Although the new studies involved animals, they amplify and extend recent human research showing that the microbiomes of active people can be very different from those of people who rarely exercise. The findings raise the possibility that, depending on how we change our internal microbes, we might end up enjoying — or avoiding — more exercise.
Gut Health Affects Overall Health
The importance of the microbiome to our health is almost indisputable at this point. The state of our intestines has been shown to affect our heart, waistline, sleep, mood, and risk of various diseases, including diabetes and certain types of cancer.
Microbiomes are also quite malleable. The number and types of our gut bugs go up and down, depending on how we live, including if and how we exercise. A study last year found that the guts of elite cross-country skiers had fewer types of bacteria after a grueling season than the microbiomes of healthy non-athletes, but more of the remaining strains found in athletes were associated with metabolic health.
While we know that exercise can influence microbiome health, it’s much less clear if the effects work the other way around, and your microbiome can influence your exercise. Anecdotally, people and lab mice on antibiotics don’t get much exercise, but the reason could be that they probably felt bad before starting the drugs, discouraging physical activity.
If you give an antibiotic to a mouse. . .
So scientists at the University of California Riverside decided to eliminate the microbiomes of mice that like to run and see how they would react. The UC Riverside mice were part of an ongoing experiment in which female mice, which ran more than their lab mates, were mated with male mice of the same confession. Over several generations, scientists have developed a strain of super-runner mice. These animals voluntarily ran on wheels about three times more than other mice. They also evolved different microbiomes.
Now, for a study published last year in Behavioral Processes, scientists gave some of these marathon mice broad-spectrum antibiotics to kill most of their gut bacteria. The animals continued to eat and otherwise live unchanged, indicating that the antibiotics had not made them sick.
But when the researchers gave the animals free access to the running wheels, the marathon running mice’s running mileage dropped sharply. They walked an average of about 21% less distance each day, numbers that barely rebounded over the next 12 days of the study.
The new experiment underscored “how much” the microbiome seemed to influence sporting animals’ willingness to run, said Theodore Garland, an evolutionary biologist at UCR, whose lab developed the marathon mice and who oversaw the new study.
How the Gut Talks to the Brain
But Garland’s study did not examine how microbiomes affect exercise motivation. For that, we need the results of another ambitious study, published in December in Nature, which looked deep into the guts and brains of mice.
In the study, scientists from the University of Pennsylvania’s Perelman School of Medicine and other institutions gathered eight types of mice, which naturally traveled different distances daily, and began genotyping them and checking thousands of molecules released in their intestines, blood and brain. before and during exercise.
They also gave some of the mice antibiotics, bred others to have no microbiomes from birth, let them run or sit, collected more tissue, transplanted samples from feces of mice happily running with robust microbiomes into mice without gut bugs – and vice versa – and kept gathering tons of genetic and tissue samples.
In the end, they had over 2.1 million data points on their mice.
What they discovered in this data was a direct communication link between the gut and the brain, activated by exercise. This communication began, they found, with the release of certain molecules in the gut during exercise, which then stimulated specialized nerves connecting the gut and the midbrain.
When activated, these nerves sent signals causing the brain to release additional dopamine, the neurochemical most associated with rewards and motivation. The scientists also found that the higher the amounts of several specific strains of bacteria in the animals’ intestines, the more dopamine their brains produced and the faster the mice ran.
Can our microbiomes affect motivation?
Indeed, the animals created a runner’s high, originating in their microbiome, that encouraged them to exercise, said Christoph Thaiss, assistant professor of microbiology at Penn Medicine and lead author of the study.
When scientists interrupted any step of the communication link, killing gut bacteria with antibiotics or chemically blocking nerve activity or dopamine release, affected animals began to run less.
“They lost interest” in exercise, Thaiss said.
He and his co-authors believe that the involvement of the gut in physical behavior may have evolved long ago in response to the availability of food. Animals that roamed and explored often – that exercised regularly – found more food, survived and reproduced, passing along the gut-brain connections that encourage exercise.
What this research means to us is still somewhat theoretical. We are not mice, although we share most of the same nervous and midbrain processes. So “it’s entirely plausible” that our motivation to exercise — or not — depends, in part, on the makeup of our gut bacteria, Garland said.
Can we boost our interest in exercise by taking probiotics or changing our diet or changing our gut bugs? “Eventually, probably,” Thaiss said. But scientists must first identify the specific types of gut bacteria implicated in exercise motivation in people, assuming they exist, he said, which could take years.
In the meantime, mouse studies suggest that exercise might be his best motivator. When we work out, we remake our microbiome, and our renovated gut can then, it seems, reinforce our desire to exercise.
But to benefit from it, the first step is to take a few first steps.
Do you have a fitness question? E-mail YourMove@washpost.com and we may answer your question in a future column.