Scientists discovered a hidden gut-brain network that can change cravings when the body needs more protein.
The body has a remarkable ability to recognize when it is missing important nutrients and adjust behavior accordingly. New research shows that the gut plays a much larger role in this process than previously understood, actively detecting protein shortages and communicating with the brain to influence food choices.
A team led by Director SUH Seong-Bae of the Center for Microbiome–Body–Brain Physiology at the Institute for Basic Science (IBS), working with researchers from Seoul National University and Ewha Womans University, has identified a previously unknown gut-brain communication system that helps animals seek out essential nutrients when protein is scarce.
Their findings reveal that the gut does far more than digest food. It continuously monitors nutritional status and can rapidly alter feeding behavior through a combination of nerve signals and hormones.
How the Gut Detects Protein Deficiency
Protein is a vital nutrient because it contains essential amino acids, which animals cannot produce on their own and must obtain through food. Scientists have long known that animals tend to crave protein-rich foods when protein is lacking, but the biological mechanism behind this behavior has remained unclear.
The researchers discovered that the gut responds to protein deficiency using two interconnected signaling pathways. One is a fast neural pathway that quickly alerts the brain when essential amino acids are in short supply. The other is a slower hormonal pathway that helps maintain protein-seeking behavior over a longer period.
To investigate how this system works, the team turned to fruit flies, which are widely used to study the neural circuits involved in feeding behavior. Combining neural imaging, behavioral testing, and genetic techniques, the researchers mapped the circuitry responsible for detecting and responding to protein shortages.
When flies were deprived of dietary protein, specialized cells in the intestine released a peptide hormone called CNMa. This molecule served two functions. First, it activated enteric neurons associated with the gut, rapidly transmitting information about amino acid deficiency directly to the brain through a gut-brain neural circuit. Second, CNMa entered the bloodstream and reached the brain more slowly, where it reinforced and prolonged the drive to seek essential amino acids.
“Our study shows that the gut is not simply a digestive organ, but an active sensory system that continuously monitors nutritional state and directly guides behavioral decisions,” said Director Seong-Bae Suh.
How Protein Shortages Change Food Preferences
The researchers found that the response was highly targeted rather than a simple increase in hunger.
Instead of causing animals to eat more of everything, the gut-brain system altered dietary priorities. Protein-related nutrients became more appealing, while interest in sugar declined.
Further investigation showed that CNMa signaling suppressed the activity of sugar-sensing brain cells known as DH44 neurons. By reducing responses to carbohydrates, the system effectively shifted feeding preferences toward foods that could provide the essential amino acids the body lacked.
Gut Bacteria Also Influence Cravings
The study also highlighted a role for the gut microbiome.
Fruit flies that lacked their normal gut bacteria displayed stronger activation of brain neurons involved in amino acid seeking. This finding suggests that gut microbes can influence feeding behavior by affecting nutrient availability and the signaling pathways that monitor nutritional status.
Evidence Found in Mammals
The researchers then examined whether similar mechanisms exist in mammals.
Experiments in mice showed that animals deprived of protein also developed a strong preference for essential amino acids, indicating that this nutrient-sensing system has been conserved through evolution.
One unexpected finding involved FGF21, a hormone widely believed to play a major role in regulating protein appetite. Even mice that lacked FGF21 continued to show the same protein-seeking behavior.
The result suggests that additional nutrient-sensing systems exist beyond those already known to science.
Implications for Obesity and Metabolic Health
The findings demonstrate that animals do not simply increase food intake when nutrients are missing. Instead, the brain can selectively adjust feeding priorities to target the specific nutrients that are lacking.
According to the researchers, understanding how the gut and brain work together to maintain nutritional balance could provide new insights into obesity, metabolic diseases, and eating disorders.
“Most current obesity and appetite-control drugs rely on gut hormone signaling, yet we still know relatively little about how naturally produced gut signals influence the brain and behavior,” said Director Seong-Bae Suh. “This study reveals fundamental principles of nutrient selection by the gut-brain axis and provides a foundation for future therapeutic strategies targeting metabolic and feeding disorders.”
The findings were published in the journal Science on May 21.
Reference: “Complex interplay of neuronal and hormonal gut-brain responses to essential amino acid deficit” by Boram Kim, Seongju Lee, Hyeyeon Bae, Shinhye Kim, Jong-Hoon Won, Dongwoo Kim, Byungkwon Jung, Makoto I. Kanai, Sung-Eun Yoon, Yangkyun Oh, Won-Jae Lee and Greg S. B. Suh, 21 May 2026, Science.
DOI: 10.1126/science.adv3355
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