UCLA psychology professor Aaron Blaisdell and his colleagues placed 32 female rats on one of two different diets for a period of six months. One group of rodents consumed their regular diet, which was made up primarily of ground corn, fish meal and other mostly unprocessed foods. The other group ate highly processed foods that contained significantly more sugar in order to simulate a human junk food diet.

After just three months, Blaisdell’s team noticed that the 16 rats in the junk food diet group had become significantly fatter than their counterparts. The study demonstrated that eating processed, sugar rich foods led to obesity, and experiments conducted with the rodents also suggested that fatigue also might have been a result of the diet.

When faced with a task in which they had to press a lever in order to receive a reward of food or water, the performance of the rats on the junk food diet suffered greatly. During a 30-minute session, those rats had to take breaks that were almost twice the length of those taken by the other group, the researchers explained.

Six months later, the study authors switched each groups’ diets, with the overweight rats being given the more nutritious diet for a period of nine days. However, they did not lose weight, and their performance in the level-pressing task did not improve. Likewise, after being placed on the junk food diet for nine days, the lean group did not experience a noticeable weight gain, and their performance in the experiment was not impaired.

According to Blaisdell, the findings of the study suggest that an established pattern of eating unhealthy food -- and not just the occasional binge -- is responsible for weight gain and the effects it has on a person’s body, and that there is no quick fix to the problem. The research could have a profound effect on how obesity is viewed in humans.

“Overweight people often get stigmatized as lazy and lacking discipline,” Blaisdell, a professor of psychology in the UCLA College of Letters and Science and a member of UCLA’s Brain Research Institute explained in a statement. “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.”

Furthermore, Blaisdell said that the results are likely applicable to people as well, since the physiological systems of the two creatures are similar. Furthermore, he and his associates found that the junk food diets made the rats hungrier, and they also grew more and larger tumors than the lean diet rodents throughout the course of the study.

“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.”