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One of the theories is women are better able to utilize fat stores for energy. When running, you have a limited amount of glycogen and virtually an unlimited amount of fat stores. When running, your body takes a percentage of glycogen and a percentage of fat for energy. For ultra's, you want that fat percentage to be as high as possible, so you don't run out of glycogen and bonk. The slower you run, the higher the fat percentage. Training at a slow pace helps your body learn to burn more fat and less glycogen. The theory goes that women on average are better able to burn fat for energy.