This is crazy:
Much of the gut's influence on the brain seems to be driven by the gut microbiome, the collection of (usually) beneficial bacteria and other microscopic organisms that reside in our digestive tracts. Introducing gut bacteria into germ-free mice has been shown to reduce anxious behaviors in the animals, and fecal transplants from humans with depression into rats ramped up depression and anxiety-like behaviors.
A study led by Thaiss that published in 2022 found that mice's gut microbiomes influence their motivation to exercise. The team found that in a diverse population of mice, those that were more inclined to run on their exercise wheels had different microbiomes than their sedentary brethren. When the researchers performed a microbiome swap via fecal transplant, the animals' exercise enthusiasm also switched. The scientists traced the difference to certain fatty acid metabolitess generated by the microbes in exercise-loving mice; these molecules in turn stimulate nerves in the gut that send signals to the brain to produce dopamine, a brain hormone associated with pleasure.
