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According to a study published in Nature Medicine, a gut bacteria called Veillonella was found in abundance in athletic people but was almost completely absent in the guts of sedentary people.
Why? Turns out that Veillonella metabolizes lactic acid as its primary food source, converting it to propionate, a short-chain fatty acid that is used to improve exercise capacity. Since athletes are continually producing lactic acid during exercise — something the microbe favors — the microbe in turn creates something that benefits the host. Yay symbiosis!