Brain imaging reveals why some people struggle to resist sweet foods

How our brains process sugar might influence our ability to resist dessert. A recent study found that individuals who exhibit a specific change in their brain chemistry after receiving sugar tend to have a harder time controlling their intake of sweet foods. The research, published in Human Brain Mapping, suggests that an overly efficient system for removing reward-related chemicals could leave some people craving more sweets.

The global rate of obesity has increased massively over the past few decades, bringing elevated risks for cardiovascular disease, diabetes, and certain cancers. The human brain plays a central role in regulating our energy balance, deciding when we feel hungry and when we feel full. Food acts as a natural reward, and the neurotransmitter dopamine is a primary driver of this system. Dopamine is a chemical messenger that helps modulate motivation, habit formation, and the pleasurable sensations associated with eating.

When we eat highly palatable foods like sweets, dopamine is released into the microscopic gaps between our brain cells. Researchers have proposed that obesity might be linked to irregularities in this dopamine system. Some suggest that certain individuals are hypersensitive to the rewarding aspects of food, leading to excessive intake. Others propose a reward deficit model, where people who are insensitive to everyday rewards overeat to boost their dopamine to normal levels.

To study this in living humans, researchers typically use Positron Emission Tomography, or PET scans. A PET scan is an imaging test that uses harmless radioactive tracers to visualize specific cellular functions and brain chemistry. Past PET scan research often focused on dopamine receptors, which act as the docking stations on brain cells that receive the dopamine signal. The results from those studies were highly inconsistent.

Some studies showed higher receptor levels in obese individuals, while others showed lower levels or no difference at all. Because dopamine receptors are difficult to interpret on their own, a research team led by Kyoungjune Pak at Pusan National University Hospital in South Korea shifted their focus to the dopamine transporter. This transporter is a protein that acts like a cellular vacuum cleaner. After dopamine has been released and done its job, the transporter sweeps the chemical out of the space between neurons and returns it to the sending cell.

Previous research by the same team demonstrated that giving humans a dose of glucose increases the activity of these dopamine transporters. When blood sugar rises, the body releases the hormone insulin to help cells absorb the energy. Insulin also appears to signal the brain to deploy more dopamine transporters. Pak and his colleagues wanted to see if this physiological response relates to how people subjectively experience and crave sweet foods.

The researchers recruited thirty-five healthy young men for a small study. Each participant visited the hospital on three separate days, fasting overnight for at least twelve hours before each appointment. This fasting ensured that recent meals would not interfere with their baseline brain chemistry or hormone levels.

During two of these visits, the participants received an intravenous infusion while resting in the scanner. On one day, they received a dose of liquid glucose, and on the other day, they received a placebo of normal saline. The participants did not know which infusion they were receiving on which day, eliminating the influence of expectation.

Following each infusion, the participants underwent a PET scan using a specific tracer designed to bind exclusively to dopamine transporters. This allowed the researchers to measure the availability of the transporters in the striatum. The striatum is a cluster of neurons located deep in the center of the brain that is heavily involved in reward, motivation, and decision-making.

On the third visit, the participants underwent a different type of PET scan that measures overall brain glucose metabolism. This scan provided a baseline map of their normal brain cellular activity. Between all these visits, the researchers tracked the participants’ body mass index, height, and weight.

In addition to the brain scans, all participants completed a twelve-item sweet taste questionnaire. This survey measures an individual’s psychological attitude toward sweets, scoring them on their sensitivity to the mood-altering effects of sugary foods. It also measures their perceived lack of control over eating sweet items. The researchers measured the participants’ blood sugar and insulin levels before and after the infusions to track the body’s physical response.

The glucose infusions successfully raised the participants’ blood sugar and insulin levels, mimicking the biological aftermath of eating a carbohydrate-heavy meal. When analyzing the PET scans, the researchers observed that dopamine transporter availability was generally higher following the glucose infusion compared to the placebo infusion. The participants essentially had more cellular vacuums ready to sweep up dopamine.

When the team compared the brain imaging data to the questionnaire responses, a consistent pattern emerged. The participants who showed higher dopamine transporter availability after the glucose infusion also scored higher on the sweet taste questionnaire. They reported being more sensitive to the mood-altering effects of sweets and having a harder time controlling their consumption of them.

The researchers did not find this relationship when looking at the baseline placebo scans. A person’s dopamine transporter levels under normal, fasting conditions did not predict their attitude toward sweet foods. Similarly, the participants’ baseline brain glucose metabolism and their body mass index did not correlate with their questionnaire scores.

The authors suggest that this biological mechanism might explain why some people struggle to resist sugary snacks. If a spike in blood sugar prompts the brain to rapidly deploy dopamine transporters, those transporters will quickly vacuum up the available dopamine. This rapid removal would abruptly end the rewarding sensation of the treat.

As a result, the individual might feel an immediate desire to eat more sweets to bring the dopamine levels back up. Their brain essentially curtails the pleasure of the sugar, creating a cycle of craving. The faster the dopamine is removed, the faster the person wants another dessert.

The authors noted a few limitations to their research. As a cross-sectional analysis, the results highlight a correlation between brain chemistry and dietary attitudes but cannot prove a direct sequence of cause and effect. It is possible that individuals with an inherent sensitivity to sweet foods simply have different underlying biological traits. Future experiments involving repeated interventions would be needed to establish exactly how these variables influence one another.

The sample size of this small study also limits the strength of the conclusions. The statistical models showed consistent directional trends, but some of the specific regional brain data were not statistically significant after adjusting for multiple comparisons. A larger cohort of participants is required to confirm the findings.

The research exclusively enrolled male participants. Past studies have shown that men and women can exhibit entirely opposite neuroendocrine responses to food-related signals. For example, previous brain imaging research found that intravenous glucose alters dopamine receptor availability differently depending on a person’s sex.

The mechanisms observed in this experiment likely have sexually dimorphic characteristics, meaning the results cannot be automatically generalized to females. Finally, because the study focused primarily on lean individuals, the findings might not apply to populations with clinical obesity. Medical professionals will need to replicate these scans in diverse groups to fully understand how sugar influences dopamine across the general public.

The study, “The Change of Dopamine Transporter After Glucose Loading Is Associated With an Individual’s Attitude Toward Sweet Foods in Healthy Young Males,” was authored by Kyoungjune Pak, Jihyun Kim, Keunyoung Kim, Seongho Seo, and Myung Jun Lee.