The concept is pretty well accepted that energy is neither created nor destroyed. Energy is transformed, converted, transported, and stored. In the human body it is the same way. Our energy comes into our body as food that we eat. Everyone knows the measure of the energy is in calories.
Calories are the energy stored in food.
As we take it into the body, energy is either transformed or stored. As it goes through the system, some energy leaves the body as fecal energy. That energy can not be counted. Other energy is lost through the urinary system. What is not lost in these two ways is available for metabolism.