Scientists at the University of California, Los Angeles (UCLA) put 32 female rats on one of two diets for six months. The first was a standard rat’s diet, consisting of relatively unprocessed food such as ground corn and fish meal. The second diet was full of high processed food that was of substantially lower quality and full of sugar — a proxy of the typical American junk food diet.
After just three months, the researchers said they observed a 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 Aaron Blaisdell, a professor of psychology in the UCLA College of Letters and Science and a member of UCLA’s Brain Research Institute who led the study.
According to Blaisdell, the experiments the researchers performed also suggest 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 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, according to the researchers.
After six months, the rats’ diets were switched, and the overweight rats were given the more nutritious diet for nine days. This, however, didn’t help reduce their weight or improve their lever responses, according to the study’s findings.
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.
But what are the implications for humans?
“Overweight people often get stigmatized 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.”
In addition, the researchers found that the rats on the junk food diet grew large numbers of tumors throughout their bodies by the end of the study. Those on the more nutritious diet had fewer and small tumors that were not as widespread.
The study was published in the journal Physiology and Behavior.