AMINO ACID
Leucine
- amino acid
An amino acid of the kind hair protein is built from. Small enough to sit inside the cuticle, and used for water binding rather than strength.
- products in our library
- 17
- of researched products
- 0.2%
- on the label, in our masks and treatments
- usually #31
- most common in
- Mask / treatment
- in water
- Water-soluble
At a glance
Four things worth knowing about how this ingredient behaves on hair.
How much of it is left on hair between washes.
What it takes to get it off the hair again.
Smaller fragments travel further into the strand.
Read from where it sits on the labels we have, not from the molecule.
| Property | Level | Out of |
|---|---|---|
| Buildup risk | Low | 1 of 5 |
| Rinse-out behaviour | Rinses clean | 5 of 5 |
| Fragment size | Smallest | 1 of 5 |
| Stiffening potential | Slight | 2 of 5 |
What it does for a formula
What Leucine does
The general function first, then how the role changes by product type where we have researched it.
Small enough to travel further into the strand than a whole protein can.
Binds water and holds it against the hair rather than adding oil.
What a humectant does depends on the air around it as much as on the formula.
Water alone removes it, so it does not accumulate between washes.
Charge
Charge is not recorded for this ingredient.
Solubility
Rinses out with water alone, so it does not build up on hair between washes.
Where it appears
Where it appears
One row per product category in our library: how many products list it, what share of that category that is, and where it typically sits on the label.
| Product type | How common | Typical position on label ? |
|---|---|---|
Mask / treatment
6 products
|
secondary · #31 | |
Conditioner
2 products
|
secondary · #32 | |
Co-wash
1 product
|
secondary · #22 | |
Dry shampoo
1 product
|
secondary · #16 | |
Finisher: anti frizz finish
1 product
|
secondary · #49 | |
Heat protectant
1 product
|
supporting · #8 | |
Leave-in
1 product
|
secondary · #38 | |
Scalp treatment
1 product
|
secondary · #50 | |
Shampoo
1 product
|
secondary · #16 | |
Styler: refresh spray
1 product
|
supporting · #8 |
1 more product type
| Product type | How common | Typical position on label ? |
|---|---|---|
Styler: smoothing cream
1 product
|
secondary · #55 |
| Product type | Products | Share of category | Median position | Earliest position | Band |
|---|---|---|---|---|---|
| Mask / treatment | 6 | 1% | 31 | 13 | secondary |
| Conditioner | 2 | 0% | 32 | 27 | secondary |
| Co-wash | 1 | 1% | 22 | 22 | secondary |
| Dry shampoo | 1 | 1% | 16 | 16 | secondary |
| Finisher: anti frizz finish | 1 | 3% | 49 | 49 | secondary |
| Heat protectant | 1 | 0% | 8 | 8 | supporting |
| Leave-in | 1 | 0% | 38 | 38 | secondary |
| Scalp treatment | 1 | 0% | 50 | 50 | secondary |
| Shampoo | 1 | 0% | 16 | 16 | secondary |
| Styler: refresh spray | 1 | 3% | 8 | 8 | supporting |
| Styler: smoothing cream | 1 | 1% | 55 | 55 | secondary |
We have not researched differences by product type yet. The reading below comes from where it sits on the labels in our library.
Position footprint
Every listing in our library, by position on the label.
| Positions | Zone | Listings |
|---|---|---|
| 1-3 | Leading | 0 |
| 4-8 | Supporting | 2 |
| 9-15 | Secondary | 1 |
| 16-25 | Lower | 3 |
| 26+ | Trace | 11 |
How to read this
US labelling rules put ingredients above 1% in descending order of amount; below 1% they may be listed in any order.
So position tells you about relative amount only above that line, and we never convert a position into a percentage.
Who should pay attention
Formulation guidance for specific hair conditions, limited to the label positions where it applies. It is not a conclusion about any finished product, or about you.
Amino acids are the smallest protein fraction and bind water as well, so they add far less stiffness than a hydrolysed protein does.
A humectant follows the air: it holds water in humid conditions and can give it up in very dry ones. Climate decides what it does more than the label does.
The lightest option in the protein family, but still protein — worth watching if protein reliably stiffens your hair.
Read from this ingredient’s chemical class. Where we have researched the ingredient itself, that reading replaces the class one.
Why it matters to shoppers
What seeing this ingredient tells you, what it does not, and the limits of our own method.
- the lightest protein option
- a light protein
- hydration rather than oil
- a rinse-clean feel
- an exact safety score (we do not publish one for any ingredient)
- how a finished product performs, which depends on the whole formula, not one line of it
- the exact percentage in a bottle, because label position never converts into a number
- a secondary amino acid, listed below the main ingredients in our masks and treatments
- it was on the printed ingredient list of every product we counted (we read labels, not marketing copy)
- that a product is right or wrong for you
We do not publish safety scores or hazard grades. Where a regulator has published a restriction or a use condition, we state it and cite it; otherwise we describe what the ingredient does and stop.
Commonly paired with
The ingredients that most often appear alongside this one in our library. We leave out the near-universal ones, because they pair with everything and tell you nothing.
- Arginine · 16
- Lysine · 13
- Butylene Glycol · 13
- Tyrosine · 11
- Histidine · 11
- Isoleucine · 10
- Phenylalanine · 10
- Valine · 10
- Aspartic Acid · 10
- Glutamic Acid · 10
The number beside a name is how many products in our library list both.
These ingredients tend to appear together to firm up damaged areas and carry the formula and let it dry.
Found in these products
The most prominent placements first. The badge is the position on that product’s own ingredient list.
Living Proof · Styler
Perfect hair Day (PhD) Style Refresh Hair Mist - 5.0 oz
#8 on the label · $6.40/oz
Living Proof · Heat protectant
Perfect hair Day Style Refresh Mist
#8 on the label · $6.40/oz
LADOR · Mask / treatment
Wonder Tear
#13 on the label · $3.12/oz
John Frieda · Dry shampoo
Brilliant Brunette Dry Shampoo
#16 on the label · $3.14/oz
Paul Mitchell · Shampoo
Tea Tree Lavender Mint Moisturizing Shampoo
#16 on the label · $2.47/oz
Davines · Co-wash
Love Curl Co-Wash
#22 on the label · $3.37/oz
Davines · Mask / treatment
LOVE CURL Mask
#26 on the label · $5.01/oz
Davines · Conditioner
LOVE CURL Conditioner
#27 on the label · $5.09/oz
Kevin Murphy · Mask / treatment
YOUNG.AGAIN.MASQUE
#29 on the label · $8.43/oz
Healing Bird · Mask / treatment
Revive Protein Deep Hair Mask Repair
#33 on the label · $2.25/oz
The Doux · Conditioner
FAST FORWARD Heat Prep Conditioner
#36 on the label · $2.24/oz
Kevin Murphy · Leave-in
SMOOTH.AGAIN
#38 on the label · $6.36/oz
Identity
What this entry is, and what our references call it.
Leucine
- CAS 61-90-5
- PubChem 6106
- humectant
- leucine
For your hair?
Whether Leucine matters for your hair depends on your hair.
17 products in our library list itWhat this ingredient does, and where it sits on the labels we have researched.
A verdict on any product that contains it, and never a safety grade.
We do not physically test your hair. Everything above comes from the ingredient lists in our library and from published references. The consultation is what turns it into a plan.
References
Sourced Backed by a manufacturer or regulatory source.
This ingredient
- Manufacturer or regulatory PubChem : Compound identity: CID 6106, CAS 61-90-5, C6H13NO2 · accessed 2026-08-28
How this page was made
- Every number above is counted from the ingredient lists in our own product library: 17 of 6816 researched products list this ingredient. Nothing on this page is written per ingredient.
- The dots are levels, not scores. Each one comes from what kind of ingredient it is (its chemical class, charge and solubility) or from where it sits on the labels we have.
- US labelling rules put ingredients above 1% in descending order of amount; below 1% they may be listed in any order. Position tells you about relative amount only above that line, and we never convert a position into a percentage.
- The prominence bands (leading, supporting, secondary) are the same ones the product pages use to read a single label, applied here to the median position across a category.
- We do not publish safety scores or hazard grades. Where a regulator has published a restriction or a use condition, we state it and cite it; otherwise we describe what the ingredient does and stop.
- Identifiers and references appear only when a published source is attached to this entry. Without one, we leave the row out rather than fill it in.
- This is cosmetic information, not medical advice, and nothing here diagnoses or treats a health condition.
- Entry last updated 2026-09-22. Product-page methodology: the product library.
