The intestinal flora affects the desire to run

Two people run through the Retiro park in Madrid. SAMUEL SÁNCHEZ

Every so often, science discovers a new connection between the bacteria that humans carry in our intestines and the rest of our bodies.

In addition to being essential for the body to assimilate nutrients, it is already known that they are related to general health.

Some of these microorganisms appear to be associated with depression, and other studies have linked greater bacterial diversity to better mental health.

Now, researchers in the United States have just discovered a connection between the gut microbiome and physical performance, specifically, the desire to exercise.

The work has been carried out in mice, but, if it could be replicated in people, they would have found an explanation for laziness.

The initial objective of the scientists from the ThaissLab, from the Perelman School of Medicine of the University of Pennsylvania (United States), was another: they wanted to confirm that intestinal bacteria influence performance during physical activity, as several recent works point out. (with mice and also with elite athletes).

On the other hand, in recent years much progress has been made in deciphering the scientific basis of the

that runners and many other athletes claim to have when they exercise.

Do bacteria have anything to do with this emotional high?

Furthermore, does having a certain microbiome encourage more going out for a run?

To answer these questions, the team led by Dr. Christoph Thaiss, a professor of microbiology at the University of Pennsylvania, gathered about 200 mice from different lineages.

They had them run, spinning on a wheel or walking on a treadmill, noting that they exhibited great diversity in their performance.

When classifying them by their genetic heritage, they found no relationship between sports performance and genes.

So they studied other possible causes: the metabolome (set of metabolites generated by the organism), its metabolism or its intestinal microbiome.

The first two did not show a correlation with the degree of physical activity, but intestinal bacteria did.

“What we discovered is a pathway that connects the gut microbiome with an area of ​​the brain that is essential for generating motivation”

Christoph Thaiss, University of Pennsylvania

The role of the gut microbiome was discovered with a radical strategy: administer a cocktail of antibiotics and see what happens.

As is known, the treatment of bacterial infections with some antibiotic has the side effect of destroying the intestinal flora.

What they observed is that the medicated mice performed much worse, up to 50% less than before and compared to their peers with whom they shared the profile.

They saw that many of them did not even approach the treadmill or get on the wheel.

Thaiss summarizes the results of his work, just published in

, the showcase of the best science: "What we discovered is a pathway that connects the gut microbiome with an area of ​​the brain called the striatum, which is essential for generating motivation."

And he details it: “The microbiome produces specific metabolites that are detected by the neurons that innervate the intestine.

These neurons are activated during exercise and molecules derived from the microbiome enhance this activation.

Sensory neurons then transmit a signal to the brain, which results in elevated dopamine levels in the striatum.

Dopamine, in turn, is the main neurotransmitter involved in generating the motivation to exercise.

This would be, for now in mice, the circuit that connects bacteria with the desire to exercise.

One of the mice used in the experiments. Susanna Kircher and Katharina Thum (Thaiss lab)

The release of dopamine, which is part of the neural reward system, during and after exercise was already known.

What is new is its role in motivation and its connection to bacteria.

“The microbiome regulates dopamine levels in the brain,” Thaiss reiterates.

“When you exercise, its levels rise in the brain, creating a feeling of pleasure and reward, often known as a

's high.

However, in the absence of a microbiome, this dopamine elevation is attenuated and therefore the rewarding feeling of exercise is reduced.”

And this is what they saw with those treated with antibiotics, who lost motivation.

To take advantage of the fact that the treated mice recovered their microbiota and activity levels within a few days of treatment, the researchers wanted to identify which bacteria were best connected to the activation of dopamine production and release.

In their detective work, they played with the antibiotic cocktail, starting with broad-spectrum compounds and gradually narrowing them down.

Thus, they were able to identify two bacteria,

and

, two habitual commensals of the digestive system.

The disappearance of these two microorganisms was coupled with less physical activity.

Both bacteria produce metabolites known as fatty acid amides.

These metabolites are the ones that stimulate the receptors of the neurons that reach the intestines.

Specifically, they activate endocannabinoid receptors (endogenous homologues of cannabis compounds) that complete the circuit, causing an increase in dopamine levels in the striatum of the brain.

And as Thaiss said before, this is where a good part of the motivation comes from.

“If replicated in humans, it could generate more feasible methods to modify motivation and mood in other areas, such as addiction and depression.

Nicholas Betley, University of Pennsylvania

University of Pennsylvania biology professor Nicholas Betley, a co-author of the study, commented in a press release that "this gut-to-brain motivational pathway may have evolved to connect nutrient availability and the status of gut bacteria to the willingness to engage in prolonged physical activity.

For Betley, "this line of research could become a completely new branch of exercise physiology."

Going further, the study authors believe that if a similar pathway exists between the brain and human gut bacteria, it could offer diet-based mechanisms to make sluggards run or improve performance in elite athletes.

Also, Betley ends,

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