PNAS: The gut microbiota determines what you want to eat by influencing the brain.

Written by | Wang Cong

Edit | Wang Duoyu

Typography | Water into the text

Humans, like other mammals, are occupied by trillions of microbes, including bacteria, viruses and fungi, collectively known as the commensal flora
.
In a sense, "man" is a multifaceted complex of the human body and the symbiotic flora
.

There are a large number of microorganisms parasitic in the human intestine, and these intestinal microbiota affect human obesity, enteritis, autoimmune diseases, response to cancer treatment drugs, and even affect human lifespan
.
A growing body of evidence reveals the extent of interdependence between humans and these gut microbiotas, and also underscores the importance of
the brain-gut axis.

We often have a thought in our minds, such as suddenly wanting to eat meat, suddenly wanting to drink milk tea, and so on
.
Even where you come from, scientists have been speculating that the gut microbiome affects a person's dietary choices and preferences, but this speculation has not been studied
in depth.


On April 19, 2022, researchers at the University of Pittsburgh published a research paper
titled "The gut microbiome influences host diet selection behavior" in the Proceedings of the National Academy of Sciences (PNAS).

The study found that gut microbes influence the host's choice
of food by producing substances that are craving different types of foods.
That is, your dietary choices may not be entirely up to you, and the gut microbiome is also involved in the choice
.


In this study, the team selected three different mice, the locust eating rat (O.
torridus), white-footed rat (P.
torridus), leucopus), mountain voles (M.
montanus), their eating habits are very different, namely carnivorous, omnivorous, and herbivorous
.

Their gut microbes were then transplanted separately to 10 mice lacking gut microbes
.
The experimental results showed that after the gut microbiome transplantation, the dietary preferences of these mice changed
.

Specifically, mice transplanted with herbivorous gut microbes chose foods with a higher protein: carbohydrate ratio, while mice transplanted with omnivorous and carnivorous gut microbes chose foods
with a lower protein: carbohydrate ratio.


The gut microbiome's influence on host behavior may sound far-fetched, but it's not surprising either
.
There is a constant "dialogue" between the gut and the brain, such as some metabolites produced during the diet that will make you feel full and stop eating
.
And gut microbes can also produce the same metabolites, so they may also "talk"
to the brain.


On April 15, 2022, a research team from the Institut Pasteur in France published a paper in the journal Science showing that hypothalamic neurons can directly sense changes in intestinal bacterial activity and adjust appetite and body temperature
accordingly.
This suggests that a direct dialogue
has occurred between the gut microbiota and the brain.


In the PNAS paper, the research team pointed out that after eating turkey or drinking milk, there will be a drowsy feeling because they are rich in tryptophan, an essential amino acid that enters the brain and is converted into serotonin, which makes people feel full, and eventually converts to melatonin, which makes people sleepy and promotes sleep
.

The study found that mice with different gut microbiomes had different levels of tryptophan in their blood, and tryptophan levels were not the same even before they were given the freedom to choose their food
.
Mice with higher levels of tryptophan in their blood also had more tryptophan-producing bacteria in their guts
.

Specifically, the gut microbes of herbivores produce more tryptophan, which causes the host's brain to produce more serotonin, limiting the host's eating and reducing carbohydrate intake
.

Brian K.
Trevelline, the paper's corresponding author, said the study provides convincing conclusive evidence that gut microbes can regulate host dietary choices by producing tryptophan, providing a rigorous experimental validation of the theoretical conjecture that microbes alter dietary choices
.
But he also pointed out that there may be dozens of signals that affect dietary choices, and tryptophan produced by microbes is just one of them
.
For example, in humans, what you ate the day before is important to what you choose to eat today, and the degree of influence may be greater than the effect of
the gut microbiome.

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