Although there is a popular perception that lazy people are more likely to eat convenient but unhealthy foods, a study conducted by researchers from the University of California-Los Angeles and published in the journal Physiology and Behavior on April 10 suggests that it may be the other way around: A diet high in processed, sugary foods may actually make people lazier.
“Overweight people often get stigmatized as lazy and lacking discipline,” lead author Aaron 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.”
The researchers fed 32 female rats either a standard diet composed mostly of unprocessed foods or a diet composed mostly of processed, sugary foods. The diets were nearly identical, however, in total fat, protein and carbohydrate content — making the amount of sugar and processed ingredients the primary difference.
A few months into the study, the rats on the processed food diet were already becoming obese, while the other rats maintained a healthy weight.
“By three months there was a statistical difference between the two groups, and from there we just saw a steady, progressive, increase in weight in the rats eating the refined diet,” Blaisdell said.
After six months, the researchers trained the rats to push a lever in order to receive a spoonful of sugar water. Over time, the researchers made the task more and more difficult by increasing the number of times that a rat needed to push the lever to receive the payoff. As the task became more difficult, rats in both groups began to get discouraged, taking long breaks between pushes of the lever or even giving up entirely. Although both groups of rats seemed to have similar levels of energy, the rats on the processed food diet took much longer breaks than the control rats.
“The biggest break a lean rat took was about 5 minutes during a 30-minute session,” Blaisdell said. “In obese rats the breaks were much longer — about 10 minutes for the longest breaks.”
Overall, the obese rats took twice as long to receive a payoff as the control rats and gave up twice as often.
“The obese rats really showed impaired motivation,” Blaisdell said. “It is as if the rat is thinking ‘This is too much work.'”
When the researchers switched the diets between the two groups, the obese rats did not lose any weight or improve their performance at the task. The researchers hypothesized that so much time on the “junk food” diet had actually changed the rats’ brain chemistry.
“A colleague of mine has found that if you impair the dopamine system in rats, they give up on harder tasks much sooner than rats that had not had an impairment,” Blaisdell said. “Diets that induce obesityare likely deregulating that dopamine system.”
In order to rule out the possibility that the “junk food” rats were simply less motivated because they were already getting so much sugar, the researchers replaced the sugar water with plain water and induced thirst in the rats. The obese rats still showed significantly less motivation than the lean ones.
Blaisdell believes that similar motivation-impairing effects probably occur in humans who eat a diet dominated by processed, sugary foods.
“Rats are a great animal model for humans because there is so much overlap in the systems that regulate appetite and metabolism,” he said.
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