A junk food diet may cause people tired and sedentary, according to a recent study.

Researchers at the University of California, Los Angeles found that being overweight makes people lazy, rather than vice versa.

For the study, scientists led by UCLA's Aaron Blaisdell placed 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 rats in the second group ate foods that were highly processed, of lower quality and included substantially more sugar - a proxy for a junk food diet.

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

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

Researchers also found 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 took substantially longer breaks than the lean rats before returning to the task, demonstrating impaired performance.  In a 30-minute session, the overweight rats took breaks that were nearly twice as long as the lean ones.

After six months, the rats' diets were switched: the overweight rats were given the more nutritious diet for nine days. This change, however, didn't help reduce their weight or improve their lever responses. Placing the lean rats on the junk food diet for nine days didn't increase their weight noticeably or result in any reduction in their motivation on the lever task.

The findings suggest that a pattern of consuming junk food, not just the occasional binge, is responsible of obesity and laziness.

The findings were recently published in the journal Physiology and Behavior.