When exercising carbohydrates an fats are oxidized to release energy used by muscles, during this oxidation 1 liter of oxygen is equivalent to approximately 5 calories. Therefore if one liter of oxygen is consumed every minute during exercise, then in an hour 60 liters of oxygen will have been used. But since one liter of oxygen is equivalent to 5 calories, then in an hour 300 calories will have burned.
Water Cycle is the answer
