them to produce strains with different exercise preferences. In hopes of identifying genetic factors that influence motivation to exercise, researchers have mounted long-term breeding experiments in both lab rats and mice. The result: some families of animals get on that exercise wheel and run and run and run. Other rodent strains would just as soon not.

Evolutionary biologist Ted Garland, now at the University of California-Riverside, has been breeding lab mice in this way for 20 years. 70 generations so far. The latest mouse products have been known to run 30 or more kilometers at a stretch. Can you run 30 kilometers? A Garland mouse can. And is in utter misery if you take away his wheel.

Addiction and dopamine

Obsessive wheel-runners like these are running addicts, and there are parallels to the rodent exercise addicts in our own species. Human runners have reported feelings of euphoria similar to the effects of psychoactive drugs. The runners develop tolerances and have to run more, and they suffer stress and depression when deprived of running. Running can even interfere with job performance and social relationships.

So it’s not surprising that the brain pathway for the neurotransmitter dopamine, the one most associated with feelings of reward and with addiction, has been associated also with pursuit of voluntary exercise. The relationship is widely accepted among researchers although there are arguments about whether a particular dopamine profile–the dopamine pathway involves several genes–is the cause or the result of exercise.

I don’t mean to suggest that dopamine signaling is the only factor that has been connected with desire to exercise. Several other genes for neuronal signaling molecules also seem to be involved. This is an enormously complex subject and by no means well understood. These several systems are associated with body composition and feeding behavior (and obesity) as well as physical activity. Some researchers have argued, however, that dopamine may be the final common signaling pathway for the action of all these molecules.

Both eating and voluntary exercise can be rewarding and invoke hedonic responses, can be addictive, and are affected by some of the same brain regions and neurotransmitter systems (dopamine, for example) Garland and his colleagues observed in a 2011 review of voluntary exercise in rodents and humans. So it makes sense that factors affecting one may also affect the other. “Hence, we may expect genetic variants that mainly affect, say, appetite or responses to (preferences for) specific dietary components also to have pleiotropic effects on the propensity for voluntary exercise.”

Rats and exercise

The Reynolds article described another long-running rodent breeding experiment for the study of genes and exercise, this one involving lab rats and carried out in the University of Missouri lab of Thomas Childs. These experiments involved two groups of rats specially bred for 8-10 generations for either high or low voluntary wheel-running.

The two groups differed not only in their attitudes toward running but in their neurons, especially neurons in the nucleus accumbens. The nucleus accumbens is the brain region that is the major site of the brain’s reward system and of dopamine activity. The high runners’ brain cells tended to be more mature than the low runners’ neurons. The researchers were able to trace these differences to differences in genes that control neuron maturation.

But here’s the good news: when the low runners did run on wheels, their brains changed to be a bit more like the high runners’ brains even though they ran much less–only a tenth as much, in fact. They developed more mature neurons.

Rats and mice are of course not like people, and rats and mice selectively bred for particular traits over many generations are especially not like people; they are not even much like your basic rat or mouse. But the work does hint that even people not predisposed to love exercise might be able to pick up the exercise habit if they worked at it for a while. The low-running rats that achieved more mature neurons after exercising suggest the possibility that exercise, which is known to change the brain, might be able to change it in ways that encourage more exercise.

Whether exercise promotes the habit of exercise has not been demonstrated conclusively, but it is hardly a new idea. In fact, it’s a staple of self-help folk wisdom. In a brief cursory search I was able to find three recent articles offering tips on how to build the exercise habit. The Huffington Post advised that deliberately recalling an enjoyable exercise experience can encourage more of it. Deborah Kotz’s Daily Dose at the Boston Globe offered six ways to get yourself to exercise, for example schedule it on your calendar.

A guest post by Canadian researchers at Obesity Panacea offered only three suggestions, one of which is to find an exercise activity that you enjoy. That seems like a sensible place to start. Self-evident, maybe, but sensible.

 

Tabitha M. Powledge is a long-time science journalist and a contributing columnist for the Genetic Literacy Project. She writes On Science Blogs for the PLOS Blogs Network. New posts on Fridays.

 

Leave a Reply

glp menu logo outlined

Newsletter Subscription

* indicates required
Email Lists
glp menu logo outlined

Get news on human & agricultural genetics and biotechnology delivered to your inbox.