New research suggests a junk food diet could encourage people to be sedentary.

Researchers put rats on either a healthy or junk food diet to see how it affected their weight, a UCLA news release reported.

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

After only three months the rats had a noticable difference in weight. The syudy suggests that fatigue could also be result of a junk food diet.

The rats were given a task in which they were required to press a lever in order to receive food or water.  The rats on the junkfood fiet took significantly longer breaks when trying to work  out how to get the foood or water reward.

After six months the rats' diets were switched ; After nine weeks their weight had not changed. 

 "There's no quick fix," Blaisdell said in the news release.

"Overweight people often get stigmatized as lazy and lacking discipline," he said. "We interpret our results as suggesting that the idea commonly portrayed in the media that people become fatbecause 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."

The team also found that the rats on the junk food diet developed a large amount of tumors.

Blaisdell himself has switched over to food "our ancestors ate" over five years ago. He tries not to eat "processed food, bread, pasta, grains and food with added sugar," the news release reported.

"I've noticed a big improvement in my cognition," he said. "I'm full of energy throughout the day, and my thoughts are clear and focused."

"We are living in an environment with sedentary lifestyles, poor-quality diet and highly processed foods that is very different from the one we are adapted to through human evolution," he said. "It is that difference that leads to many of the chronic diseases that we see today, such as obesity and diabetes," he said.