A sulphur-containing amino acid that is a major component of keratin.
Why it’s used: One of the building blocks of hair and nails.
L-cystine is formed when two molecules of the sulphur-containing amino acid cysteine link together. The body can make cysteine from methionine, so it is not classed as essential, and both forms are found in protein-rich foods such as meat, eggs, dairy, legumes and whole grains.
Cystine is a major component of keratin, the tough protein that makes up hair and nails. The sulphur in cysteine forms disulphide bonds between keratin chains, and these bonds give hair its strength and rigidity. That is why cystine is described as one of the building blocks of hair and nails, and why it appears in many hair and nail formulas.
On labels you may see L-cystine or L-cysteine. They are related but not interchangeable, so an amount of one does not translate directly into the same amount of the other. Researchers have studied cysteine-containing blends in small hair studies, but a 2018 review noted that human clinical evidence is limited, and no health claims are authorised for cystine. It is often combined with methionine, biotin and zinc, with amounts listed in milligrams (mg).
3 findings behind this ingredient. Hover a card for the detail and the source.
Double-blind, placebo-controlled trial in 80 adults aged 18 to 60 with chronic telogen effluvium or androgenetic alopecia, one capsule daily for 6 months. Hair density rose by 12.3 hairs/cm² at 6 months versus placebo.
Pilot study (36 patients) then a double-blind trial (47 patients) of 70 mg L-cystine, 7,000 mg gelatin and 18,000 IU retinol a day. The telogen rate fell 13.5% and anagen rate rose 8%, with no change on placebo.
Open-label, multicentre study in 112 adults with light diffuse hair loss taking two capsules a day for 12 weeks. Hair density and anagen hairs increased and hair loss fell by 59% from baseline; there was no placebo group.
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Cysteine is a sulphur-containing amino acid. Cystine is two cysteine molecules joined together. The body converts between them, but they are not interchangeable on labels, so an amount of one does not equal the same amount of the other.
Cystine is a building block of keratin, the protein in hair and nails. Researchers have studied cysteine-containing blends in small trials, but human evidence is limited and no health claims are authorised. It is best seen as one part of a balanced diet and routine.
Cystine and cysteine are found in protein-rich foods such as meat, eggs, dairy, legumes and whole grains. The body can also make cysteine from methionine, another sulphur amino acid, so a varied diet with enough protein usually supplies plenty.
Information only, not medical advice. If you take medication, are pregnant or breastfeeding, speak to your GP or pharmacist first.
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