In what case is the energy used by a person in the process of life released in the cells?

Energy is needed for every cell constantly. Due to the presence of energetic substances, the cell can live and perform its functions. Various substances, including carbohydrates, enter human cells. They are obtained by breaking down food. But if a person does not eat, works hard, or so on, then the cells will not die from lack of energy, since they have an emergency supply of various substances in case of unforeseen situations. Of course, we will feel hunger, fatigue, as the body will begin to signal a lack of carbohydrates.