A new study finds eating an unhealthy diet not only makes you fat, it could make you feel less motivated, too.
The study, by UCLA researchers, fed several dozen rats different diets – one healthy, one junk food – to see how they reacted, and how willing they were to work for rewards.
Rats eating junk food soon lost their motivation, The Los Angeles Times reports.
"The obese rats really showed impaired motivation," says Aaron Blaisdell, lead author of the study. "It is as if the rat is thinking 'This is too much work.'"
Researchers gave half the rats a diet of complex carbohydrates and unprocessed foods, such as ground corn and fish meal.
The other rats ate a junk food diet similar to what humans consume -- high in sugars and highly processed.
Within three months, rats on the junk food diet were significantly fatter than the rats on a healthy diet.
The unhealthy rats were also less motivated, indicating that a poor diet had a serious impact on their tiny rodent brains, CBS News reports.
When the rats were put through familiar tests – like pressing a lever for a reward of food and water, the junk food-eating rats were slower.
They also took longer breaks than the rats on the healthy diet. Researchers refer to this lack of motivation as "cognitive impairment."
At the end of six months, the researchers reversed the rats' diets. But after nine days on the less-processed foods, the obese rats showed little change in weight and no change in their response to the lever task.
Similarly, the lean rats remained lean and showed no decrease in motivation after nine days on junk food, Huffington Post reports.
The study indicates that long-term habits, more than occasional health kicks or junk food binges, are responsible for our weight and motivation.
Lead author Blaisdell says his study also suggests that current societal attitudes towards obesity should be reconsidered.
"Overweight people often get stigmatized as lazy and lacking discipline," Blaisdell says. "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, which were published in the journal Physiology & Behavior likely apply to humans because their physiological systems are similar to those of rats, he says.