How does burning calories for energy work? If someone runs on an empty stomach and burns 1500 calories, where did that energy come from? When they eventually eat something, how does the body know what to do with those calories?

Question

How does burning calories for energy work? If someone runs on an empty stomach and burns 1500 calories, where did that energy come from? When they eventually eat something, how does the body know what to do with those calories?

In: Biology

Answer 1

The liver stores about 2000 calories worth of energy in the form of glycogen. Once you burn through that, your body starts to enter a state called ketosis, where is breaks down fats for energy. When you eat again, it depends on what you eat. Carbohydrates are used more or less directly to replenish the glycogen stores in the liver and to supple the muscles. Fats and proteins are used less directly, and often stored or used to repair tissues.