03.27.11

Is the Will to Work Out Hereditary?

The reason some people hit the gym every day while others can’t bear it may be encoded in our genes. Casey Schwartz on new research that suggests the will to workout is hereditary.

We’ve all noticed that there are people in this world who will go to any length to get their daily aerobic fix. They jam running shoes and free weights into suitcases for their vacation to Bermuda. They take the stairs to their office on the 10th floor. They show up late to happy hour because they couldn’t tear themselves away from the treadmill.

What is wrong with these people?

Nothing at all. Still, the two groups eye each other like alien species: How can you? asks one. How can you not? asks the other.

Like so many things these days, the reason appears to be partially informed by genetic inheritance. Which genes really matter in determining our inborn appetite for exercise has yet to be determined, as does the extent to which they make a difference from one person to the next. But the field of exercise genetics is now turning up all kinds of findings that, put together, are beginning to advance the ball on this issue.

It started in 2006, with research from several European countries that looked at exercise patterns in identical twins, who share the same genetic make up, versus fraternal twins, who are only as genetically similar as regular siblings. Twin studies have for years been the go-to method for investigating the question of how much any one behavior or trait is determined by genes, but most of them still offer, at best, partial answers. In the case of innate motivation to exercise, these initial twin studies found a definite role for genes, with exercising tendencies more similar between identical twins than fraternal ones. But estimates varied widely about the strength of the genetic effect.

In more recent years, Dr. Angela Bryan, a professor of psychology and neuroscience with labs at both University of Colorado, Boulder, and University of New Mexico, has been working to flesh out the portrait of peoples’ varying inclinations to exercise.

In her research, Bryan works on the assumption that genes, physiology, and psychology all work in tandem to push people toward or away from the gym. Genes affect the way different people respond to the physiological changes that exercise causes, like increased heart rate, raised body temperature, and feelings of exertion--which in turn affects how likely they are to do it again.

In other words, according to Bryan’s findings, it seems that one person’s pain is another one’s pleasure.

The way that exercise affects your mood during and immediately after a workout is perhaps the most crucial predictor of whether or not you’re likely to keep doing it.

“Two people might feel the exact same pain running up a steep hill. One of them says, this is horrible, I don’t want to do this. The other one says, I’m building so much muscle, I’m so excited to be working this hard, I can tell my body’s getting stronger,” says Bryan. “So it’s the interpretation of those physiological responses that seems to be pretty important in terms of how people view their exercise behavior.”

In a 2007 study, Bryan focused in on one gene in particular--the gene for BDNF, a peptide that works throughout the nervous system, influencing the growth of new brain cells as well as skeletal muscles. In animals, research suggests that BDNF has a lot to do with voluntary movement; according to Bryan’s research, the same seems to be true of humans.

Examining 64 subjects, Bryan found that what form of the BDNF gene a person had was significantly linked to whether or not exercise put them in a good mood.

Indeed, says Bryan, the way that exercise affects your mood during and immediately after a workout is perhaps the most crucial predictor of whether or not you’re likely to keep doing it.

“People who exercise, by and large, they talk about doing it because they enjoy it. It’s not,  ‘I’m doing this so I won’t get cancer in 50 years,’” she says.  “That’s why we’re looking at some of the underlying genetic and physiological variables that might be associated with that intrinsic motivation. Because if we could figure out who’s got more of it and who’s got less of it, then we can potentially develop different interventions for those kinds of people.”

Ann Caldwell Hooper, a graduate student in Angela Bryan’s lab, ran her own study—which she presented at the Society of Behavioral Medicine Conference last year—looking at the question of intrinsic motivation to exercise.

She put her subjects on the treadmill for 30 minutes of observed exercise.  However, at the end of the prescribed time, she said to each participant,  “Your 30 minutes of exercise are complete. You now have a choice: you can either begin a 5-minute cool down, or keep going at this pace for 5 more minutes and then begin your cool down. It’s totally up to you.”    

Caldwell Hooper was looking to get around problem that often confuses the scientific study of peoples’ desire to exercise--when subjects come into a lab for these studies, they’re exercising because that’s what they’re being instructed to do. Caldwell Hooper found that about half of her sample chose to keep going for the optional 5 minutes, and half didn’t. When she compared the genetic testing results between the two groups, she found that those with one form of the BDNF gene were significantly more likely to have continued running than those without it.

Bryan’s research dovetails with another major research effort currently under way at the Pennington Biomedical Research Center in Baton Rouge, Louisiana. In a study coming out this month in the Journal of Applied Physiology, Dr. Claude Bouchard and his colleagues used a tightly controlled exercise intervention on nearly 500 subjects over the course of five months. They discovered that the extent to which their subjects did or did not become more fit was significantly determined by exactly 21 tiny variations on snippets of their DNA.

Even though Bryan and Bouchard’s findings suggest that some people are graced with a genetic advantage when it comes to benefiting from and sticking to a rigorous exercise regime, it’s also true that old fashioned perseverance can go a long way in making up the difference.

Geralyn Coopersmith, a certified strength and conditioning specialist and the director of the Equinox Fitness Training Institute notes that even the sedentary among us can build up a physiological desire to exercise, just by sticking to a consistent routine.

“When you ask these people, did you like it the first time, they’ll say God no, it was awful, I was nauseous, I was sore for four days--they’ll tell you all kinds of horror stories,” she says. “And if somehow they were able to push through the initial period, invariably they’ll tell you, oh my God, I can’t imagine my life without it.”

Casey Schwartz is a graduate of Brown University and has a Masters Degree in psychodynamic neuroscience from University College London. She has previously written for The New York Sun and ABC News. Currently, she's working on a book about the brain world.