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18-MEA (18-methyleicosanoic acid)
The fatty acid on the outer surface of virgin hair that makes it water-repellent and smooth, lost with weathering and chemical treatment.
What it is
A branched fatty acid attached to the outer surface of the cuticle. It is a structural part of hair, not a typical cosmetic ingredient. See also hair porosity.
How it works
It gives the surface a water-repellent, low-friction finish. When Kao researchers removed it, wet fibres stuck together and tangled, and the disorder carried over into the dry state. Tanamachi 2010 Weathering and chemical treatments such as bleaching strip it, which is part of why damaged hair feels rough and tangles.
What the studies show
- Removing 18-MEA caused entangled wet and dry fibre alignment in a Kao laboratory study. Tanamachi 2010
- Hair from 10 patients with maple syrup urine disease, which lacks 18-MEA, showed rougher cuticle surfaces and breaks in the cement between cells. Smith 2005
- LG researchers attached a mimic (16-methylheptadecanoic acid) to stripped hair and reported that it survived washing and restored water-repellent behaviour and moisture properties. This was in vitro and company-authored. Song 2024
How it's used in studies
It is not a consumer ingredient. Mimics are at patent or in-house stage.
Side effects and safety
No safety issues for the natural lipid.
Bottom line
Knowing it exists explains why damaged hair feels rough and tangles, and why light conditioning agents and silicones that mimic its slip help. True replacement is not available to consumers. We are fairly sure of the biology and not sure about any repair product.
Fun facts
Hair from people with maple syrup urine disease, a metabolic disorder, lacks 18-MEA, which gave scientists a natural experiment in what the lipid does. Smith 2005
Studies 3
Tanamachi H et al. 18-MEA and hair appearance. J Cosmet Sci. 2010
Result: Loss of 18-MEA makes wet hairs stick together, tangle and dry in a disordered, frizzy arrangement.
Read the study ↗Smith JR, Swift DJ. Maple syrup urine disease hair reveals the importance of 18-methyleicosanoic acid in cuticular delamination. J Cosmet Sci. 2005
Result: Absence of 18-MEA was linked to rougher cuticle delamination surfaces and discontinuities in the cell membrane complex.
Read the study ↗Song SH et al. Biomimetics through bioconjugation of 16-methylheptadecanoic acid to damaged hair. 2024
Tested: attachment of an 18-MEA mimic
Result: The mimic stayed on after washing and restored water-repellent behaviour and moisture properties in vitro (company-authored).
Read the study ↗Related
- CeramidesWaxy lipids from the cement between hair cells; a lab study with ceramide-containing liposomes restored some lipids and break resistance, but retail products lack hair trials.
- Hair porosityPorosity describes how easily water moves in and out of a hair fiber; in practice it is mostly a marker of damage, and the popular float test does not measure it.
- Silicones in hair careSilicones coat hair to cut friction and add shine without repairing it; buildup is real but manageable, and the scare claims are unsupported.