Cysteine
Also known as L-cysteine
Cysteine is a conditionally essential, sulfur-containing amino acid the body makes from methionine. It is the key ingredient in glutathione, the body's master antioxidant, and its sulfur bonds give shape to proteins like keratin in hair and nails.
Why it matters
What it does in your body
- Building block of glutathione, the master antioxidant
- Forms disulfide bonds that shape proteins
- Component of keratin in hair, skin, and nails
- Sulfur source for the body
Intake guidance
How much you need
There is no separate requirement, because the body makes cysteine from the essential amino acid methionine, and the two are counted together as the sulfur amino acids. Dietary cysteine spares some methionine, and it becomes conditionally essential in premature infants and certain illnesses.
What affects it
What affects your intake
Methionine supply
Cysteine is made from methionine, so adequate methionine covers cysteine for most people.
Glutathione demand
Oxidative stress and illness raise the need for cysteine to keep making glutathione.
Diet source
Meat, eggs, dairy, and alliums like garlic and onions are good sources.
In practice
Good to know
Good to know: the supplement form N-acetylcysteine (NAC) is used medically to replenish glutathione, thin mucus, and treat acetaminophen overdose, and it is studied for a range of conditions. For everyday health, protein foods supply cysteine along with the methionine it comes from, and deficiency is not a concern for people eating enough protein.
Where to get it
Best food sources
The bottom line
Cysteine builds the body's master antioxidant and is made from methionine, so protein-eaters get enough. Its supplement form NAC has specific medical uses.
Track cysteine automatically
Cysteine is one of the 92 nutrients Vora reads from a photo of your meal, so you can see where you stand without weighing or logging by hand.
Sources and method
The classification, roles, and guidance on this page come from the references below, drawing on the Institute of Medicine (now the National Academies) Dietary Reference Intakes and reputable nutrition science.
- Institute of Medicine (2005), Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids
- National Library of Medicine, MedlinePlus
This page is for general education, not medical advice. Reference intakes are population averages for healthy people and do not replace guidance from your doctor or a registered dietitian, especially if you are pregnant, managing a health condition, or considering supplements.