EXERCIZE: Mouse Study Hints at Why Obese People Struggle to Exercise.

“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.

That’s a nasty catch-22 to overcome, a natural tendency towards inactivity triggering a weight gain which causes even less activity.