Food and Behaviour Research

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10 April 2014 - New study shows how junk food diets prompt laziness

Though most of us have experienced that lazy feeling after eating a large meal, researchers from California say their new study indicates that consistently eating processed foods and being overweight makes people tired and inactive, contrary to some beliefs maintaining laziness leads to obesity.

Researchers, from the University of California-Los Angeles (UCLA), have published their results in the journal Physiology and Behavior.

They conducted their study in rats, whose physiological systems are very similar to those of humans. Placing them in two groups, the team then fed the 32 female rats one of two diets for a period of 6 months.

The first group ate a normal rat's diet of unprocessed foods - such as ground corn and fish meal - whereas the second group was fed a lower-quality, processed diet with much more sugar. This diet served as a proxy for a junk food diet, the team says.

Observing changes over 3 months, the team noticed the rats on the junk food diet became obese, whereas the group that ate a normal, unprocessed diet did not.

These results are hardly surprising, but when the researchers gave the rats a task of pressing a lever in order to receive a reward of food or water, they found that rats on the junk food diet took much longer breaks than the lean rats. In a 30-minute session, the obese rats took breaks nearly twice as long as the lean ones.

Lead researcher Aaron Blaisdell says their findings suggest that lethargy may arise from consuming a junk food diet.

'Long-term diet patterns contribute to cognitive impairments'

Interestingly, the investigators switched the two groups' diets after 6 months. This time, the obese rats were fed the more nutritious diet for 9 days, while the lean rats were fed the processed junk food diet.

Despite eating the more nutritious diet, the obese rats' weight did not improve, nor did their responses to the lever task. Additionally, when the lean rats ate junk food for 9 days, their weight did not noticeably increase, and their responses to the lever task did not become impaired.

This result suggests that a long-term pattern of eating junk food - not short-term changes - is to blame for obesity and cognitive impairments, the researchers say.

These findings highlight not just the health implications of obesity, but also social perceptions that may be incorrectly ascribed to it, as Blaisdell says:

"Overweight people often get stigmatized as lazy and lacking discipline. 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."

He adds that either "the highly processed diet causes fatigue or the diet causes obesity, which causes fatigue."