Why Coffee Tastes Bitter at First Then Becomes a Habit
- A new study identifies a slow dopamine pathway linking the gut to the brain.
- The pathway explains how we learn to like foods that were unpleasant at first, like coffee.
- It responds to internal body signals and its effect lasts several minutes.
- The process occurs in the anterior basolateral amygdala.
- Blocking the pathway prevents learning a preference for a calorie-linked flavor.
- The response is stronger on the left side of the brain in humans and animals.
- The discovery may pave the way for treatments that target food preferences, not just hunger.
Why do many people find coffee bitter at first, then end up unable to go without it? And why are we drawn to chocolate and foods rich in sugar and fat?
The answer may not lie in taste preferences alone. It may lie in a biological learning process that happens between the gut and the brain. This process helps shape our relationship with food over time, according to experts.
A recent report highlighted a study titled “The Neural Basis of Flavor and Nutrient Learning.” Researchers at the University of California, San Francisco, conducted it, and Nature journal published it. The study describes a slow dopamine pathway that connects the digestive system to the brain.
What Is New in This Study?
Scientists have long known that dopamine guides behavior. It teaches the brain what is worth pursuing. However, most research has focused on its fast signals, which occur within seconds. Those signals respond to external cues such as seeing a favorite food.
The newly discovered pathway responds to internal signals originating in the body. Its effect lasts longer. This difference is the core of the discovery.
“The slow dopamine system helps us learn to love certain foods over time. The researchers were able to locate this process and understand how it happens. When the brain links coffee’s flavor to caffeine’s stimulant effect, its taste begins to improve for that person.”
— Zachary Knight, lead researcher
Signals That Begin in the Gut
The digestive tract contains sensors that detect nutrients such as sugar and fat. These sensors send signals to the brain. After food reaches the intestine, dopamine release begins gradually. This response can last several minutes.
To observe this process, the researchers ran experiments measuring dopamine release in different brain regions. They delivered nutrients directly to the stomach, including sugar solutions and a milkshake-like liquid. Bypassing the mouth isolated the brain’s response to nutrients from the influence of taste.
| Criterion | Fast Pathway | Slow Pathway |
|---|---|---|
| Source | External cues | Internal body signals |
| Timing | Seconds | Several minutes |
| Example | Seeing a favorite food | Nutrients reaching the gut |
| Function | Guiding immediate behavior | Learning to prefer beneficial foods |
The Amygdala Is the Center of the Process
Results showed a strong and sustained dopamine response in a region of the amygdala. This region is known as the anterior basolateral amygdala. It was already known to release dopamine. However, its function there was not clearly understood before this study.
To confirm the pathway’s importance in forming food preferences, the team ran additional experiments. They showed that disrupting the related neural signals prevents learning a preference for a new flavor linked to calories. This did not affect normal food intake.
In contrast, activating dopamine fibers in the same region after tasting a calorie-free flavor led to a learned preference for it. It was as if the neural signal substituted for nutrients reaching the body.
Similar Results in Humans
The evidence was not limited to animal experiments. Brain imaging studies in humans showed a similar pattern. Psychologist Dana Small of Yale University reanalyzed data from an earlier study using positron emission tomography. The study included 13 men who consumed a milkshake or a calorie-free drink.
Results revealed dopamine release concentrated in the left side of the basolateral amygdala. This occurred after nutrients reached the intestine. In a separate experiment, Mark Tittgemeyer and colleagues used functional magnetic resonance imaging to study 38 adults. They observed increased activity in the same brain region after glucose intake.
The Puzzle of Left-Right Asymmetry
One striking finding was that the dopamine response was stronger on the left side of the region than on the right. This appeared in both humans and animals. It is unexpected, because neural circuits responsible for basic functions such as feeding are generally thought to work similarly on both sides of the brain.
The reason for this difference remains under investigation. Knight’s laboratory is currently working to understand it.
Can We Change Our Food Preferences?
Discovering this pathway opens new horizons for understanding how our food preferences form. It also opens the door to influencing them in the future.
One question scientists are studying is whether this mechanism relates to drugs that mimic a hormone produced by the gut. These drugs produce noticeable changes in eating behavior. However, Knight’s team did not test these drugs in the study. Therefore, their effect on the newly discovered dopamine pathway remains unknown.
The researcher proposes a hypothesis. These drugs may reduce dopamine signals triggered by nutrients reaching the body. That could make certain foods less able to reinforce the desire to eat them over time.
Why Does This Discovery Matter?
The discovery matters because it may pave the way for future treatments. These treatments would not merely affect feelings of hunger or fullness. They would also target the learning process that makes some foods more attractive than others.
Knight said that understanding the stages linking nutrient sensing in the gut to learning in the brain could one day allow precise control of these mechanisms. That could help change people’s preferences for different types of food.
Research Team and Funding
Researchers from the University of California, San Francisco; the Howard Hughes Medical Institute; the Max Planck Institute for Metabolism Research; and Yale University participated in the study. Authors include Anna Hakimi, Longhui Qiu, Zhengya Liu, Kwan Li, Jingkun Zhang, Jaun Choe, Violeta Aboudia, Heiko Backes, Bojana Kuzmanovic, Mark Tittgemeyer, and Dana Small.
The research was supported by the National Institutes of Health in the United States. The researchers declared no conflicts of interest.
