Greasy and sweet foods are unhealthy – and yet they have a stimulus for many of us that we are only too happy to give in to. But why is it so difficult for us to ignore fries, chocolate and the like? A study now suggests that high-fat, high-sugar snacks condition our brains to want more and more of them. The results help to understand how our Western diet contributes to the development of obesity.
Fat and sugar provide our body with a lot of energy – often too much given that we are usually sedentary and have an abundance of food. But even though we rationally know that sweets, fast food and similar snacks are unhealthy, these same foods activate the reward system in our brain. We already react to signals that indicate food, such as the logo of our favorite donut shop. Even when we’re not hungry, just looking at it makes us crave donuts.
How snacks shape taste preferences
But why exactly do we like unhealthy foods so much? “Our propensity for high-fat, high-sugar foods, the so-called Western diet, could be innate or it could develop as a result of being overweight. But we think that the brain learns this preference,” explains Sharmili Edwin Thanarajah from the Max Planck Institute for Metabolism Research in Cologne. Together with her team, she examined this hypothesis on 57 normal-weight test subjects.
All subjects received two small portions of yoghurt daily for a period of eight weeks in addition to their normal, self-chosen diet. One group received a product with a lot of fat and sugar, the other group one with little fat and sugar and a high protein content. The calorie content of the yogurt was the same for both groups. Before, during and after the eight weeks, the research team recorded the taste preferences of the test subjects, determined their weight and various metabolic parameters and measured their brain activity using functional magnetic resonance imaging. With the help of a special device, the research team gave the subjects small amounts of a greasy-sweet milkshake or alternatively water in their mouths during the brain scan. A visual cue showed in advance what the subjects could expect.
Conditioning to sweet and greasy
“In the group that had consumed the high-fat, high-sugar yogurt in the previous weeks, the cue of milkshake elicited increased activity in different areas of the brain associated with the reward system compared to water,” the research team reports . Test persons who had received the high-protein yoghurt instead showed no significant effects in response to the milkshake cue. “Our measurements of brain activity have shown that the brain rewires itself through the consumption of high-fat and high-sugar snacks. It subconsciously learns to prefer rewarding food,” says Thanarajah’s colleague Marc Tittgemeyer. “Due to these changes in the brain, we will subconsciously always prefer foods that are high in fat and sugar.”
In taste tests, after the eight-week intervention, the test persons from the fat-sugar group showed an increased preference for products with a high content of fat and sugar. On the other hand, they were less fond of low-fat or low-sugar foods. However, this shift in preferences did not go hand in hand with dulled feelings: If they rated how fatty or sweet a product is, their assessment did not change compared to the first measurement at the beginning of the study.
The research team concludes that the altered preferences are not due to changes in taste perception but to changes in the brain that has been conditioned to fat and sugar for eight weeks. However, the current study does not reveal whether corresponding changes can be reversed by not eating excessive amounts of fat and sugar. The authors also point out that due to the small sample size, the results must be interpreted with caution. It is also conceivable that subjects who are overweight or underweight would show different reactions or that snacks other than yoghurt would lead to different results. In the current study, the test subjects did not gain more weight than the subjects in the control group, and their blood values, such as blood sugar or cholesterol, did not change either.
Source: Sharmili Edwin Thanarajah (Max Planck Institute for Metabolism Research, Cologne) et al., Cell Metabolism, doi: 10.1016/j.cmet.2023.02.015