Summary: That instinctive “never again” response after food poisoning isn’t just a memory — it can be encoded through a direct line of communication between the immune system, fat tissue, and the brain. A new study in the fruit fly Drosophila reveals how pathogen detection triggers a fat-to-brain signal that enforces long-term avoidance of contaminated food.
Researchers from the Universities of Bonn and Tohoku and University Hospital Bonn mapped a previously unrecognized pathway in which immune sensors activate neurons that, in turn, engage adipose tissue to produce neurotransmitters. Those signals then alter brain circuits responsible for learning and odor-driven food choices. The findings, published in the journal Neuron, outline a bidirectional fat–brain axis for pathogen avoidance and suggest conserved principles that may apply to mammals, including humans.
Key Findings
- Innate attraction then learned avoidance: Unexposed flies initially prefer the smell of food contaminated with a pathogenic bacterium, indicating an innate olfactory attraction. After ingestion and illness, however, flies learn to avoid that odor.
- Fat tissue as a signaling hub: Adipose tissue (the fat body in flies) is essential for converting peripheral immune signals into a chemical message that modifies brain circuits. Fat cells synthesize dopamine in response to neuronal input and release it to reinforce avoidance learning.
- Neural-immune-fat interaction: Pathogen-detecting receptors on neuromodulatory neurons respond to harmful bacteria, releasing octopamine (an analogue of adrenaline). These octopaminergic neurons innervate the fat body, where octopamine triggers dopamine production that is then sent to the brain.
- Metabolic state matters: Starvation reduces fat reserves and likely lowers fat-derived dopamine production. The authors hypothesize that when animals are hungry, a weakened fat-to-brain avoidance signal could make them less selective, increasing the willingness to risk contaminated food for survival.
- Potential human relevance: Mammalian adipose tissue also produces signaling molecules that influence appetite and brain function. Disruption of organ–brain communication may contribute to eating disorders and metabolic diseases, making this pathway a candidate for further research in higher animals.
Source: University of Bonn
How the study was done
The researchers offered flies a choice between two identical food sources: one contaminated with the pathogenic bacterium Pseudomonas entomophila and the other with a benign Pseudomonas strain. Naïve flies showed a preference for the odor of the contaminated food, but after consuming the pathogenic bacteria and activating their innate immune response, they learned to avoid that food source.
Molecular and imaging experiments identified immune receptors that respond specifically to components of the harmful bacterial cell wall. Many of these receptors are expressed on specialized neurons located near the fly’s pharynx. Activation of these neurons triggers release of octopamine, which travels through neuronal projections to the head fat body.
In the fat body, octopamine initiates intracellular signaling that leads to dopamine production. Dopamine is then transported to the brain, where it acts on mushroom body output neurons through Dop1R1 receptors. Two-photon calcium imaging showed that pathogen ingestion changes odor-evoked activity in these learning circuits, linking peripheral immune detection to persistent neural plasticity and behavioral avoidance.
Biological and practical implications
This work positions adipose tissue as more than an energy reserve: it functions as an active signaling organ that converts immune alerts into neuromodulatory cues. By sending dopamine to learning centers in the brain, the fat body helps solidify aversive memories that guide future food choices — a clear survival advantage when pathogens are present in the environment.
The study also raises important questions about how metabolic state interacts with risk assessment. If reduced fat reserves blunt the fat-derived avoidance signal, hungry animals may accept higher infection risk to avoid starvation. This trade-off may be a widespread feature of animal behavior and metabolism and could have parallels in human conditions where the brain–organ communication network is disrupted.
Participating institutions and funding
This collaborative study involved researchers from the Universities of Bonn and Leipzig, Tohoku University (Japan), and University Hospital Bonn.
Funding: German Research Foundation (DFG), the iBehave Network of North Rhine-Westphalia, and the Human Frontier Science Program Organization.
Frequently Asked Questions
Q: Why would my fat cells care about what I eat?
A: Fat cells act as an internal monitor of nutritional and health status. When immune sensors detect a pathogen, the fat tissue helps relay that information to the brain by producing signaling molecules. This study shows fat is an active organ in survival signaling, not just an energy store.
Q: Could this explain cravings for unhealthy food when I’m hungry, even if it makes me sick later?
A: Possibly. The researchers propose that low fat reserves weaken the dopamine-based avoidance signal. When hungry, the body may lower its avoidance threshold because the immediate risk of starvation outweighs the potential cost of a transient illness.
Q: Does a stomach infection actually change the brain?
A: Yes. Immune detection of pathogens can trigger a cascade of signals that alter neural activity and learning circuits to prevent repeated ingestion of harmful food. This mechanism appears to be conserved across species and serves as a protective, adaptive response.
Editorial notes
- This article was edited by a Neuroscience News editor.
- The original journal paper was reviewed in full for accuracy.
- Additional explanatory context was provided by editorial staff.
About this research
Author: Johannes Seiler, University of Bonn
Source: University of Bonn
Contact: Johannes Seiler, University of Bonn
Image: Image credited to Neuroscience News
Original research (open access): A Bidirectional Brain-Fat Body Axis for Pathogen Avoidance — Yujie Wang, Jean-François De Backer, Aurélie Muria, Ayako Abe, Kokoro Saito, Helen Holvoet, Mareike Selcho, Hiromu Tanimoto, and Ilona C. Grunwald Kadow. Neuron. DOI: 10.1016/j.neuron.2026.03.026
Abstract
A Bidirectional Brain-Fat Body Axis for Pathogen Avoidance
Ingesting pathogens threatens survival and has driven the evolution of avoidance behaviors across species. The mechanisms linking immune detection to behavioral change have remained incompletely understood. This study identifies a bidirectional communication pathway between fat tissue and the brain in Drosophila melanogaster that mediates pathogen avoidance. Immune receptors and a specific antimicrobial peptide are required in both the fat body and neuromodulatory neurons to suppress pathogen intake. Pathogen-sensing octopaminergic neurons innervate the fat body, activating calcium signaling through an octopamine receptor and triggering fat body dopamine release. Dopamine then acts via Dop1R1 receptors in mushroom body output neurons to drive avoidance behavior. Two-photon calcium imaging demonstrates that pathogen ingestion alters odor responses in these neurons, linking immune activation to persistent behavioral change. These findings reveal an immune-to-brain communication loop in which fat tissue and innate immunity coordinate to adapt behavior and improve survival during infection.