Protein sparing (amino acid sparing) is the process by which the body derives energy from sources other than protein. Such sources can include fatty tissues, dietary fats and carbohydrates. Protein sparing conserves muscle tissue. The balance between digestible protein (DP) and digestible energy (DE) in the diet is a key factor. Decreasing dietary DP/DE ratio results in an increase of protein conservation. The amino acids are not catabolized for energy, and are conserved in the body in a greater ratio.
The amount of protein used in the body is influenced by the percentage that is digestible by the body, and the total quantity of protein fed to the body. Bodybuilding and other strength training promotes the utilization and conservation of protein's amino acids in the body. Using alternate energy sources lessens the amount of amino acids that will be metabolized for energy. An increase of protein in the diet does not lead to greater protein efficiency, more protein will be lost, but a greater amount of protein will be conserved in the body through sheer volume, staying a step ahead of the metabolization of amino acids for energy.
This supposedly happens roughly two days after a water fast, according to fasting proponents.