During an ultramarathon, the body primarily burns carbohydrates, which are the body's preferred source of quick energy. However, the body's carbohydrate stores (glycogen) are limited and can become depleted, especially during prolonged exercise such as an ultramarathon. Once the glycogen stores have been used up, the body switches to burning fat for energy. This is why ultra runners often consume high-fat foods and supplements during the race to maintain their energy levels.
