L-Cysteine

L-cysteine is the natural form of cysteine, a sulfur amino acid found in meat and eggs. FDA lists cysteine (L-forms) as GRAS for animal feed under good manufacturing or feeding practice (21 CFR 582.5271). It goes into fur and into the antioxidant glutathione. For a cat with bladder stones or another medical condition, ask your veterinarian about its diet as a whole.

Source: govinfo.gov

L-cysteine is added to help a recipe meet a cat's sulfur amino acid needs. Cats use these amino acids heavily, because fur is mostly keratin and keratin is high in cysteine. Adding a little L-cysteine lets a maker balance a formula without raising total protein.

The L names the mirror-image form of the amino acid that is found in proteins. Amino acids can exist in L and D forms, which have the same atoms in opposite shapes. L-cysteine is the form the cat's body uses. FDA's GRAS listing for cysteine in animal feed covers the L form. Pettings has separate pages for cysteine and L-cysteine because labels use both names.

Source: govinfo.gov

Cysteine is one of the three amino acids the body uses to build glutathione, an antioxidant inside cells. A cat eating a complete food already gets cysteine from meat, and published proof that extra L-cysteine in food raises glutathione in cats is limited.

Pettings gives L-cysteine 4 out of 5 and lists it as an amino acid. It appears on 2 of the 149 dry cat foods we list. It scores the same as cysteine and cystine. Each food's page shows how it scores overall.