Obesity and Laziness: Scientists Explain Biological Causes of Inactivity in Obese Individuals

A new study revealed that altered dopamine receptors, and not just the excess weight alone, might be the reason why obese individuals tend to move a little or be mostly inactive.

The study, published in the journal Cell Metabolism, showed that inactivity in obese people can be caused by a dysfunction in their dopamine systems.

"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," said Alexxai V. Kravitz, an investigator in the Diabetes, Endocrinology, and Obesity Branch at the National Institute of Diabetes and Digestive and Kidney Diseases and senior author of the study, in a press release.

For the study, the researchers used mouse models. They separated the mice into two groups. The first group was given a high-fat diet. On the other hand, the second group was given a standard diet and served as the control.

The researchers observed that the mice in the first group gained higher body weight by the second week. By the fourth week, the mice on a high-fat diet spent less time moving and got around more slowly. The researchers found that the first group of mice moved less even before they gained substantial amount of weight, suggesting that the excess weight along is not responsible for the inactivity.

After taking a look into six different components in the dopamine signaling pathway, the researchers found that the obese, inactive mice had deficits in the D2 dopamine receptors. To further investigate the connection between inactivity and weight gain, the researchers studied lean mice that were engineered to have the same deficit in D2 receptor.

Lean mice with the same deficit did not gain weight more readily on high-fat diet, despite their lack of inactivity. This suggests that weight gain was compounded once the mice start moving less.