Cystine
Also known as L-cystine
Cystine is simply two cysteine molecules joined by a sulfur bond, the stable form cysteine takes in many proteins. It gives strength to keratin in hair, skin, and nails and helps maintain the shape of many proteins.
Why it matters
What it does in your body
- Forms the disulfide bridges that hold proteins in shape
- Structural strength in keratin (hair, skin, nails)
- A storage and transport form of cysteine
- Supports glutathione and antioxidant systems
Intake guidance
How much you need
There is no separate requirement. Cystine and cysteine are interchangeable in the body and are covered by the sulfur amino acids, which the body makes from methionine. Dietary protein supplies both.
What affects it
What affects your intake
Relationship to cysteine
Cystine is the oxidized pair of two cysteines, and the body converts freely between the two forms.
Keratin demand
Hair and nails are rich in cystine bonds, which is why sulfur amino acids are tied to hair strength.
Diet source
The same high-protein foods that provide cysteine provide cystine.
In practice
Good to know
Good to know: cystine is essentially cysteine in its paired form, so the two share sources, roles, and status. Its disulfide bonds are what make hair and nails strong, which is why sulfur amino acids appear in hair and nail supplements. A rare genetic condition called cystinuria causes cystine kidney stones, but that is a medical issue, not a dietary one.
Where to get it
Best food sources
The bottom line
Cystine is the paired form of cysteine that strengthens hair and nails, and protein foods supply both. There is no dietary requirement to track separately.
Track cystine automatically
Cystine 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.