Overweight mice may provide a hint as to why it's so hard to start -- and stick to -- your New Year's resolution to exercise more.
Researchers found signs that the brains of obese mice may encourage inactivity.
"We know that physical activity is linked to overall good health, but not much is known about why people or animals with obesity are less active," said study senior author Alexxai Kravitz.
Kravitz is an investigator in the diabetes, endocrinology, and obesity branch of the U.S. National Institute of Diabetes and Digestive and Kidney Diseases.
"There's a common belief that obese animals don't move as much because carrying extra body weight is physically disabling. But our findings suggest that assumption doesn't explain the whole story," Kravitz said.
Kravitz has theorized that the brain chemical dopamine is key to inactivity in mice.
"Other studies have connected dopamine signaling defects to obesity, but most of them have looked at reward processing -- how animals feel when they eat different foods," Kravitz said.
"We looked at something simpler: Dopamine is critical for movement, and obesity is associated with a lack of movement," he said. His team wondered if problems with dopamine signaling alone could explain the inactivity.
For the study, researchers fed normal and high-fat diets to mice. The mice on the high-fat plan put on weight and slowed down. But they slowed down before adding pounds, raising questions about why things happened in that order.
One possible answer: The researchers found that the obese and slow-moving mice had less of a "receptor" that processes dopamine.
Further research suggested that weight gain was compounded by their inactivity.
The study results appear in the Dec. 29 issue of the journal Cell Metabolism.
"In many cases, willpower is invoked as a way to modify behavior," Kravitz said in a journal news release. "But if we don't understand the underlying physical basis for that behavior, it's difficult to say that willpower alone can solve it."
Learning more about the physiological reasons why obesity is associated with less activity might also help reduce some of the stigma that obese people face, the researchers said.
However, the results of animal studies aren't necessarily applicable to humans. Much more research will be needed before anyone can say there's a direct connection between dopamine and obesity-related activity levels.