High-fat foods are comforting, said David Lau, M.D., Ph.d., of the University of Calgary. Brain scans back this up—eating fat “lights” up different parts of the brain. This may suggest that fat-rich foods are so “feel good” they could become addictive.
“Fat-rich foods can actually cause chemical reactions in the brain in a similar way to illicit drugs, ultimately leading to depression as the ‘comedowns’ take their toll,” said lead researcher Dr. Stephanie Fulton.
Recent studies increasingly reveal that obesity is linked to a higher risk of depression, said Fulton, but exactly what the underlying biological mechanisms are between the two remains unknown. Fulton and her co-author, Sandeep Sharma, wanted to investigate whether a high-fat diet might affect the brain’s emotion and reward circuits.
For the new study, the researchers studied mice already prone to obesity. One group was fed a diet high in fat, particularly saturated fat, the other low-fat chow.
After 12 weeks, the rodents were given a series of behavioral tests, including “anxiety” tests measuring how they react to a new environment. Stressed animals tend to freeze, or run off to a corner, rather than explore.
Mice given the high-fat diet were much less active, avoided open areas and did little exploring.
In a swim test used to measure “behavioral despair” — a test also widely used by drug companies to screen new anti-depressants — mice had to swim in a glass cylinder filled with water for six minutes.
“Animals that give up quickly — they stop swimming and just float and stop trying to pull themselves out of the beaker — that’s (a sign of) self-helplessness,” Fulton said.
Mice on the high-fat diet “actually gave up” and attempted fewer escapes, she said.
When the researchers studied the rodents’ brains, they found higher levels of corticosterone, a stress hormone. They also saw a difference in the expression of proteins responsible for signaling among neurons in areas of the brain regulating emotions and reward.
The type of fat might make a difference, said Fulton. Other research has shown that food high in saturated fat — such as hamburgers, bacon, pork sausages, cheese, butter, and ice cream — cause inflammation in the body, including the brain, and that this inflammation may lead to “negative mood states.”
Fulton’s lab found evidence that rodents consuming the same total amount of fat, but “good fat” like olive oil, experience less anxiety.
The researchers haven’t ruled out the prospect that the extra fat gained by the mice on the high-fat diet affected their performance and “increased immobility times” during the swim test.
Furthermore, the researchers are unsure how to reconcile their results with findings from other studies. Other teams have reported that rats fed high-fat diets are less anxious and more docile.
But that’s only the case in the short term, Fulton said. Animals, including humans, exposed to a stressful situation, or even long-term, moderate stress, “will have a reduced physiological stress response” — meaning they’ll feel a sense of relief — “when given the opportunity to eat high-fat food,” said Fulton, a principal investigator at the Centre Hospitalier de l’Universite de Montreal and a member of the Montreal Diabetes Research Centre.
“In the short-term high-fat food feels comforting, but in the long-term, and with increasing adiposity (fat mass) it is having negative effects on mood. We know that diet is a large contributor to the obesity epidemic throughout the world,” Fulton added.
Foods high in saturated fats and sugar are particularly abundant, she said.
In addition to obesity’s well-known associations with high blood pressure, cancer, and Type 2 diabetes, “we really need to consider mental disorders,” she said.
Lau, editor-in-chief of the Canadian Journal of Diabetes and chair of the diabetes and endocrine research group at the University of Calgary, said the story is much more complex.
“We still don’t understand why obese people are more depressed — is it related to body image (or other issues)?” Lau said. “Basically what they saw was some association,” he said, not cause-and-effect.
It’s an interesting hypothesis-generating observation, said Lau, “but it needs a lot more work. More research is needed, especially in humans, to better understand how nutrient signals affect the hedonic brain pathways.”
The research appears in the International Journal of Obesity.
Source: Universite de Montreal