Protein Treatment for Hair: What It Does and When You Need One
Hair is mostly protein, yet the protein in a mask never becomes part of its keratin. What a treatment does instead, how long it stays, and how to tell whether your lengths need one.
Direct answer
A protein treatment is a mask, conditioner or leave-in carrying hydrolyzed protein, such as keratin cut into small pieces. The pieces cling to hair, damaged hair most, and smaller ones get inside. In lab tests they raised strength modestly, especially wet; most of it washes off over later shampoos, though that is rarely measured. None of it rebuilds hair's keratin.
Key takeaways
- Hair is 65 to 95% protein by weight, depending on how much water it holds, but applied protein never becomes part of its keratin.
- Size decides where protein goes. In one industry-funded lab study, mid-sized keratin reached the cortex of relaxed hair, large pieces left a surface film, and the smallest got in without strengthening it.
- In a small study of a realistic five-minute rinse-off on bleached hair, only hydrolyzed keratin beat the same conditioner without protein, and only when tested wet.
- "Too much protein breaks hair" has not been shown: we found no study that measures protein overload, and in one lab study on relaxed hair, the keratin treatments that made it stiffer also reduced early breaks.
- A salon keratin treatment is a heat-set straightening service, not a protein treatment.
What is a protein treatment for hair?
A protein treatment is any hair product that adds hydrolyzed protein: protein cut into short chains, called peptides, small enough to cling to hair and, at the smaller sizes, slip inside. It comes as a mask, a rinse-off treatment, a conditioner, a shampoo, a leave-in or a salon service.
Hair-care hydrolysates come from keratin, wheat, collagen, elastin, milk, almond and silk, in small amounts: under 1% in Robbins' textbook example of a conditioner, and 0.5 to 2% at the levels suppliers recommended in one study. In most conditioners and masks, the conditioning base does much of the work you feel.
What does a protein treatment do to hair?
It sticks to hair, and to damaged hair most. In lab tests, most protein treatments made damaged fibers measurably, if modestly, stronger straight after use, most clearly when wet.
Proteins carry many charged and polar groups that can bond to hair, and chemically treated hair, bleached hair above all, picks up more of them. A review in the International Journal of Trichology suggests why: damaged hair carries a stronger negative charge, the protein pieces a positive one, and that attraction may also reduce frizz and friction.
Size decides where the protein goes. Smaller pieces diffuse into hair more easily, and there is no sharp size limit. The clearest test is an industry-funded 2021 study by Malinauskyte and colleagues, who soaked relaxed, very curly hair in keratin of three sizes. The mid-sized pieces, around 2,600 daltons, reached deep into the cortex, the fiber's load-bearing core. The largest, around 75,000 daltons, only just entered the outer layers of the cortex and left a film on the surface. Both made the hair stronger. The smallest got in too and swelled the fibers, yet did not strengthen them, so getting inside a hair is not the same as helping it.
Those soaks lasted hours. Daniels and colleagues at London College of Fashion tested something closer to a shower: a conditioner left on damaged hair for five minutes, then rinsed. On bleached hair, the conditioner base alone did not significantly improve strength over untreated hair. Compared with that base, only the keratin version was significantly stronger, and only when tested wet; wheat and collagen did no better. Each result rests on six fibers, measured before any washing.
What a protein treatment cannot do is turn damaged hair back into healthy hair. The hair shaft is not alive and cannot repair itself, and applied amino acids "do not replace or restore the damaged molecule structure," as the same review puts it. Bleach turns many of the disulfide bonds in keratin into cysteic acid, which no conditioner turns back. A protein treatment cannot fuse a split end, and it does not change your curl pattern. For how those bonds work, see what hair bonds are.
Is keratin a protein, and is a keratin treatment a protein treatment?
Keratin is a protein; a salon keratin treatment is not a protein treatment. Keratins are the structural proteins hair is built from, and depending on how much water it holds, hair is 65 to 95% protein by weight.
On a product label, "keratin" usually means hydrolyzed keratin, most often from wool rather than human hair.
Keratin smoothing (sold as Brazilian keratin, progressive brush and other names) is a semi-permanent straightening service. An aldehyde such as formaldehyde or glyoxylic acid goes onto the hair and is set with a blow-dryer and a flat iron, which in one review's description passes over the same spot 15 to 20 times. The keratin in the product is not what straightens the hair. A 2022 review of 33 articles describes the claim that it diffuses into the cortex and restructures it, then adds: "there is no evidence of this effect."
Ask what is in it before you book. The US Food and Drug Administration says these products "often contain formaldehyde," which, when the solution is heated, "is released into the air as a gas" and "can cause serious irritation of your eyes, nose, and lungs." Labels have been wrong: of seven smoothing brands sold in South Africa in 2012, six contained formaldehyde, five of them labeled formaldehyde-free. The FDA advises checking labels for formaldehyde, formalin and methylene glycol, and asking your salon.
How long does a protein treatment last?
Until it washes out: most of it is gone over the following shampoos, though few studies have measured how fast. It is a deposit on and in the fiber rather than new hair, so the effect is strongest right after use.
A dermatology review describes high-molecular-weight hydrolyzed proteins entering through defects in the cuticle and temporarily filling fractures and fissures "until the next cleansing." In a 2023 lab study on permed hair, a keratin-and-serine solution formed only a protective film outside the cortex that washed off easily, and by an alkali-solubility test its effect had not lasted through five washes. It lasted that long only after the researchers added an enzyme that links the proteins together.
That is why protein products are used again and again: the damage stays, the deposit washes away, and repeat treatments only top it up.
How do I know if my hair needs protein?
Check its history, then compare your lengths with your roots. Most guides read a stretch test as a verdict (stretches then breaks means protein, snaps means moisture), but it is a rough elasticity check with no validated reading, and no study we found supports that split. A home check can roughly gauge how your lengths compare with new growth that has not yet been bleached, relaxed or ironed. Your roots are the control.
A self-check with one shed hair:
- Count the history: bleach, relaxer, perm or regular flat-ironing on the same lengths, the kinds of damage protein treatments have been tested on. If none applies, your hair is probably not a candidate (see the second path below).
- Wet a shed hair long enough to include new growth. Pull gently on a short section near the root, then on the processed length. If the length stretches much more easily, that fits a loss of wet strength, which in hair tracks the loss of disulfide bonds; a fingertip test can suggest this but cannot measure it. If it stretches like chewing gum and stays long, read our gummy hair guide first.
- Compare wet and dry. Water weakens damaged hair far more than healthy hair: after bleaching, wet strength falls far more than dry strength. Lengths that feel normal dry but soft and weak wet fit that pattern.
- Think back to your last protein product. If your hair felt stronger for a few washes, that suggests your lengths respond to protein, although you felt it rather than measured it. If it felt stiff, rough or straw-like, start with the third path below; stiffness alone does not say what caused it.
Laboratories use an instrument instead; see how hair tensile testing works.
If steps 2 and 3 showed lengths weaker than new growth, the check cannot say whether bleach, relaxer or heat did most of it, or what to do first. The free hair consultation works on those two questions. In about a minute you answer questions on your color history, heat tools, washing routine and how your hair behaves, and it scores porosity and damage. It then asks for your email before it shows your Damage Profile, along with a routine in order. It is a guide to care, not a medical diagnosis.
Is protein overload real, and can too much protein break your hair?
Some protein treatments have been measured to make hair stiffer. Protein overload, as a condition that breaks hair, has not been shown. "Protein overload" is the salon and online name for hair that turns stiff, rough or straw-like after protein-heavy products, and searches of Scopus and PubMed for "protein overload" and "protein-moisture balance" in hair research returned nothing.
The usual version runs: protein filler can help, but too much can actually break your hair. The first half has some support, since a dermatology review says larger hydrolyzed proteins can temporarily fill cracks in damaged hair until the next wash. For the second half we found no measurement of protein making hair brittle or causing it to break. In the relaxed-hair study, the treatments that made hair stiffer also cut its early breaks, and none of the protein treatments made it break more.
What has been measured is stiffness, and it may be what people feel. In the relaxed-hair study, mid-sized and large keratin made the hair stiffer in dry air, and in humid air the large pieces still did; the large pieces also left a film visible under an electron microscope. In the London College of Fashion study, wheat protein formulated with wheat starch gave the best dry strength, which the authors suggest may come from starch that "stiffens with drying." Hair that breaks more after a protein product is a different signal; run the self-check above.
Damage itself can also be read as overload: a straw-like feel has more than one possible cause, covered in our hair breakage guide. Either way, the third path below applies.
Is protein good for high porosity hair?
It can, when the porosity comes from damage, though the measured gains are modest. Bleached and otherwise damaged hair is more porous, carries more negative charge and takes up more protein than intact hair, and that is what the advice rests on.
Two cautions. The studies tested damaged hair rather than porosity types, so "high porosity needs protein, low porosity cannot take it" has no evidence as a rule. And more porous does not simply mean more protein binds. In one study, perming let a small dye reach five times deeper into hair (25 µm against 5 µm), yet weakened the binding of one extracted keratin fraction, while another bound as well as it did to unpermed hair.
In a lab study on bleached hair, keratin proteins made the fibers less permeable. Like any protein deposit, expect that to fade with washing.
How porosity is measured is covered in the science of hair porosity, why bleaching raises it in is bleached hair high porosity, and daily care in our high porosity hair guide.
Do protein treatments help Black, coily and relaxed hair?
In one lab study, mid-sized and large keratin made relaxed hair stronger, though the smallest did not, and that study is the best evidence we found. It is the Malinauskyte study above, on very curly South African hair relaxed with sodium hydroxide.
Relaxing left the hair weak in humid air: its break stress was about a third lower at 80% humidity than at 20%, while unprocessed hair from the same source showed no significant difference. Mid-sized and large keratin raised the relaxed hair's break stress, most in humid air, and in dry air cut early breaks from 15 per 50 fibers to 5 or 6. Unprocessed hair was still more than twice as strong as untreated relaxed hair in humid air, so the treatment won back a fraction of what relaxing took. The soaks lasted 24 hours, far longer than any rinse-off, and the work was funded by Croda, an ingredient supplier, and TRI Princeton.
Curly hair is often described as "protein sensitive." The same study measured something that may relate: mid-sized and large keratin made relaxed hair stiffer in dry air. If protein leaves your curls stiff, trust that response over any rule; the third path below applies.
The results apply to relaxed hair. They say nothing about how unprocessed curls or coils respond, or about any group of people.
Protein vs moisture treatment: what is the difference?
A protein treatment deposits protein on and in the hair; a moisture or conditioning treatment leaves conditioning agents on the surface, where they bind most to damaged hair and can lower friction. Many conditioners contain a little protein too.
The advice you will see most often is "protein plus moisture," or alternating the two, on the idea that protein dries hair out. That is a simplification. Proteins have long been used in cosmetics partly because they bind water, and in lab tests keratin proteins helped bleached hair hold water, not lose it. None of the studies we found tested a protein-moisture balance or an alternating routine, so treat it as stylist practice.
Whether a bond repair treatment also contains protein depends on its formula; our guide to bond repair treatments covers that category.
| Protein treatment | Moisture or conditioning | Bond repair treatment | Keratin smoothing | |
|---|---|---|---|---|
| What the label shows | Hydrolyzed keratin, wheat, collagen or silk protein | Positively charged conditioning agents, such as cetrimonium chloride | Ingredients aimed at hair's bonds; the chemistry varies by brand | Formaldehyde, formalin, methylene glycol or glyoxylic acid, though labels have left formaldehyde off; set with a flat iron |
| What it acts on | Binds to hair, damaged hair most; small pieces get inside | The surface; binds most to damaged hair and can lower friction | Aimed at the bonds that chemical services break | The hair's shape |
| How long it lasts | Mostly washes out over the following shampoos; rarely measured | Much of a deposited conditioning polymer washes off at the next shampoo | Depends on the chemistry | Semi-permanent, through many washes |
| Who it suits | Bleached, permed, relaxed or heat-damaged lengths | Any hair, reapplied at every wash | Chemically processed hair | Used to straighten hair or smooth curls; see the formaldehyde note above |
| What it cannot do | Turn bleach-damaged bonds back, or fuse split ends | Much for strength alone: in one small study, the base did not significantly strengthen bleached hair | Restore a split end to how it was | Rebuild hair with the keratin in the product; there is no evidence of that |
Not sure which column is yours?
Which works best: a protein mask, shampoo, conditioner or salon treatment?
No study we found compares formats head to head, or a salon treatment with a mask at home. In lab work, how much protein hair picked up depended on the protein itself, its concentration, how long it stayed in contact and the rest of the formula, so longer contact gives it more chance to bind, though binding is not the same as helping.
For a mask or rinse-off treatment, the closest test is the London College of Fashion study: a five-minute conditioner with a positively charged base.
A shampoo gives protein the least time and also washes deposits away, though how much protein it removes is less studied. An older radiolabel study still found wheat amino acids, from fully hydrolyzed wheat protein, getting into hair from both a shampoo and a conditioner.
A leave-in is not rinsed out, so what it deposits can stay until your next wash; see what leave-in conditioner does.
A salon protein treatment uses the same kinds of hydrolyzed protein. Ask what is in it, and whether a flat iron is part of the service; aldehyde chemistry set with a flat iron is keratin smoothing.
How do you use a protein treatment?
Shampoo, apply it to the lengths only, leave it on for the time on the label, rinse and condition; repeat when your hair feels weaker again, and skip it if your hair has never been processed.
If your lengths are bleached, relaxed, permed or heat-damaged
- After shampooing, apply it to the lengths and not the scalp; rinse, then condition unless the treatment is a conditioner.
- Between treatments you can use a conditioner or leave-in with a little protein. Each deposit washes out, and no study we found tested whether this helps.
- We found no study of frequency either. After one treatment, count how many washes the difference lasts, and use that, with the label, as your starting interval.
- Have split ends trimmed; nothing makes a split end whole again.
Protein is one step in a longer routine, set out in our guides to bleached hair, heat-damaged hair and how to repair damaged hair. For the order that fits your own lengths, the free consultation will find your Damage Profile.
If your lengths have never been processed
You probably do not need a protein treatment. The studies above tested damaged hair, and intact hair has fewer of the damaged, charged sites that protein binds to. Condition, handle it gently when wet, and save your money.
If your hair turned stiff or straw-like after protein
- Stop protein products for a few washes.
- Wash once with a clarifying shampoo. Shampoo removes much of what conditioners leave on the surface, though how much protein it removes has been studied less.
- Condition at every wash.
- Bring back one protein product at a time, and note how your hair feels after a few washes.
These steps are stylist practice; none of the studies we reviewed tested them. Gummy hair needs the plan in the gummy hair guide instead.
Where does protein fit in a hair routine?
Only on lengths the self-check showed to be weaker than your roots, as one step among several; never-processed hair usually needs none.
Whatever you decide about protein, if your lengths are bleached, the Complete Reconstruction System is ANATOMY's three-step routine of shampoo, conditioner and leave-in. It is made in Switzerland and patented. It is built on click chemistry, the reaction class recognized by the 2022 Nobel Prize in Chemistry, and was independently tested at SGS proderm in Germany, on bleached hair. Because that testing was on bleached hair, this guide makes no claim for it on relaxed, permed or heat-damaged lengths, and the paths above apply whichever products you use.
If your lengths have been processed and you are unsure which path fits, the free consultation can show you your Damage Profile.
Frequently asked questions
How do I spot protein on an ingredient label?
Look for "hydrolyzed" before keratin, wheat protein, collagen, silk, milk or almond protein, and for quaternized versions. Protein is usually a small part of the formula. Rarely, protein ingredients cause allergic reactions; anyone with a wheat allergy or reactive skin should check labels and ask a dermatologist.
Can I use protein and a bond repair treatment in the same routine?
None of the studies we reviewed tested protein and bond repair products together, or alternating them, so treat combinations as practice. They are aimed at different things, and a bond repair product may already contain hydrolyzed protein, so check its label. If you use both, add one new product at a time to see which one caused any stiffness.
Can a protein treatment repair split ends?
No. No product restores a split end to how it was, and only a trim takes the split away. Our split ends guide covers what else you can do.
Can you use a protein treatment on bleached or colored hair?
Yes. Much of the lab evidence comes from bleached hair, and none of the bleached-hair studies we found reported a loss of strength. For colored hair, one study added a silk and milk protein mix to a blonde dye: combing improved, and breaking strength did not change.
Which protein is best: keratin, wheat, collagen or silk?
In the London College of Fashion study, hydrolyzed keratin, the smallest and most hair-like, did most for wet strength; wheat protein with wheat starch did most for dry strength; collagen gave the most consistent result on heat-damaged hair. For silk we found no independent strength trial of silk protein alone.
References
- Robbins CR (2012). Chemical and Physical Behavior of Human Hair, 5th edition. Berlin, Heidelberg: Springer Source
- Malinauskyte E, Shrestha R, Cornwell PA, Gourion-Arsiquaud S, Hindley M (2021). Penetration of different molecular weight hydrolysed keratins into hair fibres and their effects on the physical properties of textured hair. International Journal of Cosmetic Science 43(1):26-37. Funded by Croda and TRI Princeton Source
- Daniels G, Nicholson S, Grant-Ross P, Tamburic S (2016). An ex vivo comparison of the tensile strengthening properties of protein derivatives on damaged hair. IFSCC Magazine 2016(3):1-6 Source
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- US Food and Drug Administration. Hair smoothing products that release formaldehyde when heated. Content current as of 15 October 2024 Source