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.

The researchers, from the University of California-Los Angeles (UCLA), will publish 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.

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Long-term consumption of junk food – highly processed food that is high in sugar – contributes to not only obesity but also cognitive impairment.

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.”

Because the study was performed in rats – which have similar physiological systems to humans – the team believes their findings will apply to humans.

Fast facts about US obesity

  • Nearly 35% of adults are obese.
  • Preventable conditions related to obesity include heart disease, stroke, type 2 diabetes and certain cancers.
  • In 2008, the estimated annual medical cost of obesity was $147 billion.

Another finding from the study shows that the rats on the junk food diet had a large number of tumors in their bodies by the end of the study, whereas those on the nutritious diet had fewer, smaller tumors.

Obesity has become a public health epidemic in the US over the last decades, where prevalence has increased to over 35% in adults and 17% in children.

Carrying with it myriad health implications, obesity has been linked to heart disease, stroke and certain cancers.

“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,” Blaisdell says. “It is that difference that leads to many of the chronic diseases that we see today, such as obesity and diabetes.”

In other nutrition news, Medical News Today recently reported on a study that suggested consuming one serving of legumes each day can keep “bad cholesterol” at bay.