The mass that is neither incorporated into the animal’s body nor passed as waste is converted directly into energy. Even though the mass of a given food item decreases when it is eaten, the Law of Conservation of Mass-Energy remains unbroken. This is how animals derive energy from their food so that they can carry out all of their biological processes.
Animals have two different types of metabolism. Catabolism occurs when they break food items down into their constituent components. Some of these components, primarily carbohydrates and fats, are further degraded to release the energy contained in their molecular bonds. Other components, primarily proteins and amino acids, are diverted into another type of metabolic pathway called anabolism, or anabolic metabolism. Anabolism is the process by which animals build new tissue, which enables growth and repair.
Therefore, while an animal may eat 1 pound of food, and yet only produce 3/4 pound of waste, no mass or energy has been destroyed. Instead, a portion of the mass was broken down to release the energy that it contained, locked inside its chemical bonds. Different types of macromolecules contain differing amounts of energy, which is usually measured in calories or kilocalories (kilocalories is written as Calories). Fat contains the most energy of any macromolecule, totaling 3500 kilocalories per pound.