Dr. Stephanie Fulton of the University of Montreal's Faculty of Medicine and colleagues fed one group of mice a low-fat diet, which was 11 percent fat, and a high-fat diet, which was 58 percent fat, to a second group over six weeks, monitoring how the different food affected the way the animals behaved.
The high-fat diet caused the waist size in the latter group to increase by 11 percent, but the mice were not yet obese. The research team used a variety of techniques to evaluate the relationship between rewarding mice with food and their resulting behavior and emotions. The researchers then looked at the brains of the mice and found them physically altered.
One of the molecules in the brain examined was dopamine, which enables the brain to reward with good feelings -- encouraging people to learn certain kinds of behavior, Fulton said.
This chemical is the same in humans as it is in mice and other animals.
In turn, CREB is a molecule that controls the activation of genes involved in the functioning of our brains, including those that cause the production of dopamine and it contributes to memory formation.
"CREB was much more activated in the brains of higher-fat diet mice and these mice also have higher levels of corticosterone, a hormone that is associated with stress. This explains both the depression and the negative behavior cycle," Fulton said in a statement. "A change of diet then causes withdrawal symptoms and a greater sensitivity to stressful situations, launching a vicious cycle of poor eating."
The findings were published online in the International Journal of Obesity.