The rate-limiting step to making carnosine
Carnosine is a dipeptide stored inside muscle, formed by joining beta-alanine and histidine. Of the two building blocks, beta-alanine is normally in shorter supply, so it is the rate-limiting precursor: how much carnosine the muscle can build is governed largely by how much beta-alanine is available. Supplementing beta-alanine therefore raises muscle carnosine, whereas eating extra histidine alone does little.
This is the core reason beta-alanine, rather than carnosine itself, is the supplement of choice, since orally ingested carnosine is largely broken down before reaching muscle.
Carnosine as an intramuscular acid buffer
During high-intensity effort, hydrogen ions accumulate and acidity rises, which contributes to fatigue. Carnosine helps buffer those hydrogen ions inside the muscle, and higher carnosine content is associated with better performance in efforts lasting roughly one to several minutes. The benefit is specific to that high-intensity window rather than to maximal single efforts or long endurance.
This is general educational information, not medical advice. Consult a qualified professional before starting a supplement regimen.