Eating fries, burgers, and chicken nuggets are guilty pleasures most of us indulge in whether we’re seeking comfort food, trying to save a few bucks, or are on-the-go and unable to eat something that’s healthy and nutritious. Regularly chowing down into the succulent, high saturated fat foods — often main culprits of heart disease — can affect our physical and mental health more than previously thought. According to a recent study published in the journal Physiology & Behavior, eating junk food, not being overweight, makes people lethargic and fat.

"Overweight people often get stigmatized as lazy and lacking discipline," said Aaron Blaisdell, leader of the study, and professor of psychology in the University of California, Los Angeles (UCLA) College of Letters and Science, and a member of UCLA’s Brain Research Institute, in a news release. Blaisdell and life scientists believe that the commonly portrayed idea in the media that people become fat because they are lazy, is false. High-fat diets have been known to cause metabolic and cognitive effects, often attributed to the diet’s high content of fat when compared to unrefined low-fat diets. However, little is known about the impact of refined versus unrefined food on cognition.

To investigate the effects of a refined low-fat diet and an unrefined diet on body weight, and cognitive ability, Blaisdell and his colleagues used lab rats in their study. It is known junk food diets make humans and rats hungrier. A sample group of 32 female rats were divided into two groups — one fed a diet of relatively unprocessed foods, and the second 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. During the task, the rats on the junk food diet demonstrated impaired performance compared to their healthier counterparts. In a 30-minute session, the overweight rats took breaks that were twice as long as the lean ones.

Three months into the experiment, the researchers unsurprisingly found the rats on the junk food diet grew increasingly fatter than the other group, Science Daily reported. However, a more interesting finding was that the obese rats’ performance of the lever task became impaired, in which the researchers referred to this lack of motivation as “cognitive impairment.”

Within six months, the rats’ diet was reversed. After nine days of consuming 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 did not show a decrease in motivation after nine days on junk food. "There's no quick fix," Blaisdell noted, when he realized long-term habits have a more profound effect on weight and motivation than the occasional health kicks of junk food binges.

The findings of this study show healthy people become overweight, rather than overweight people become less healthy. “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,” Blaisdell said.

In a similar 2013 study published in the journal Brain, Behavior, and Immunity, researchers found a diet high in fat and sugar may restrict cognitive abilities after just one week in a group of lab rats. The rats on the junk food diet had signs of inflammation in their brain’s hippocampal area — a cerebral center associated with spatial memory. This suggests that the inflammatory responses recorded in obese people may not be limited to fat tissue. "What is so surprising about this research is the speed with which the deterioration of the cognition occurred," said Margaret Morris, research co-author and professor from the University of New South Wales In Australia, in the news release.

With over 25 percent of Americans consuming fast food every day, according to the Palo Alto Medical Foundation, and close to 50,000 fast food chains across the U.S., doctors must warn their patients about the serious complications of a high-fat diet. The efficient service, low prices, and casual atmosphere can become more than you bargained for when it comes to your health.


Blaisdell AP, Fan B, Fast CD, et al. Food quality and motivation: A refined low-fat diet induces obesity and impairs performance on a progressive ratio schedule of instrumental lever pressing in rats. Physiology & Behavior. 2014.

Beilharz J, Maniam J, and Morris M. Junk food can harm memory in a week. Brain, Behavior and Immunity. 2013.