Final answer:
After 3 hours of exercise, the body primarily uses fatty acids and ketone bodies for energy. Muscle glycogen is used up quickly, followed by blood glucose and increased fat metabolism. If exercise continues for extended periods, protein can also be broken down for glucose.
Step-by-step explanation:
After 3 hours of exercise, the main sources of energy for the body are fatty acids and ketone bodies. When muscle glycogen stores are depleted, which happens within the first few minutes of high-intensity exercise, the body then transitions to using blood glucose and fatty acids as the primary sources of energy.
Glucose can still contribute to energy production through the bloodstream, particularly if it's been recently ingested through diet. If exercise continues for an extended period or at lower intensity, fatty acid metabolism increases, and the body begins to utilize these fats more predominantly. Eventually, if the body is subjected to starvation or continued long-duration exercise, it will start producing ketone bodies from fatty acids as an alternative energy source.
Proteins are usually conserved during exercise but can be broken down to glucose in situations of energy deficit or extended duration activities. In summary, following exhaustive exercises like those spanning 3 hours, fat stores and any circulating blood glucose become the principal sources of energy for the body, with ketone body production coming into play thereafter.