AMINO ACID
Proline
- amino acid
An amino acid from hair protein, used in amino-acid conditioning blends.
- products in our library
- 155
- of researched products
- 2%
- on the label, in our shampoos
- usually #33
- most common in
- Shampoo
- 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 Proline 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.
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 ? |
|---|---|---|
Shampoo
31 products
|
secondary · #33 | |
Mask / treatment
30 products
|
secondary · #30 | |
Leave-in
29 products
|
secondary · #29 | |
Conditioner
24 products
|
secondary · #25 | |
Heat protectant
12 products
|
secondary · #34 | |
Scalp treatment
9 products
|
secondary · #35 | |
Styler: curl cream
4 products
|
secondary · #46 | |
Styler: smoothing cream
4 products
|
secondary · #42 | |
Finisher: serum
2 products
|
secondary · #46 | |
Styler: lotion
2 products
|
secondary · #36 |
6 more product types
| Product type | How common | Typical position on label ? |
|---|---|---|
Styler: mousse
2 products
|
secondary · #19 | |
Styler: volumizer
2 products
|
secondary · #32 | |
Co-wash
1 product
|
secondary · #24 | |
Finisher: anti frizz finish
1 product
|
secondary · #56 | |
Styler: hairspray
1 product
|
secondary · #27 | |
Styler: refresh spray
1 product
|
secondary · #55 |
| Product type | Products | Share of category | Median position | Earliest position | Band |
|---|---|---|---|---|---|
| Shampoo | 31 | 2% | 33 | 12 | secondary |
| Mask / treatment | 30 | 5% | 30 | 12 | secondary |
| Leave-in | 29 | 4% | 29 | 12 | secondary |
| Conditioner | 24 | 2% | 25 | 6 | secondary |
| Heat protectant | 12 | 6% | 34 | 12 | secondary |
| Scalp treatment | 9 | 3% | 35 | 19 | secondary |
| Styler: curl cream | 4 | 2% | 46 | 21 | secondary |
| Styler: smoothing cream | 4 | 5% | 42 | 23 | secondary |
| Finisher: serum | 2 | 1% | 46 | 38 | secondary |
| Styler: lotion | 2 | 6% | 36 | 27 | secondary |
| Styler: mousse | 2 | 1% | 19 | 17 | secondary |
| Styler: volumizer | 2 | 1% | 32 | 24 | secondary |
| Co-wash | 1 | 1% | 24 | 24 | secondary |
| Finisher: anti frizz finish | 1 | 3% | 56 | 56 | secondary |
| Styler: hairspray | 1 | 0% | 27 | 27 | secondary |
| Styler: refresh spray | 1 | 3% | 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 | 1 |
| 9-15 | Secondary | 9 |
| 16-25 | Lower | 52 |
| 26+ | Trace | 93 |
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.
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
- a rinse-clean feel
- help on very dry or damaged hair
- 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 shampoos
- 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.
- Alanine · 145
- Serine · 143
- Arginine · 143
- Glycine · 141
- Threonine · 140
- Isoleucine · 133
- Phenylalanine · 133
- Valine · 133
- Aspartic Acid · 133
- Histidine · 133
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.
Found in these products
The most prominent placements first. The badge is the position on that product’s own ingredient list.
Suave · Conditioner
Bonding Infusion Repairing Conditioner
#6 on the label · $0.25/oz
Eva NYC · Shampoo
Take Care Healthy Hair Shampoo
#12 on the label · $1.40/oz
Kevin Murphy · Leave-in
NIGHT.SHIFT
#12 on the label · $15.68/oz
Maria Nila · Mask / treatment
Bond Builder
#12 on the label · $8.82/oz
Suave · Heat protectant
Essentials Heat Protection Shield Spray
#12 on the label · $0.38/oz
Eva NYC · Conditioner
Take Care Healthy Hair Conditioner
#13 on the label · $1.40/oz
Eva NYC · Shampoo
Reset Ready Weekly Detox Clarifying Amino Acid & AHAs Shampoo
#14 on the label · $1.40/oz
hOURS haircare · Conditioner
The Daily Conditioner
#15 on the label · $4.00/oz
Kristin Ess · Conditioner
Moisture Rich Curl Conditioner
#15 on the label · $1.20/oz
PATTERN · Leave-in
Repair360® Leave-in Foam
#15 on the label · $8.18/oz
JVN · Conditioner
Nurture Hydrating Conditioner
#16 on the label · $2.40/oz
JVN · Mask / treatment
Revive Rapid Repair Bond Gloss
#16 on the label · $6.00/oz
Identity
What this entry is, and what our references call it.
No public-domain structure depiction is available for this entry.
Proline
- proline
For your hair?
Whether Proline matters for your hair depends on your hair.
155 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
Seed entry, sourcing in progress Written from our reference work; primary sourcing is still being attached.
No published source is attached to this entry yet.
How this page was made
- Every number above is counted from the ingredient lists in our own product library: 155 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.
