I think therefore I am, and I think I am satisfied after a good meal. But, according to new research, it may be the bacterial organisms living in my gut that decide when I am full.

When E. coli have had enough to eat, they send a signal that’s similar to a human hormone that tells our brains that we’ve had enough to eat.

Humans respond to their signals when we decide we are full, a new study published in the journal Cell Metabolism suggests.

Learn More: What Is the Human Microbiome, and Why Does It Matter? »

Bacteria’s Response to Food

French researchers, led by Sergueï Fetissov, Ph.D., a researcher at the Institute for Research and Innovation in Biomedicine at Rouen University, watched as E. coli living in rat guts responded after they introduced a nutritional liquid.

The researchers knew that E. coli, which make up about 1 percent of the bacteria in the average human’s gut, produce a protein called ClpB that’s similar to a human satiety hormone.

The researchers noted that the bacteria used sugar to multiply. After 20 minutes, their growth stabilized and they produced more of the protein ClpB.

It may also be that bacterial signals drive the release of the post-meal hormones GLP-1 and PYY. E. coli bacteria live in the colon. There, they come into close contact with the cells that produce the post-meal hormones.

The cells responded differently to the proteins E. Coli produced when they were actively proliferating and those they produced when their growth frenzy had stopped.

What’s more, when researchers influenced the rats’ bacterial parasites directly, without feeding the animals, the rodents ate less.

“This work is seminal in showing that colon bacteria participate in telling the brain to stop eating,” said Maria Gloria Dominguez-Bello, Ph.D., an associate professor in the Human Microbiome Program at New York University School of Medicine. She was not involved with the French study. “It opens questions as to how many other bacteria have this protein also signaling satiation.”

It could even begin to explain why some foods don’t reliably trigger people’s “enough” signal. In the study, the E. coli produced more of the proteins when they got a nutritional syrup containing protein than when they got pure sugar.

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Different Digestion

Humans also feel most full 20 minutes after eating, and feel more satisfied after eating protein or fat. Those similarities intrigue microbiome researchers.

“The study may also imply that foods that are highly digestible by human enzymes, and absorbed in the small intestine (such as processed foods), do not lead to satiation signals, while foods with components that bypass human digestion and feed colon bacteria (fibers) may increase satiety,” Dominguez-Bello said.

The research is too preliminary to answer open questions about the relationship between an individual’s gut bacteria populations and their weight, cholesterol, and blood sugar.

“Bacterial proteins produced after nutrient-induced growth can be one type of molecular effectors, as we showed in our study,” Fetissov said. But “we cannot exclude that other mediators might be involved.”

The researchers are also planning to study how other bacteria respond to what the host eats, and whether other bacterial proteins might be involved, Fetissov said.

Nevertheless, the study begins to sketch an enormous terra incognita in our current understanding of beneficial gut bacteria.

A number of studies have shown that obese and metabolically imbalanced people have different types of bacteria living in their digestive tracks than their leaner, healthier peers. But scientists don’t yet know how or why that happens.

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