Mixed
During exercise, skeletal muscle synthesizes and releases alanine from glucose-derived pyruvate to transport amino groups to the liver, establishing a glucose-alanine cycle that supports hepatic gluconeogenesis without causing generalized hyperaminoacidemia.
During exercise, your muscles convert glucose byproducts (pyruvate) into alanine to send to your liver for energy production. This is a normal, healthy metabolic process called the glucose-alanine cycle. It does not mean your muscles are breaking down; it means they are efficiently recycling fuel. You do not need to intervene with extra protein to stop this; it is a natural regulatory mechanism.
The data suggest that (a) synthesis of alanine in muscle, presumably by transamination of glucose-derived pyruvate, is increased in exercise probably as a consequence of increased availability of pyruvate and amino groups; (b) circulating alanine serves an important carrier function in the transport of amino groups from peripheral muscle to the liver, particularly during exercise; (c) a glucose-alanine cycle exists whereby alanine, synthesized in muscle, is taken up by the liver and its glucose-derived carbon skeleton is reconverted to glucose.
Why this rating
Controlled human study with invasive catheterization measuring arterial-venous differences across specific beds (leg and splanchnic), providing high-fidelity metabolic data.
Source
Amino acid metabolism in exercising man
Philip Felig et al. · Journal of Clinical Investigation · 1971
DOI 10.1172/jci106771
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