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When mice are given the opportunity to exercise, they (like humans) vary considerably in their enthusiasm for doing so. A team at the University of Pennsylvania found that some “active” mice will run nearly 20 miles on a wheel in a 48-hour window, whereas other “lazy” mice barely will use the wheel.
The investigators found no genetic differences that explained the very different behavior of active and lazy mice. Instead, the behavioral differences were caused by differences in the gut microbiome. Active mice had an abundance of bacterial species that produced particular types of fatty acids. These fatty acids set in motion a series of signals that led to increased dopamine in the brain's reward centers when the animals exercised. The resulti…