A new research suggests that consistently eating processed foods and being overweight makes people tired and inactive.
The researchers, from the University of California-Los Angeles (UCLA), conducted their study in rats, whose physiological systems are very similar to those of humans, and hence, they believe their findings will apply to humans.
Lead researcher Aaron Blaisdell says their findings suggest that lethargy may arise from consuming a junk food diet.
Another finding from the study also 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.
The survey also 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 several countries, including in the US 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," the researcher adds.