Obesity and sedentary behavior: Which is chicken, which is egg?

December 30, 2016 by Melissa Healy, Los Angeles Times
This is an image of a weight scale. Credit: CDC/Debora Cartagena

If you dread the prospect of hauling your lazy rear end to the gym more often in 2017, new research suggests the extra weight you've been carrying around may be to blame.

That may sound obvious. But the relationship between excess weight and inactivity is anything but. Behind the twin epithets "fat and lazy" lie some unkind - and unclear - beliefs about whether inactivity leads to or whether weight gain leads to inactivity.

A new study offers evidence that diet-induced obesity alters the brain's functioning in ways that suppress the natural impulse to move around.

Many other powerful factors influence our inclination to exercise - not least having the safe spaces, leisure time and social encouragement to do so. But new research in confirms that obesity disrupts the proper functioning of a key docking station for dopamine, a brain chemical that affects our moods, appetites and motor control.

The result: The chubby mice became couch potatoes.

When researchers fattened mice up on high-fat chow, they saw the activity of a specific class of dopamine receptor in the brain's striatum (a center of movement control and reward-seeking behavior) fall. Along with that change, they observed that the obese mice adopted more sedentary habits than their lean peers.

When researchers experimentally turned down or knocked out that brain receptor's activity in lean mice who were fed normal chow, those mice too lost the impulse to run on their running wheels or zip around their cages. They did not, however, become obese.

And when researchers took and experimentally "turned up" the receptor's faulty signaling, they saw the chubby mice step up the frequency of their .

The new research, published Thursday in the journal Cell Metabolism, suggests that inactivity is not a natural cause of obesity. The activity of the affected dopamine receptor varies considerably among mice and presumably in humans, and it's clearly not the case that the lazy among us all get fat.

Rather, inactivity appears to be a downstream consequence of excess weight, the new findings suggest. As such, it may not only encourage further weight gain. Since we know that regular exercise can prevent or mitigate the effects of obesity-related diseases, lack of exercise may foster the development of such conditions as Type 2 diabetes, hypertension and worrisome cholesterol.

The research also suggests that carrying may subtly interfere with the rewards we are meant to get by physical activity.

Not all of us feel joy at the prospect of mounting the elliptical or stair climber for a solid aerobic workout. But free of the spandex and the guilt and the goading of muscled trainers, properly fed experimental mice will happily run for hours on a running wheel. It's when they avoid the wheel that lab technicians wonder what's wrong.

Humans are likely no different - though now we have cars and computers and televisions to induce us to stay and sit.

About 30 percent of Americans 6 years old and over are thought to live a completely sedentary lifestyle, meaning they do not engage in any . And 8 in 10 American adults don't meet government recommendations for activities that build aerobic fitness and strength. Adults who are inactive pay $1,437 more per year in health care costs than physically active adults.

That's a problem for all Americans. But it's a particular loss for the roughly 1 in 3 who are obese and who need to exercise if they are to drive down their added risk of diabetes and heart disease. Restoring their zeal for movement could go a long way to helping protect such people from those diseases. But knowing what's sapping that zeal may be a first step.

Explore further: Inactivity in obese mice linked to a decreased motivation to move

More information: Cell Metabolism, Friend et al: "Basal ganglia dysfunction contributes to physical inactivity in obesity" http://www.cell.com/cell-metabolism/fulltext/S1550-4131(16)30596-4 , DOI: 10.1016/j.cmet.2016.12.001

Related Stories

Inactivity in obese mice linked to a decreased motivation to move

December 29, 2016
Starting a regular program at the gym is a common New Year's resolution, but it's one that most people are unable to stick with for very long. Now a study done in mice is providing clues about one of the reasons why it may ...

Does good-tasting food cause weight gain?

December 15, 2016
Does eating good-tasting food make you gain weight? Despite the common perception that good-tasting food is unhealthy and causes obesity, new research from the Monell Center using a mouse model suggests that desirable taste ...

Brain changes after menopause may lead to lack of physical activity

July 28, 2016
As women enter menopause, their levels of physical activity decrease; for years scientists were unable to determine why. Now, researchers from the University of Missouri have found a connection between lack of ovarian hormones ...

Fat reduction did not reduce incidence of colon and liver tumors in mouse studies

November 4, 2016
Losing weight may not protect against colon and liver cancer, even though obesity is associated with increased risk of certain types of gastrointestinal malignancy. Researchers from the University of South Carolina found ...

Exercise eases arthritis in obese mice even without weight loss

September 27, 2011
Adding another incentive to exercise, scientists at Duke University Medical Center have found that physical activity improves arthritis symptoms even among obese mice that continue to chow down on a high-fat diet.

Brain receptor regulates fat burning in cells

January 12, 2016
Scientists at the Gladstone Institutes have discovered an unusual regulator of body weight and the metabolic syndrome: a molecular mechanism more commonly associated with brain cells. Lowering levels of P75 neurotrophin receptor ...

Recommended for you

Study finds 90 percent of American men overfat

July 24, 2017
Does your waist measure more than half your height?

Are sugary drink interventions changing people's behaviour?

July 19, 2017
An evaluation of efforts designed to reduce how many sugary drinks we consume shows some success in changing younger people's habits but warns they cannot be the only way to cut consumption.

Young adult obesity: A neglected, yet essential focus to reverse the obesity epidemic

July 18, 2017
The overall burden of the U.S. obesity epidemic continues to require new thinking. Prevention of obesity in young adults, while largely ignored as a target for prevention and study, will be critical to reversing the epidemic, ...

Weight gain from early to middle adulthood may increase risk of major chronic diseases

July 18, 2017
Cumulative weight gain over the course of early and middle adulthood may increase health risks later in life, according to a new study led by researchers from Harvard T.H. Chan School of Public Health. They found that, compared ...

Study finds children carry implicit bias towards peers who are overweight

June 23, 2017
Even children as young as 9 years old can carry a prejudice against their peers who are overweight, according to a new study led by Duke Health researchers. They might not even realize they feel this way.

Mother's obesity boosts risk for major birth defects: study

June 15, 2017
Children of obese women are more likely to be afflicted by major birth defects, including malformations of the heart and genitals, according to a study published on Thursday.


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.