Why is ingesting carnosine directly ineffective?

Ingested carnosine breaks down in the body into beta-alanine and histidine on absorption from the gut and only micro amounts of beta-alanine are found in the blood stream. Research has shown that enough histidine is naturally present in the body to meet the demands of muscles for synthesis of carnosine, and that the limiting factor is beta-alanine. Beta-alanine supplementation augments the small amounts available from (1) synthesis in the liver, and (2) from the ingestion of meat and fish. Supplemental beta-alanine combines with the naturally occurring histidine to increase the levels of carnosine.