Alanine glucose cycle

See also: Beta-alanine

During physical exercise, the glucose-alanine cycle represents a very important metabolic pathway, which allows the liver to obtain glucose starting from an amino acid, alanine, coming from the active muscle.

Intense and prolonged physical exertion leads to depletion of blood glucose levels and an increase in the blood concentration of lactic acid. The muscle is thus forced to increase the oxidation of fatty acids and amino acids for energy purposes, in particular those with branched chain (BCAA). The carbon skeleton of the latter is used in the muscles to produce energy through the krebs cycle. while the amino group is first transferred to glutamate and then to pyruvate, resulting in the formation of alanine. This amino acid is then released into the blood and conveyed to the liver, which, in turn, removes the amino group and uses the pyruvate thus obtained to form glucose, according to a process called gluconeogenesis.The newly formed glucose is then put back into circulation, with the aim of ensuring a constant supply of sugar to the brain.

In turn, the muscle can capture blood glucose and metabolize it for energy; at the end of glycolysis two pyruvate molecules are thus obtained, which can enter the krebs cycle or be used to synthesize as many lactic acid (in anaerobic conditions) or alanine. At this point the cycle can begin again.

The amino acid alanine, therefore, in addition to being a normal constituent of proteins, acts as a transporter of nitrogen from the peripheral tissues to the liver. At this level, in fact, the amino group, which represents the toxic molecule of amino acids, can enter the cycle of "urea and be eliminated in the urine without creating too much damage to the body.

In skeletal muscle the synthesis of alanine is directly proportional to the intracellular concentration of pyruvate, which increases, for example, when there is a "high degradation of fatty acids for energy purposes, with consequent slowdown of the krebs cycle and the formation of ketone bodies."

Similarly in anaerobic conditions: pyruvate, not being able to be oxidized in the krebs cycle, is converted partly to alanine and partly to lactic acid. The latter is released into the circulation together with alanine and, similarly to it, transported to the liver, where it is used as a gluconeogenetic precursor (Cori cycle).

For all these reasons, the glucose-alanine cycle and the Cori cycle, although also occurring in conditions of rest, are activated in a particular way during intense muscular exercise.

The glucose-alanine cycle is also stimulated by the "increase in plasma levels of glucocorticoids (cortisol) in response to a stressful event of physical origin (fasting, illness, surgery, intense effort) or psychic (performance anxiety, etc.).