Mammals do have a reserve source of amino acids that they can call up on during periods of metabolic stress. Cellular proteins such as muscle protein are hydrolyzed to provide amino acids. Some of the released amino acids are degraded to provide energy, while others are used as building blocks to replace indispensable proteins. However, such self-cannibalization exacts a steep metabolic price and is only a stopgap method of survival (see the Clinical Application-Kwashiorkor).