Junk food diet leads to low levels of activity.

junkA study by UCLA researchers has confirmed the obvious, a diet high in junk food leads to low levels of activity and increased risk of diabetes and obesity. The research involved rat models who were placed on one of two diets for six months. The first, a standard rat’s diet, consisted of relatively unprocessed foods like ground corn and fish meal. The ingredients in the second mirrored a human’s junk food diet and contained highly processed food with lower quality and included substantially more sugar.

In three months the researchers noticed a significant difference in the amount of weight the rats had gained with the junk food rats becoming obese. The obese rats also demonstrated substantially greater fatigue than their normal diet counter parts. 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.

“One diet led to obesity, the other didn’t,” said Blaisdell, a professor of psychology in the UCLA College of Letters and Science and a member of UCLA’s Brain Research Institute.

When the junk food diet rats were placed on a normal diet for nine days after six months there was no reduction in weight or increase in activity level. The time period on a normal diet may have been too short to counter act the significant physiological effect of the junk food diet. 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.

“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.”

The findings can be compared to humans who have physiologically similar systems to rats. Junk food diets make humans and rats hungrier, Blaisdell said. “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.”


Aaron P. Blaisdell, Yan Lam Matthew Lau, Ekatherina Telminova, Hwee Cheei Lim, Boyang Fan, Cynthia D. Fast, Dennis Garlick, David C. Pendergrass. 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; 128: 220 DOI: 10.1016/j.physbeh.2014.02.025

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