Cysteine

Cysteine is a sulfur-containing amino acid found in meat and eggs. FDA lists cysteine (L-form) as GRAS for animal feed under good manufacturing or feeding practice in 21 CFR 582.5271. Cats get most of their cysteine from the animal protein in their food. If your cat has bladder stones or another medical condition, check with your veterinarian about its diet.

Source: govinfo.gov

Cysteine is added to round out the sulfur amino acids in a recipe. Hair is mostly keratin, and keratin is high in cysteine and cystine, so cats use a lot of these amino acids for their coat. The body also uses cysteine to make glutathione, an antioxidant inside cells. Adding a small amount lets a maker reach its target without more meat.

Cystine is two cysteine molecules joined by a sulfur bond. The body can switch between the two forms, so on a cat food label they do the same nutritional job. AAFCO counts cystine together with methionine in one sulfur amino acid total. The spelling differs by one letter, and labels sometimes use either name for what is close to the same thing.

Source: aafco.org

Cats need sulfur amino acids, and cysteine is one of them. AAFCO cat food nutrient profiles set a combined methionine plus cystine minimum of 0.40% of dry matter for adult maintenance and 1.10% for growth and reproduction. Most complete foods reach that from animal protein, so added cysteine is a small top-up rather than the main source.

Source: aafco.org

Pettings gives cysteine 4 out of 5 and lists it as an amino acid. It appears on 1 of the 149 dry cat foods we list. It is uncommon as a named ingredient because meat already supplies it. Each food's page shows how it scores overall.