An astonishing new report indicates that there’s a very good reason why your New Year’s resolution to lose weight may be doomed at the start. A study examining the behavior of mice who were given high-fat diets found that they were way less motivated to exercise, even before they started putting on pounds, suggesting a chemical imbalance in the brain may be causing people to get caught in a vicious cycle.

The study found that the brains of fat mice encouraged inactivity. Scientists had thought that obese animals don’t move around as much because they extra weight makes it hazardous physically, but now it appears that it has something to do with the brain chemical dopamine, according to a statement from Cell Press, which published the study.

Scientists split mice into two groups for the study: one group was fed a normal diet, while the others had a high-fat diet. Mice on the high-fat diet started slowing down their activity even before packing on the pounds, indicating that these slow-moving mice had less of a receptor for processing dopamine, and the weight gain only made the situation worse for them over time.

“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,” says the study’s senior author Alexxai V. Kravitz, an investigator in the Diabetes, Endocrinology, and Obesity Branch at the National Institute of Diabetes and Digestive and Kidney Diseases–part of the National Institutes of Health. “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.”

“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 says. “We looked at something simpler: dopamine is critical for movement, and obesity is associated with a lack of movement. Can problems with dopamine signaling alone explain the inactivity?”