It’s no secret that running is exhausting, but an early-stage research paper is exploring just how much it impacts the body. According to an article by RunnersConnect, prolonged endurance running, such as marathons, may deplete the body’s carbohydrate stores, leading it to burn fat for energy. A preprint research paper recently submitted to bioRxiv suggested that the body turns somewhere unexpected to find its fat cells: the brain. The preprint reports found that brain tissue — specifically myelin — may be used as fuel in marathon running.