According to Pearson Higher Education, the alanine cycle, or glucose-alanine cycle, is a process by which muscles can cycle the by-products of glucose breakdown to the liver to be turned back into glucose or flushed out of the body. This metabolic cycle occurs in the muscles, and it aids muscle cells that are using energy during periods of high activity, but do not have enough oxygen to complete aerobic respiration.
Wikipedia explains that muscle cells typically use glucose to make a molecule called ATP, which is the main type of energy used by cells. In order to do this, the muscle cells need to break down glucose in a process called glycolysis. Glycolysis produces ATP and another molecule called pyruvate. Pyruvate is combined with ammonia by the cell to make alanine, which is an amino acid that can be transported through the blood to the liver.
Once alanine reaches the liver, the liver cells can remove the ammonia from the alanine to make pyruvate again, and turn the pyruvate back into glucose, which is transported back to the muscle cells so that they can break them down for more energy. It is important for the alanine cycle to function because otherwise poisonous ammonia builds up in the muscle cells and the glucose available for energy is rapidly depleted.