A nonessential AA, alanine is produced in large amounts within skeletal muscle during exercise. Much of the amino nitrogen released from BCAA metabolism in muscle is utilized to produce alanine via transamination of pyruvate. During states of stress such as aerobic exercise, there is a net flux of alanine from muscle tissue into the blood for transfer of nitrogen from skeletal muscle to the liver. In the liver, alanine is deaminated back to pyruvate and ammonia. Pyruvate can enter liver gluconeogenesis for the production of glucose, and the fate of ammonia is mainly in the formation of urea for excretion via the kidneys. This process, often called the glucose-alanine cycle, allows the skeletal muscles to eliminate ammonia via a gluconeogenic precursor that the liver can return to the circulation as glucose to support continued muscular contractions.11 To keep up with glucose demand in times of physiological stress such as exercise, alanine can become vital in helping to maintain blood glucose via gluconeogenesis in the liver. Ultimately, this helps maintain glucose delivery to muscle and to decrease skeletal muscle catabolism.8 Alanine production in skeletal muscle is linear with exercise intensity.8 Alanine and glutamine combined account for approximately 68% of the AAs released from skeletal muscle during exercise and rest.12 Pitkanen et al. showed that alanine levels in muscle tissue decreased 30% at rest after resistance training13 and increased approximately 27% in plasma.14 A loss of alanine and other AAs from muscle during intense exercise, or due to a prolonged energy deficit, represents a temporary state of net muscle protein catabolism.15
As to its involvement in sports, alanine has generally not been studied or used alone as an ergogenic substance. However, its role in energy metabolism and cell swelling (cellular hydration) has led to its incorporation in different dietary supplement mixtures for athletes attempting to maximize PS, minimize skeletal muscle protein catabolism, or improve performance (see Section 15.7.3).
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