New research from the University of California, Los Angeles suggests lifestyles -- specifically eating habits -- make people overweight and then leave them tired and sedentary, not the other way around.

Life scientists led by Aaron Blaisdell, a professor of psychology in the UCLA College of Letters and Science and a member of UCLA's Brain Research Institute, put 32 female rats on one of two diets for six months -- the first, a standard rat's diet, consisting of relatively unprocessed foods like ground corn and fish meal, and the second, a diet comprising highly processed foods of lower quality, as well as substantially more sugar.

The foods in the second diet served as a proxy for human junk food diet, according to the group's findings, which are currently posted online and are scheduled for publication in the April 10 print edition of the journal Physiology and Behavior.

After three months, the researchers observed a notable difference in the amount of weight the rats had gained: the 16 rodents on the junk food diet were significantly fatter.

"One diet led to obesity, the other didn't," said Blaisdell.

As part of the study, the rats were required to press a lever to receive rewards of food or water.

The rats on the junk food diet demonstrated impaired performance, taking substantially longer breaks than the lean rats before returning to the task.

In one 30-minute session, the overweight rats took breaks that were nearly twice as long as the lean ones.

Then, after six months, the diets of the two study groups were reversed, with the overweight rats given more nutritious diets for nine days and the leaner rats made to eat junk food for the same amount of time.

As it turned out, the dietary flip-flop didn't help improve the weight or lever responses of the rats in the original junk food group. Likewise, feasting on junk food for nine days didn't have any appreciable negative impact on the weights or lever performances of the rats originally on the healthier diet.

The research, Blaisdell said, suggests an ongoing pattern of consuming junk food, not just binging occasionally, is responsible for obesity and cognitive impairments.

"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, who believes his findings with the lab rats are very likely to apply to humans as well.

Junk food diets make humans -- and rats -- hungrier and less healthy overall, he said.

Researchers found that rats on the junk food diet grew a large number of tumors throughout their bodies by the end of the study, while those on the more nutritious diet suffered fewer and smaller tumors that were also not as widespread.

Blaisdell, 45, said he changed his diet more than five years ago to eat "what our human ancestors ate."

He indicated he now eats meats, seafood, eggs, vegetables and fruits, while avoiding processed food, bread, pasta, grains and food with added sugar. As a result, Blaisdell said, he's experienced dramatic improvements physically and mentally.

"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," Blaisdell said. "It is that difference that leads to many of the chronic diseases that we see today, such as obesity and diabetes."

Better health is obtainable by anyone, said Blaisdell, as long as they commit to living and eating healthfully for the long-term.

"There's no quick fix," he said.