A new UCLA psychology study provides evidence that being overweight makes people tired and sedentary — not the other way around.

Life scientists led by UCLA’s Aaron Blaisdell placed 32 female rats on one of two diets for six months. The first, a standard rat’s diet, consisted of relatively unprocessed foods like ground corn and fish meal. The ingredients in the second were highly processed, of lower quality and included substantially more sugar — a proxy for a junk food diet.

After just three months, the researchers observed a significant difference in the amount of weight the rats had gained, with the 16 on the junk food diet having become noticeably fatter.

“One diet led to obesity, the other didn’t,” said Blaisdell, a professor of psychology in the UCLA College of Letters and Science and a member of UCLA’s Brain Research Institute.

The experiments the researchers performed, Blaisdell said, also suggest that fatigue may result from a junk food diet.

As part of the study, the rats were given a task in which they were required to press a lever to receive a food or water reward. The rats on the junk food diet demonstrated impaired performance, taking substantially longer breaks than the lean rats before returning to the task. In a 30-minute session, the overweight rats took breaks that were nearly twice as long as the lean ones.

Does a junk food diet make you lazy? UCLA psychology study offers answer | UCLA