Cystine

Cystine is the paired form of the amino acid cysteine, found naturally in proteins such as meat and hair. FDA lists cystine (L- and DL-forms) as GRAS for animal feed under good manufacturing or feeding practice in 21 CFR 582.5273. It is added in small amounts. If your cat has bladder stones or another medical condition, its veterinarian should guide the diet.

Source: govinfo.gov

Cystine is added to meet the cat's need for sulfur amino acids. These amino acids go into keratin, the protein that makes up fur and claws, and into glutathione, an antioxidant the body makes. A recipe that uses some plant protein may run low on them, and a small dose of cystine closes that gap.

AAFCO groups them because some or all of the need for cystine can be met from extra methionine, though the reverse is not true. Its cat food nutrient profiles list methionine alone and a separate methionine plus cystine total, with an adult maintenance minimum of 0.40% of dry matter for the combined figure and 0.20% for methionine alone. Extra cystine cannot make up for missing methionine.

Source: aafco.org

Cystine is one of the building blocks of keratin, the protein in fur, so a cat short on sulfur amino acids can grow a poorer coat. A cat already eating a complete food that meets AAFCO profiles gets enough for normal hair. Extra cystine beyond that has not been shown to make a coat shinier. Coat changes can also come from illness, so a veterinarian should look at a dull coat.

Pettings gives cystine 4 out of 5 and lists it as an amino acid. It appears on 1 of the 149 dry cat foods we list. Pettings scores cystine and cysteine the same because they do the same job. Each food's page shows how it scores overall.