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How the Standard American Diet Affects Your Brain

March 20, 2024

A Q&A With Mireille Serlie

Mireille Serlie, MD, PhD, professor of medicine (endocrinology and metabolism), investigates the brain areas involved in food intake and glucose metabolism, the molecular pathways underlying insulin resistance, and the relationship between the brain and obesity.

Her recent research found that people with obesity have an impaired brain response to the detection of nutrients in the stomach, helping to explain why some people consume too many calories and have trouble losing weight and keeping it off.

In a Q&A, Serlie discusses the effects of eating patterns on the brain, the risks of being frequently exposed to high-sucrose and high-fat foods, and recommendations for adopting healthy eating habits.

How does repeated consumption of unhealthy foods impact the brain?

Research shows multiple changes in brain circuitries occurring when animals are put on a Western diet and that these changes affect feeding behavior. In neuroimaging studies in humans, we found differences in brain areas involved in food intake between people with a healthy weight and those with obesity. We don’t know exactly when these changes in the brain occur and when they become more permanent. But we do know that specific eating patterns and diet compositions are associated with weight gain, suggesting specific nutrients play a role.

In a study in which we gave young and healthy people more calories than they needed for a certain number of weeks, we found that snacking throughout the day impacted the serotonergic system in the hypothalamus, a brain structure heavily involved in food intake regulation. Interestingly, consuming the same number of calories in a three-meal-per-day pattern did not affect this system. This shows that eating patterns have a body-weight-independent effect on our brains.

What does that mean for people eating the average American diet, high in saturated fats and added sugars?

High-sucrose or high-fat foods are potent triggers for the brain’s reward system. The brain's response to seeing, smelling, and eating these foods varies from person to person. A strong brain response is associated with more food intake, and repeated exposure and consumption of unhealthy foods can change that brain response, resulting in chronic overeating. We don’t yet know why some people gain weight in a particular environment, and others don’t. Many factors add up: type of food, how often you eat it, how long you have been eating it, your genetics, socioeconomic environment, mental health, medication use, and level of cognitive control.

But a lot of people are affected by obesity, suggesting that many people cannot resist food. This is not surprising if you look at the issue through the lens of evolution. Food means survival, and the brain is wired to keep us eating. If you are in a high-risk environment—meaning there is unhealthy food available 24 hours a day—the brain can be affected in a way that causes you to keep eating more and more.

A recent study showed that even pictures of unhealthy high-fat or high-sucrose foods can affect the brain. How?

Signals of food availability trigger a response in the brain that motivates you to get that food and eat it—it’s part of survival. These responses are normally stronger when you are hungry. In people with obesity, the brain response to these pictures of food is stronger compared to people with a healthy weight. The visual cues of food can induce a dopamine response, and some foods, including unhealthy foods, induce higher responses than others.

What do you recommend to those who want to adopt healthy eating habits?

Many factors play a role in how the brain reacts to food. Since unhealthy food has a larger effect on the brain, food choice is important. But eating patterns and timing of food intake are also important. We have done studies showing that the timing of food intake affects the brain and how it reacts to visual food cues.

For instance, when we studied participants with obesity on a hypocaloric diet, we found that there was an increase in brain dopamine transporters in those who ate most of their calories in the morning. This suggests timing food intake to follow a circadian rhythm can help you lose weight. I tell my patients to have breakfast and not to eat after 8 p.m.

I also advise my patients not to snack between meals because snacking affects the brain’s serotonin system, and lower serotonin transporters are also found in people with obesity, suggesting that snacking is a risk factor for weight gain.

If you have obesity and want to lose weight, you can't rely on the signals your brain is giving you. One of our studies showed that the presence of nutrients in the stomach induces brain activity changes in lean people, but these brain responses were not measurable in individuals with obesity. This lack of brain response affects the ability of people with obesity to regulate food intake because if the brain does not register food in the gastrointestinal tract, people will keep eating. So, it’s also a matter of incorporating this knowledge into your behavior. Set a certain goal before you start dinner, and do not eat more than the food on your plate. And, of course, the new obesity medications are beneficial in reducing food intake.

There’s still a lot to learn. Some people have a body mass index of 30 and stay that way their whole life. For others, body mass index increases to 50 or 60. Why do some people have progressive obesity while others sharing that same environment do not gain weight at all? Future studies will help us understand why the brain reacts to food cues and food intake differently in various groups of people.

Yale School of Medicine’s Section of Endocrinology & Metabolism works to improve the health of individuals with endocrine and metabolic diseases by advancing scientific knowledge; applying new information to patient care; and training the next generation of physicians and scientists to become leaders in the field. To learn more about their work, visit Endocrinology & Metabolism.

Submitted by Serena Crawford on March 19, 2024