http://www.latimes.com/features/health/la-he-exercise-genes7-2009sep07,0,5714533,full.story
"Fitness fanatic or
couch potato? Blame your DNA"
Studies suggest that people
might have a genetic predisposition to enjoying exercise.
by Christie Aschwanden
Freelance writer
christie@nasw.org
www.christieaschwanden.com
for the Los Angeles Times
September 7, 2009
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.
The result was a strain of
high-runner mice that run as many as eight hours per night.
Garland's next step was to
find out what caused the mice to want to run. He found clues in the brain.
In a study published in 2003,
his group showed that high-runner mice and regular mice respond differently to
stimulants such as cocaine and Ritalin. Regular mice would run more when plied
with the stimulants. "But we've never found a drug that will increase
running in high-running mice," he says. Whatever those drugs do in the
brain seemed to be already turned on in the high-runner mice.
Because cocaine and Ritalin
alter levels of the brain chemical dopamine, a neurotransmitter involved in
pleasure and reward, the drugs' different effects on the two breeds suggest
high-runner and regular mice may process dopamine differently in the brain --
and that may dictate how much pleasure they get out of running.
Other studies have also
linked physical activity to dopamine.
For instance, a 1998 study
showed that mice deficient in a receptor involved in processing dopamine, the
D2 receptor, are less active than those with normal D2 receptor levels.
More recently, Lightfoot and
his colleague Amy Knab found that two other dopamine-related genes were less
active in their high-runner mice.
Says Knab, who is an exercise
physiologist at Appalachian State University, "There's something
inherently different in the dopamine systems of the high-runners versus
low-runners."
Human studies have also
linked exercise frequency to dopamine. Bouchard's research team studied
physical activity levels in a sample of 721 volunteers from 161 families in
Quebec, Canada. They found that variations in the dopamine D2 receptor gene
correlated to physical activity levels in women, but not men.
It's a start
Bouchard says the study is an
intriguing start -- but he speculates that there are many more genes that
influence exercise inclination.
Environment still plays a
major role in how much someone exercises, though. "You can't blame being
lazy on your genes," Knab says.
In fact, a twins study
published last year suggests that environment trumps genetics when it comes to
the kind of exercise needed for good health.
When University of Washington
exercise physiologist Glen Duncan and his colleagues examined data from the
university's twin registry they found that genetics did predict the propensity
to exercise up to 60 minutes per week.
But at 150 minutes or more --
the amount of exercise that public health officials recommend -- "the
genetic component went away and the environment was the bigger factor,"
Duncan says. For example, if people walk into a building and see a set of
stairs first thing, they will probably take them. But if there's an escalator
front and center, they'll take that instead, he says.
Researchers are now trying to
tease out the ways that genes and the environment combine to turn one person
into a marathon runner and another into a couch potato. By doing so, they may
discover more effective ways to encourage exercise among those not naturally
inclined.
"It's really hard to
change people's physical activity levels," physiologist Joey Eisenmann at
Michigan State University says.
"There are a lot of
people working on interventions to increase physical activity, and for the most
part they haven't been shown to be highly effective. As we learn more about
genetic factors, that may shed light on why these programs don't work as well
as we'd like."
Some of this research may
eventually lead to more individualized approaches to fitness.
Or -- failing that --
researchers may even learn to enhance exercise's gratifying effects with drugs.
"Some day," Garland
says, "we could be giving people pills to make it more pleasurable to
run."
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