There are surely many people who, when they first drank coffee, felt, "I simply can't stand the bitterness that lingers on my tongue." This is the kind of bitter taste the brain instinctively tries to avoid. Yet, just as many people love drinking coffee, there are plenty of people who come to prefer bitter flavors. Recent research has revealed that this may be due to the action of the neurotransmitter dopamine.
Scientists finally discover why we learn to love bitter coffee | ScienceDaily
https://www.sciencedaily.com/releases/2026/11/261108233100.htm
In a study published in the scientific journal Nature on November 4, 2026, a research team at the University of California, San Francisco, revealed the existence of a previously unclear dopamine pathway connecting the digestive system and the brain. The research team explained that this is "a clue to explaining the mechanism by which people come to prefer a particular taste after repeatedly tasting it."
Dopamine is a neurotransmitter involved in motivation, reward, and learning. It has long been known that dopamine helps the brain determine "whether an experience is worth repeating." However, much of the previous research has focused on dopamine signals that arise within seconds of encountering a stimulus associated with reward, such as "seeing something one likes."
By contrast, research at the University of California, San Francisco, focused on a different mechanism that acts more slowly. This pathway responds to information coming from within the body and helps the brain learn, after food is placed in the mouth, about the benefits that food provides to the body. Dr. Zachary Knight, the physiology professor who led the study, explained: "This study showed that this slow-acting dopamine system promotes learning to come to like a particular food over time. We have also learned where and how that learning occurs." "When the brain links the taste of coffee with caffeine's stimulating effect, coffee will continue to taste much better."
The "fast-acting dopamine" that has been widely studied helps us respond to opportunities for reward. By contrast, slowly acting dopamine has received little attention until now.
When food enters the digestive tract, sensors detect nutrients such as sugar and fat. When that information reaches the brain, dopamine responses gradually intensify and may continue for several minutes. To investigate this process, Dr. Knight placed nutrients directly into the stomachs of mice and observed dopamine activity throughout the brain.
By introducing liquids similar to sugar water and milkshakes directly into the stomach, the research team was able to separate out the effect of taste and examine the action of the nutrients themselves. As a result, they found that strong dopamine responses persisted for a long time in the amygdala's anterior basolateral region (anterior BLA). The amygdala is a region involved in emotional processing and learning and is responsible for linking experiences with their outcomes. It had long been known that dopamine was released in this region, but why it was released there was not known.
The study next investigated whether dopamine signals were actually involved in the learning that causes animals to prefer a particular taste.First, the newly identified dopamine pathway was blocked in mice. The mice continued to eat as usual, but were unable to learn to associate a newly encountered taste with the calories it could provide.
Next, the research team exposed mice to a non-caloric taste and then artificially activated the dopaminergic nerve fibers in the anterior BLA. The mice came to prefer the non-caloric taste despite receiving no nutrition from it. In other words, stimulating the dopamine pathway allowed them to “reproduce the learning signals normally produced by nutrients.”
These experiments showed that this dopamine system plays an important role in helping the brain learn which tastes indicate that food provides nutrition.
Furthermore, the research team is investigating whether a similar response occurs in humans. Dr. Dana Small, a collaborator of Dr. Knight and a psychologist at Yale University, analyzed positron emission tomography (PET) data from 13 men. PET is an imaging technique that examines the distribution and movement of substances within the body, and the participants consumed either a milkshake or a non-caloric beverage. Analysis confirmed that dopamine release following nutrient consumption was concentrated particularly in the anterior BLA on the left side of the brain.
In a separate study using functional magnetic resonance imaging (fMRI), which measures brain activity based on changes in blood flow, researchers examined 38 adults who consumed glucose, a type of monosaccharide, and confirmed increased activity in specific brain regions. Unexpectedly, the strength of the response differed between the two sides of the brain. In both mice and humans, the dopamine response to nutrients is stronger in the left anterior BLA than in the right.
Brain networks responsible for essential life-maintenance functions such as eating are generally thought to operate relatively symmetrically on the two sides, so this lateralization came as a surprise to the research team. It remains unclear why the left side plays the larger role as of the time this article was written. Dr. Knight’s laboratory is investigating the cause of this left-right difference.
The implications of this discovery extend beyond hunger and satiety. Many approaches to changing eating behavior focus on “suppressing appetite” or “increasing the feeling of fullness after a meal.” However, the newly identified dopamine pathway suggests that the process by which the brain learns food preferences may itself become a target for future treatments. Dr. Knight said, “If we understand the process from when nutrients are detected in the gut until the brain learns, we may be able to adjust part of that process and gradually change preferences for different foods.”
