New study reveals that fluctuating—not just intense—brain signals play a hidden role in overeating

A recent study found that people with higher body mass index and tendencies toward disinhibited eating experience more fluctuating brain responses to food rewards. The findings, published in Translational Psychiatry, suggest that inconsistent reward signals in the brain may play a role in overweight and overeating. Meanwhile, individuals with binge eating disorder showed specific inconsistencies in how much they consciously desired food from moment to moment.

Binge eating disorder involves recurrent episodes of eating large quantities of food while experiencing a loss of control. People with this condition frequently experience high rates of overweight and obesity. Previous research into eating disorders has primarily focused on the average strength of brain signals when people encounter food cues.

One major psychological theory suggests that repeated exposure to highly rewarding foods sensitizes the brain’s motivational pathways. Over time, this sensitization could cause food cues to trigger heightened arousal and cravings. However, hunger and satiety naturally fluctuate.

If eating for pleasure repeatedly overrides the body’s natural fullness signals, a person’s response to food might become highly variable rather than uniformly strong. Instead of just looking at average brain activity, researchers suspected that fluctuations in these signals from moment to moment might better explain erratic eating patterns. To test this idea, cognitive neuroscientist Nils B. Kroemer, psychology researcher Mechteld M. van den Hoek Ostende, and their colleagues at the University of Tübingen in Germany investigated whether variability in reward processing is higher in people with binge eating disorder.

The researchers focused on the nucleus accumbens, a region deep in the brain that plays a central role in processing rewards, anticipating pleasure, and driving motivation. They also looked at the dorsolateral prefrontal cortex, a brain area involved in cognitive control, impulse regulation, and weighing the costs and benefits of a potential action.

The researchers recruited 79 women for the study. The sample included 35 participants diagnosed with binge eating disorder, 21 with subsyndromal binge eating disorder who experienced less frequent episodes, and 23 control participants with no history of binge eating. The groups were matched so that their average body mass index was similar, allowing the researchers to separate the effects of the eating disorder from the effects of body weight.

During the first session, participants completed a behavioral experiment called a grip force effort allocation task. They were asked to squeeze a specialized handgrip device to earn either monetary rewards or small snack foods. The researchers calibrated the task to each person’s maximum grip strength.

This calibration ensured that the relative effort required was identical for everyone, regardless of their baseline physical fitness. In some trials, the required effort was explicitly shown on a screen, making the physical cost obvious. In other trials, the difficulty was hidden to create uncertainty.

After each round, participants rated how much they subjectively wanted the reward and how much effort they felt they had exerted. The points earned during the task were later exchanged for actual cash or snack calories. This allowed the researchers to quantify the physical effort participants would exert for a specific prize.

The researchers found that participants with binge eating disorder showed higher variability in their trial-by-trial ratings of wanting food. Their conscious desire for the food fluctuated widely from one moment to the next, especially during the uncertain trials. This high variability in subjective desire was not seen when they played for money.

However, their actual physical effort to obtain the rewards did not vary in the same way. The researchers noted that this disconnect might mean people with binge eating disorder rely more on external cues or habitual responses to decide how much effort to expend. This could cause their physical behavior to remain steady even while their internal feelings of desire fluctuate.

For the second phase of the study, 59 of the women returned to the laboratory after an overnight fast. They completed a similar version of the handgrip task while lying inside a functional magnetic resonance imaging scanner. This machine tracks blood flow in the brain, providing an indirect measure of neural activity in real time.

The researchers specifically measured the brain’s activity during the anticipation phase, which occurred exactly when the participants first saw the reward cue but before they started squeezing the handgrip. They calculated how much the neural signals in the nucleus accumbens and the dorsolateral prefrontal cortex fluctuated across the trials.

Trial-by-trial fluctuations in the nucleus accumbens were positively associated with a higher body mass index. Across all groups, participants with higher body weight exhibited more variable reward signals in this brain region. Participants who reported higher levels of disinhibited eating on a standard eating behavior questionnaire also displayed greater variability in the nucleus accumbens.

The researchers found a similar pattern in the dorsolateral prefrontal cortex. A higher body mass index was linked to more fluctuating activity in this cognitive control region. To ensure these fluctuations were not just random brain noise, the researchers checked control regions in the temporal lobe that are not involved in reward processing.

Those control regions did not show the same variability, indicating the effect was specific to the reward and control centers. The results for binge eating disorder specifically were less definitive. While participants with the disorder showed slightly elevated variability in the nucleus accumbens compared to the control group, the evidence was not statistically significant.

The highly variable brain responses were more closely tied to overall body weight and a general tendency to eat without restraint, rather than a clinical diagnosis of the eating disorder. The study’s design includes several limitations that affect how the results can be applied. The research only included female participants, meaning the observed patterns might not generalize to men.

Sex hormones can influence eating behaviors and reward processing, which the study could not account for. The laboratory setting also tightly controlled when participants ate and minimized changes in their mood. Real-world binge eating episodes are often triggered by shifting emotional states and metabolic changes, which the sterile experimental environment was designed to eliminate.

Additionally, the researchers did not measure actual food consumption during the experiment. The study measured the willingness to work for food and the brain’s anticipation of it, rather than how much participants actually ate. Future studies tracking daily food intake would be needed to link these fluctuating brain signals directly to binge eating episodes.

Finally, slight head movements inside a brain scanner can sometimes look like variable neural activity. People with higher body weight sometimes move slightly more in scanners. While the researchers used mathematical models to correct for head movement, imaging experts often view residual motion artifacts as a potential complication in imaging research.

The study, “Obesity is associated with greater variability of reward signals in the nucleus accumbens,” was authored by Mechteld M. van den Hoek Ostende, Anne Kühnel, Monja P. Neuser, Thomas Dresler, Jennifer Svaldi, and Nils B. Kroemer.