To hack your motivation to exercise, you may need to adjust your gut microbiome
Exercise more. It’s usually my (and many others’) best New Year’s resolution.
But it’s drizzling with a bone-chilling wind howling outside. And I’m wrapped in a fuzzy blanket on the couch with a cup of hot cocoa and the latest Netflix show. My resolve is rapidly waning.
According to a new study in Nature, I could get a motivational boost from a surprising source: my gut microbes. In a turn-de-force study, a team from the University of Pennsylvania found that changing millions of gut bugs in your microbiome can get you off the couch and motivated to exercise—at least if you’re a mouse.
The results alone sound like pseudoscientific nonsense. But the study dug deep: the team looked at how and why gut microbes encourage mice to run and keep running. Cruxet is a chemical produced by the microbiome that sends a signal from the gut to the brain, triggering a flood of dopamine that is released into the ventral striatum – the brain’s “motivation center” – in turn sparking a desire to exercise.
I have said this often: mice are not men. But the study pushes the relatively new field of gut-brain interaction into new territory. Can the gut directly influence the brain’s motivations and desires? By hunting for the molecules in the gut that spur the brain on wishes to be physically active, the study gave us a first answer: yes.
“If these findings are relevant to humans, they raise the question of whether targeting gut bacteria can improve the mental processes involved in the decision to exercise across individuals, whether elite athlete or not,” said neuroscientists Drs. Gulistan Agirman and Elaine Y. Hsiao at the University of California, Los Angeles, who were not involved in the study.
The training dilemma
We all know exercise is good for us. Thousands of studies have shown that regular exercise helps with everything from weight management to reducing the risk of heart disease and boosting mental health and mood, and even fighting aging and dementia.
So why is it that despite knowing the benefits, it is still there so Hard to get motivated?
Mindset—that is, your psychology—was originally thought to be the main culprit, Agirman and Hsiao explained. But the new study suggests that the gut microbiome can also give you a powerful motivational boost.
The gut-brain connection is one of the most influential discoveries of the past decade. The brain does not exist in a vacuum. Rather, molecules and hormones from the body can significantly affect function. Chemicals released from the liver, for example, boost memory function in aging mice after exercise, giving birth to more new neurons in the dentate gyrus—the “nursery” of the hippocampus, a region critical for memory.
A major source of these systemic molecules is the gut microbiome. Its symbiotic microbes thrive inside our intestines, helping to digest nutrients and support metabolism. A decade ago, neuroscientists surprisingly found that they also affect the brain. Wiping out the bacteria with antibiotics, for example, increases depressive symptoms in mice. Subsequent studies found that certain microbes release chemicals when they digest food, which activates the vagus nerve, a major signaling pathway that runs from the gut to the brain.
They also help the body to respond to exercise. Specific groups of bacteria in the gut have emerged “as key regulators of exercise performance,” Agirman and Hsiao said. Usually this happens through chemicals secreted by microbes to generate energy, or those that help eliminate molecules that lead to physical exhaustion, such as lactate. The new study wondered: could the gut microbiome directly shape our desire to exercise by influencing brain function?
Mice generally love to run. But like humans, depending on their genetics and physiology, they have different inclinations – some like to run fast, others long, and some not at all.
To understand why, the team started with nearly 200 mice specifically bred to encourage a diverse genetic background and collected their body data. These included genetic sequencing, metabolic profiling and sequencing of the RNA in their stool – an established method for measuring a gut microbiome profile.
Overall, the team collected over 10,500 data points for each mouse and about two million in total.
The mice then ran on a treadmill or running wheel. The latter is a treat, as (anyone who has a hamster or other rodent knows) they happily jump on and run considerable distances every night – more than nine miles per day.
But there were also couch potatoes. These fluffballs were happy to relax, barely touching the wheel during a two-day test period.
Surprisingly, the mice’s genetic signatures had very little effect on their motivation to run. To widen the hunt, the team turned to machine learning to analyze molecules in their blood, their metabolism and their gut microbiomes to see if individual differences corresponded to running performance.
The answer raised eyebrows: the only factor that predicted a mouse’s willingness to run was its gut bacteria. That suggested that “gut bacteria drive exercise performance,” Agirman and Hsiao said.
But correlation is not causation. In the next tests, the team wiped out the microbiome of a group of athletic mice using antibiotics, turning them into couch potatoes. By contrast, mice raised in a germ-free bubble—which naturally lack gut bacteria—transformed into marathon runners when transplanted with intestinal defects from their naturally fit peers.
A brainy link
Why does the gut microbiome have anything to do with motivation?
The answer appears to be dopamine. Often called the “joy chemical,” dopamine has various roles in the brain, including flagging errors that don’t fit predictions and directing smooth movements. But its best-known role is to combine movement and reward, which happens in a deep lump of the brain called the ventral striatum, part of the brain’s “reward center.”
Digging into the mice’s microbiome data, the team found that athletic mice had a population of gut bugs that were particularly good at excreting fatty acid amides (FAA). These chemicals acted as “keys” and then activated a receptor “lock” – the CB1 receptor that pricks the outside of a specific type of sensory neuron inside the gut (yes, the gut has neurons, and yes, the CB1 receptor is also the target of marijuana’s main chemical components). These specialized neurons then send electrical signals directly through the spinal cord into the brain’s striatum, flooding it with dopamine.
In contrast, mice without gut bacteria did not have this dopamine peak. A little more investigation found that their brains had a high level of an enzyme that quickly chews up dopamine, essentially killing their “runner’s high.” But giving them a dose of FAA as a dietary supplement or transferring FAA-producing gut bacteria into their guts increased their running games.
The authors “have shown that the circuits involved in the motivation needed to maintain physical activity in mice are modulated by gut microbes,” Agirman and Hsiao said.
New Year’s resolution
To be clear, these results are in mice. We don’t know if they persist in humans. But they provide new clues to long-standing questions, such as why runner’s high feels good even when you’re in physical pain. I wouldn’t be surprised if the gut bug chemicals are bottled up in pre-workout motivational elixirs – but again, buyer beware!
Zooming out, the study adds to a growing pantheon of evidence that our microbiomes directly influence brain function, particularly for mood and motivation. But our gut does not control our desires.
“While it is tempting to consider the human implications of this research, gauging the practical relevance of these findings will require extensive further consideration,” Agirman and Hsiao said. “A variety of other factors influence motivational states in humans, requiring a variety of strategies to strengthen motivational and reward circuits in adverse environments.”
Image credit: Wokandapix from Pixabay