The study by University of California - Los Angeles provides evidence that being overweight makes people tired and sedentary - not the other way around. (Agencies)
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, researchers observed 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.
The experiments the researchers performed also suggest that fatigue may result from a junk food diet, Blaisdell said.
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.
After six months, the rats' diets were switched and the overweight rats were given more nutritious diet for nine days.
This change, however, didn't help reduce their weight or improve their lever responses.
The reverse was also true: 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.
These findings suggest that a pattern of consuming junk food, not just the occasional binge, is responsible for obesity and cognitive impairments, Blaisdell said.
"There's no quick fix," he noted.
"Overweight people often get stigmatised as lazy and lacking discipline," Blaisdell said.
"We interpret our results as suggesting that the idea commonly portrayed in the media that people become fat because they are lazy is wrong. Our data suggest that diet-induced obesity is a cause, rather than an effect, of laziness. Either the highly processed diet causes fatigue or the diet causes obesity, which causes fatigue," said Blaisdell.
Blaisdell believes the findings are very likely to apply to humans, whose physiological systems are similar to rats'.
Junk food diets make humans - and rats - hungrier, he said.
The study appears in the journal Physiology and Behaviour.
The study by University of California - Los Angeles provides evidence that being overweight makes people tired and sedentary - not the other way around.