A new study out of the University of California-Los Angeles suggests that eating a consistent diet of processed foods and carrying extra weight causes people to become tired and inactive—not the other way around. But on the upside, it indicates that a few processed food binges isn’t enough to impair our motivation.

Researchers fed 32 rats two different diets—one a normal diet of foods like ground corn and fish meal, the other a processed diet with ingredients like corn starch, sucrose, and soybean oil. After three months, the rats on the processed diet had grown obese; they also took much longer breaks during tasks than the leaner rats.

After six months, the rats switched diets for nine days. The obese rats’ weight and responses to tasks didn’t improve, but the leaner rats’ weight stayed about the same, and their responses to the tasks weren’t impaired.

Though the study was performed with rats, lead researcher Aaron Blaisdell says it’s very likely the results apply to humans, as we share some of the same neurophysiological and hormonal mechanisms.

“Humans who are obese likely suffer from impairments in motivation, either driven by a reduced capacity to be motivated by rewards, or by higher amounts of fatigue and lethargy, or both,” he says. “Thus, once an individual becomes obese through lifestyle factors (e.g., poor diet), then it creates a situation where it is more difficult to exert effort to reduce the obesity.”

But the fact that the leaner rats didn’t suffer after nine days on the processed food diet also indicates that a few processed food binges aren’t likely to cause immediate lethargy and motivation problems. Unfortunately, though, it’s also unlikely that a short period of clean eating will erase the damage from regular junk food consumption.

“It might take more than a few days to a week of eating a clean diet to reduce the motivational impairments induced by the previous chronic consumption of a junk food diet,” Blaisdell says.