Your hair is roughly 90% keratin — a tough, protective protein. And keratin is unusually rich in one amino acid: cystine. If keratin is the brick wall of your hair, cystine is the mortar that locks it together. So it makes sense that supplying this building block has been studied for hair strength and growth.
What L-cystine does
L-cystine is formed from two cysteine molecules linked by a “disulfide bond.” Those bonds are exactly what give hair its structural strength and resilience. Without enough of these sulfur-containing amino acids, the body simply cannot build robust keratin.
The clinical evidence
A 90-day randomized, placebo-controlled trial in 60 healthy women looked at L-cystine supplementation for hair and nails (clinical study). Compared with placebo, the L-cystine group saw:
- Anagen (active growth-phase) hair rate up ~10% versus ~3% for placebo.
- Hair-pull test down ~40% — meaning noticeably less shedding — versus ~11% for placebo.
- Higher hair density and faster nail growth.
Earlier double-blind research also found that a cystine-containing supplement increased the proportion of growing-phase hairs in women with temporary shedding (telogen effluvium) — a meaningful, real-world outcome.
Why women 40+ may benefit
Building keratin is resource-intensive, and hair is one of the first tissues to lose out when protein or amino-acid supply is tight — something that can become more common with age, dieting, or stress. Supplying L-cystine directly gives the follicle the specific raw material it needs to form strong strands.
Strength you can feel
One thing users often notice with amino-acid support is not just more hair, but stronger hair — less breakage, a bit more resilience, and healthier-looking nails as a bonus (they are made of keratin too).
The bottom line
If saw palmetto works on the hormonal side, L-cystine works on the structural side — providing the keratin building blocks for stronger strands and supporting the active growth phase. It is a foundational reason it sits in our Hair Density 40+ formula.
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Hair Density 40+
Saw palmetto, L-cystine, tocotrienols, zinc and biotin — a multi-nutrient formula to support hair density and help reduce everyday shedding from the inside out.
Shop Hair Density 40+ →These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. This article is for educational purposes only and is not medical advice. Consult your healthcare provider before starting any supplement, especially if you are pregnant, nursing, or taking medication.