Does junk food make you lazy?

A diet rich in processed foods and fat -- and the extra weight that comes along with it -- may actually cause fatigue, a lack of motivation and decreased performance, according to a recent study involving lab rats.

It's a generally accepted fact that a sedentary lifestyle can lead to obesity. But according to the study led by biologist Aaron Blaisdell at UCLA in California, obesity can also lead to a sedentary lifestyle, indicating something of a vicious cycle. If the effects seen in the rats in the study can be applied to humans, excessive consumption of processed and fat-rich foods affects our motivation as well as our overall health.

For the study, published in the April 10 issue of Physiology & Behavior, Dr. Blaisdell and his team of researchers divided a sample group of 32 female rats into two groups. The first group was fed a diet of relatively unprocessed foods, while the second was given a "junk food" diet of highly processed foods rich in sugar and saturated fat. All of the rats were required to complete a basic task -- pushing a lever -- to receive a food or water reward.

Three months into the experiment, the researchers observed, unsurprisingly, that the rats on the junk food diet had grown significantly fatter than the others. The more interesting finding, however, was that these obese rats' performance of the lever task had become impaired, as they took much longer breaks than the lean rats between performing the task. The researchers refer to this lack of motivation as "cognitive impairment."

At the end of six months, the researchers reversed the rats' diets. But after nine days on the less-processed foods, the obese rats showed little change in weight and no change in their response to the lever task. Similarly, the lean rats remained lean and showed no decrease in motivation after nine days on junk food. According to the researchers, these findings indicate that it is long-term habits, rather than occasional health kicks or junk food binges, that are responsible for our weight and motivation.

The researchers indicate that the findings are very likely to apply to humans, whose physiological systems are similar to those of rats. For Blaisdell, the study suggests that current societal attitudes towards obesity should be reconsidered.

"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."