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Dietary carbohydrates are all broken down to glucose in the blood. The body then takes it up and stores it as glycogen (glycogenesis). When you start to work out, and your body needs energy, it breaks down glycogen to form glucose (glycogenolysis) and then further breaks the glucose down into pyruvate (glycolysis) if the activity is intense enough to provide your cells with ATP. If the exercise is reasonably moderate and long duration, pyruvate is converted into acetyl-CoA (and joined to oxaloacetate, a by-product of carbohydrate metabolism) and shuttled into the Krebs cycle inside the mitochondria to produce a mass amount of ATP for endurance exercise. The last pathway for the body to get ATP and glucose is through gluconeogenesis—basically, a backup system for supplying glucose to the brain, in which the liver converts stored glycogen into glucose, or converts fats and protein into glucose. This is what typically happens during low-carbohydrate diets. If there are times when gluconeogenesis can’t happen, the liver will create ketone bodies.