So-called Western diets that are high in fat, ultra-processed foods and sugar are in large part responsible for the rising tide of obesity worldwide.
Beyond the many empty calories they provide, they promote weight gain through a cycle of overeating and modifying the brain’s motivation and reward center, called the dopaminergic system. However, not much is known about how obesity actually changes this system. It’s not clear if these brain changes are caused directly by repeated consumption of unhealthy foods; secondary to obesity or overweight; or whether they are genetic, predisposing a person to weight gain.
We are now closer to an answer. The findings of researchers from the Max Planck Institute for Metabolism Research in Germany and Yale University suggest that a high-fat/high-sugar diet may actually teach the brain to prefer these foods.
“Our tendency to eat high-fat and high-sugar foods could be innate or develop as a result of being overweight. However, we think the brain learns this preference,” said Sharmili Edwin Thanarajah, lead author on the study and a scientist at the Max Planck Institute, in a statement.
The brains of those on a high-fat, high-sugar yogurt became more readily stimulated by sugar and fat. This response increased when they were given cues that they were about to get a milkshake. At the same time, their desire for low-fat food decreased.
The study involved 49 healthy, normal-weight participants who were divided into two groups. Twice a day, every day for eight weeks, one group ate a high-fat/high-sugar yogurt in addition to their regular diet; the other ate a low-fat/low-sugar yogurt in addition to their regular diet.
Those assigned to the high-fat/high-sugar group did not gain more weight compared to those in the low-fat/low-sugar control group during the study period. Their blood cholesterol and blood sugar values did not change, either.
What did change were the brains of those who had been given the high fat/calorie yogurt. Participants’ brain activity was measured at baseline and at the end of the study period using functional magnetic resonance imaging.
After eight weeks of high-fat, high-calorie yogurt, the dopaminergic system in the brains of participants in that group had become upregulated — more readily stimulated by sugar and fat. This response increased when they were offered a high-fat/high-sugar milkshake and when they were given cues that they were about to get the milkshake. At the same time, their desire for low-fat food decreased.
No change in dopaminergic response was observed in the control group when they were offered the shake or in response to cues it was coming soon.
This is similar to what happens with obesity, researcher Dana Small, a professor of psychiatry at the Yale University School of Medicine, told TheDoctor. “People with obesity and overweight often have stronger reward responses to those unhealthy foods than those who have a healthy weight.” That suggests that changing the food environment and limiting the availability of high-fat/high-sugar foods should become important ways to reduce the rising tide of obesity, according to the researchers.
The brain’s preference for high-fat, high-sugar foods will continue even after the study’s end. “New connections are made in the brain, and these connections do not dissolve so quickly,” Marc Tittgemeyer, a senior author on the study and a scientist at the Max Planck Institute, said in a statement. The researchers want to know if these diet-induced changes in the brain are reversible.
“The brain is very plastic and learns how to navigate and make decisions in an environment to help it adapt to that environment,” said Small. Going forward, she said it would be interesting to see if different effects are observed for high-fat diets alone versus high-sugar diets or high-carbohydrate diets. “Some evidence in animal studies suggests that high-fat diets cause different changes in the brain than diets that are high in sugar, and we would like to tease that apart.”
The study is published in Cell Metabolism.