You know an unhealthy diet can make you fat, but new research suggests it can sap your motivation too.
In a study published in the journal Physiology & Behavior, researchers at UCLA found that rats fed a diet low in fat but high in simple sugars and refined flour were not only more obese than rats that had a better diet, but also less willing to work for a reward.
"The obese rats really showed impaired motivation," said Aaron Blaisdell, who studies animal cognition at UCLA, and is the lead author of the study. "It is as if the rat is thinking 'This is too much work.'"
For this study, Blaisdell started with 32 female rats. Half of the rats were given a healthful rat-food diet with complex carbohydrates. The other half were given what the scientists consider a junk food diet -- low in fat, but high in simple carbs.
The two diets had about the same amount of fat, protein and total carbohydrates. But two months into the experiment the rats eating the refined diet had already started to look noticably larger than the other rats in the study.
"By three months there was a statistical difference between the two groups, and from there we just saw a steady, progressive, increase in weight in the rats eating the refined diet," said Blaisdell.
After six months, the researchers ran a test to see if a poor diet has an adverse effect on motivation. One at a time, the rats were moved to a new environment where they were taught to press a lever to access a small spoonful of sugar water.
Once the rats had learned the behavior, the scientists made it successively more difficult for the rats to get the sugar water. The first time it took just one push of the lever to get the sugar water, the next time it took three pushes, then six pushes, and so on. In another, similar experiment the number of times the rat had to hit the lever went from one to five to 10 and so on.
Both sets of rats eventually grew tired of the task -- sometimes taking long breaks between lever pushes, and sometimes giving up completely. But the researchers found the obese rats took significantly longer breaks between pushes and gave up on the task twice as fast as the svelte, healthy rats.
"The biggest break a lean rat took was about 5 minutes during a 30-minute session," said Blaisdell. "In obese rats the breaks were much longer -- about 10 minutes for the longest breaks."
To see if the obese rats were simply less impressed by the sugar water because of the refined sugar in their diet, the researchers ran the experiment again, this time restricting the rats' water so they were thirsty by the time they came into the test environment. This time, a lever push was rewarded with plain water. The results, however, remained the same.
Anecdotally, Blaisdell said the two groups of rats seemed to have the same energy level -- so it's not that the obese rats had grown more lethargic overall than the thin rats. Instead, he thinks it is possible that the refined diet is changing the chemistry inside these rats' brains.
"A colleague of mine has found that if you impair the dopamine system in rats, they give up on harder tasks much sooner than rats that had not had an impairment," he said. "Diets that induce obesity are likely deregulating that dopamine system."
Blaisdell said he still has more work to do to confirm that thesis. For example, down the road, he might run this experiment again, and then euthanize the two groups of rats and compare how many dopamine receptors they have in their brains.
In the meantime, Blaisdell said, it is not such a stretch to say that what is true for rats may be true for humans as well.
"Rats are a great animal model for humans because there is so much overlap in the systems that regulate appetite and metabolism," he said.
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