Cysteine

Cysteine is a sulphur-containing amino acid that is not strictly essential, because the body can manufacture it from [methionine](/glossary/methionine), another sulphur amino acid. For that reason nutritionists usually assess the two together, expressed as methionine plus cystine in a food's amino-acid profile. Dietary cysteine has a sparing effect: by covering part of the sulphur requirement, it reduces how much methionine must be diverted to make it. Cysteine earns its keep structurally, since it is a major component of keratin, the protein that builds hair and claws, which ties its supply directly to coat and nail quality. It is also the precursor of glutathione, one of the body's most important cellular antioxidants, and in species able to make taurine it feeds into that pathway too. An adequate supply of sulphur amino acids therefore underpins skin, coat and antioxidant defence all at once. A shortfall can blunt growth and dull the coat. The best sources are animal proteins such as egg, meat and fish, with some plant proteins contributing as well. As one of the building blocks behind [taurine](/glossary/taurine) and a partner to the [essential amino acids](/glossary/essential-amino-acids), cysteine is part of why animal protein suits dogs and cats so well. See the [Petipedia glossary](/glossary).

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