Hair peptides: care, scalp products and scientific evidence

Peptides appear in hair products in very different forms. They can be components of protein hydrolyzates, specific sequences designed to interact with keratin, ingredients in shampoos and conditioners, or molecules experimentally studied for their effects on hair follicles and growth-related pathways.

However, all these applications should not be treated as equivalent.

A conditioner containing peptides can alter the surface or mechanical properties of existing hair without causing new growth. A peptide that binds to keratin in a laboratory study may be interesting from a formulation technology perspective, but this does not yet translate to a visible benefit for users. On the other hand, a molecule that affects cells associated with the hair follicle in culture can provide mechanistic data while remaining far from a clinical study of a topical scalp serum.

The basis for this distinction is the biological purpose of the action.

The visible hair shaft is a keratinized fiber composed mainly of dead, highly ordered material. The hair follicle, on the other hand, contains living cells that participate in the production of this fiber. The scalp, in turn, is a living tissue surrounding the follicles and creating their environment.

The product may affect one of these elements without exerting a proven effect on the others.

Research on peptides used in the context of hair is therefore best analyzed in at least three separate categories:

1. conditioning of the hair fiber and its impact on material properties;
2. research on formulation, ingredient deposition, and scalp products;
3. research on hair follicles and hair growth.

The route of exposure, the composition of the finished product, the research model, the comparison group, the study population, and the measured endpoint determine what conclusions can be reasonably drawn from a given publication.

If you want to learn the chemical basics of this group of compounds first, see also: peptides – what they are, how they work, and how they are classified.

How do peptides occur in hair products?

Protein hydrolysates contain mixtures of fragments

Hydrolyzed keratin, hydrolyzed wheat protein, and hydrolyzed collagen are examples of protein-derived ingredients that may contain peptides.

Hydrolysis breaks down larger proteins into smaller fragments. However, this process does not necessarily lead to the formation of a single, homogeneous peptide sequence or molecules of uniform size.

The hydrolysate may contain a mixture of:

• peptides of varying length;
• free amino acids;
• larger protein fragments;
• chemically modified fragments;
• and other accompanying ingredients.

Therefore, even two hydrolyzates obtained from the same protein can differ significantly in molecular weight distribution, peptide sequences, electrical charge, solubility, stability, and affinity for the hair.

„Hydrolyzed protein” is therefore a compositional category, not a full description of biological or cosmetic action.

In one study, enzymatically obtained feather keratin hydrolyzates were characterized, including the size distribution of the peptides present in them. Next, the material was applied in a mild shampoo and a rinse-off conditioner. The researchers evaluated the properties of the hair fibers treated with the preparations and observed deposits on their surface using microscopy. [1]

It was a study of a specific hydrolyzate and a specific formulation system.

It does not constitute proof that all keratin shampoos stimulate the growth of new hair.

The method of obtaining the hydrolyzate matters

Hydrolysates can be produced or modified using various chemical and enzymatic processes. These processes can affect fragment size, amino acid composition, charge, solubility, stability, and the way they interact with keratin.

Therefore, scientific data should be related primarily to the actually characterized material used in the experiment, rather than exclusively to the known protein name.

The result obtained for one enzymatically produced keratin hydrolysate cannot be automatically assigned to all cosmetic ingredients designated as „hydrolyzed keratin”.

The same principle applies to collagen and wheat protein hydrolysates.

Defined peptides represent a different research approach

Some studies focus on a specific peptide sequence selected or designed to interact with keratin or another specific biological target.

This is a fundamentally different approach than studying a broad hydrolysate containing many different fragments.

In one of the experiments, 1,235 decapeptides derived from human hair proteins were tested for binding to an isolated keratin fraction. [2] The researchers found correlations between the peptide composition and their binding properties.

These results support the concept that peptide-keratin interactions may depend on the sequence and physicochemical properties of the molecule. They also show why „peptides” should not be treated as a single, uniform category of ingredients.

However, merely binding to isolated keratin does not mean that a given peptide:

• penetrates undamaged hair;
• remains stable in the finished cosmetic product;
• remains bound after washing;
• improves the mechanical properties of the hair;
• changes its appearance under real-world usage conditions;
• it reaches the hair follicles;
• or stimulates hair growth.

Binding is one experimental step. It is not the final clinical result.

Peptide sequence matters

Two peptides can contain the same number of amino acids and yet have different sequences and properties.

A decapeptide simply means a peptide composed of ten amino acid residues. This information alone does not tell us which amino acids are present in its structure, in what order they are arranged, what charge the molecule has, what structure it adopts, or how strongly it interacts with keratin.

Therefore, general statements such as „peptides bind to damaged hair” require clarification.

Some peptides may exhibit useful interactions under specific experimental conditions. However, this does not mean that all peptides share this property to the same degree.

Modifying the ingredient can alter its deposition on the hair

Ingredients used in hair care can be chemically modified to alter their interaction with the negatively charged or chemically altered hair surface.

In a study on wheat protein hydrolyzate, enzymatic treatment was evaluated, followed by cationization of the material and its behavior in shampoo-related experiments. [3]

For the tested system, an improvement in certain properties related to the hair surface was observed.

However, the results apply to a specific modified material. They cannot be automatically transferred to unmodified wheat protein, another wheat hydrolyzate, keratin hydrolyzate, collagen peptides, or a scalp serum not related to this study.

An older study simulating the application of radiolabeled collagen hydrolysates also showed that their retention on hair depended on the properties of the hydrolysate, exposure conditions, and other formulation ingredients. [4]

This means that the ready-made formulation is of key importance when interpreting the results.

Surface deposition is not the same as penetration

A substance can deposit on the outer surface of the hair without deeply penetrating its interior. Similarly, the presence of material on the hair surface does not mean that it reaches the hair follicle.

These are separate issues.

Embedding data can support claims regarding coating, slip, surface modification, or temporary conditioning.

Claims regarding deeper structural change, on the other hand, require evidence relating to the specific site of action. In the case of hair follicles, yet another chain of evidence is needed.

A multi-peptide serum is a specific formulation, not a standard method of action

The term „multi-peptide” generally means that a product is intended to contain more than one peptide ingredient.

However, it does not specify what exact peptides these are, what concentrations they occur in, what their purity and stability are, in what medium they have been placed, what their biological purpose is, or what clinical effect has been demonstrated.

The product may additionally contain solvents, humectants, plant extracts, proteins, amino acids, conditioning polymers, preservatives, and other ingredients. Each of these can affect the scalp sensations or the appearance of the hair.

Peptide counts should also not be confused with the number of amino acids. A pentapeptide consists of five amino acid residues, but this does not mean that the preparation contains five different peptides.

So the most important questions are:

What exactly is in the formulation?

What is its intended purpose?

What effect is declared?

What studies actually test this formulation and this effect?

Selected studies do not confirm the general principle that a greater number of peptides leads to greater hair growth.

Hair fiber conditioning and breakage

Existing fibers and living follicles require different measurements

The visible part of the hair does not regenerate through living cellular processes. It is a structure made of keratin.

However, a cosmetic product may alter its surface or physical properties. Potential effects may include coating, deposition of ingredients, improvement of slip, electrostatic interactions, change in friction, influence on water interactions, or other chemical interactions with the damaged fiber.

Such changes can have a real cosmetic significance, but they are not biological regeneration.

Therefore, the term „hair repair” requires clarification. Depending on the study, it may mean a smoother hair cuticle appearance, reduced combing force, improved performance in tensile strength tests, lower breakage under specific conditions, the coating or filling of damaged areas, or a specific chemical interaction.

This does not automatically mean the complete restoration of the original hair structure, the permanent reversal of all chemical damage, the repair of every broken bond, or the stimulation of new follicle formation.

The study of feather keratin hydrolysate utilized fiber measurements and microscopy. [1] Its results therefore fall primarily into the category of material properties and conditioning, rather than research on clinical hair regrowth.

Cosmetic „repair” and biological regeneration are different concepts

In living tissue, repair usually means a biological process involving cells.

Once grown out of the follicle, the hair fiber is not capable of such independent regeneration.

Cosmetic „repair” therefore usually means improving the measurable or visible properties of the hair. This does not mean that the effect is insignificant. Reducing friction or breakage can improve the behavior and appearance of existing hair.

However, it answers a different question than the regeneration of the hair follicle.

Lower breakage can improve the perception of density

Hair can appear fuller if you manage to keep more of its existing length.

Visual volume can also be influenced by fiber alignment, surface coating, an increase in diameter resulting from material deposition, reduced brittleness, styling, and a change in surface texture.

These mechanisms differ from increasing the number of actively growing follicles.

Breakage refers to the physical snapping of the hair shaft at a certain point. Hair loss, on the other hand, means the release of the hair from the follicle.

For the user, both phenomena may look similar because in both cases hair is visible during washing or brushing. From a scientific point of view, however, they should be separated.

This measurement method sets the scope of the possible conclusion.

Broken hairs and hairs that have fallen out of the follicle should not be automatically counted together.

A broken hair may be just a fragment of the shaft. A naturally shedding hair, on the other hand, comes from the follicle.

Without proper evaluation, hair collected from a hairbrush, drain, or clothing can fall into both categories.

Therefore, statements such as „reduced hair loss” can be difficult to interpret if the study does not specify exactly what was counted.

An anti-breakage product can reduce the number of visible hair fragments without affecting the hair growth cycle. Such an effect may still be cosmetically beneficial, but it should be correctly described.

Microscopy and strength testing answer different questions

Microscopy can show the appearance of the hair cuticle, deposits, surface irregularities, cracks, or the coating pattern. Mechanical tests, on the other hand, determine how fibers behave under defined forces.

A smoother hair appearance does not automatically mean greater tensile strength if appropriate mechanical measurements have not been performed.

In the study of keratin hydrolysate, deposits were observed near the junctions of the hair cuticles and interpreted as potentially related to surface effects. [1] In the study of modified wheat protein, surface properties in a specific system were also analyzed. [3]

None of these results should be presented as a permanent repair of every type of hair damage.

Testing conditions can strongly influence the results

Hair fibers differ in, among other things, diameter, history of bleaching and coloring, exposure to high temperature, chemical straightening, or environmental factors.

Laboratory conditions also matter: humidity, temperature, wet or dry testing, washing method, preparation concentration, exposure time, and the number of application cycles.

Therefore, a favorable laboratory result should always be interpreted together with the conditions under which it was obtained.

The durability of the effect requires separate evidence

Immediate improvement after using the preparation does not mean a lasting effect.

The embedded material may be partially or completely removed during subsequent washing. The properties of the fiber may also change again after shampooing, thermal styling, bleaching, brushing, or environmental exposure.

Durability claims require testing involving multiple treatments or wash cycles.

A single measurement taken directly after applying the preparation does not prove a lasting „repair”.

Disulfide bond research does not validate every bond repair type product„

Keratin contains a peptide backbone, but cysteine residues can also form transverse disulfide bonds. These are chemically distinct structures.

Therefore, the claim regarding the interaction with disulfide bonds should not be automatically described as „rebuilding peptide bonds.”.

You can read more about the chemistry of these compounds here: peptide bonds – structure and significance.

In a study on a bio-inspired peptide, the designed PepACS material was evaluated in the context of hair perming and potential hair repair. [5] The authors described interactions with keratin sulfhydryl groups, the preservation of selected strength properties, and microscopic observations regarding cuticle damage.

This is specific data concerning a single designed peptide system.

They do not mean that every retail product advertised as „peptide repair,” „bond repair,” „molecular repair,” „bond building,” or „inner repair” uses the same chemistry and delivers the same results.

„Hair repair from within requires evidence regarding the site of action

The statement that the preparation works „from the inside” suggests a specific site of action for the material.

To scientifically justify such a designation, it would be necessary to distinguish between surface deposition, penetration into the cuticle layers, localization in the deeper parts of the fiber, specific chemical interaction, and durability after washing.

Merely observing the hair surface is not enough to confirm deep internal repair.

Scalp treatments and related claims

The scalp product works in a different environment

Scalp serum is applied primarily to the living skin, although it may also come into contact with the hair fibers.

Declared effects may concern scalp moisturization and comfort, visible flaking, greasiness, hair loss, hair density, diameter, or growth.

Before interpreting the study, these endpoints should be separated.

A reduced sensation of scalp dryness is not a measurement of hair follicle activity. The reduction of visible buildup is not proof of new hair growth.

Similarly, the ability of an ingredient to bind to keratin does not mean it reaches the living cells of the follicle.

Topical peptide delivery is a separate scientific issue

For a topically applied peptide to affect a biological target in the scalp or hair follicle, several consecutive stages may be important:

the peptide is in the formulation → remains stable → reaches the correct site → maintains proper integrity → interacts with the biological target → a measurable hair effect appears.

Proof of one stage does not automatically confirm all subsequent ones.

Direct exposure of cultured cells to the peptide bypasses many practical problems associated with delivering the substance from a topically applied product.

Therefore, the results from cell cultures should not be described as if the user recreated the same conditions by applying serum to the scalp.

Follicle research may indicate promising mechanisms

The study of the five-amino-acid peptide GLYYF analyzed the activation of adiponectin receptor 1. [6]

The study involved experiments on cells related to human hair follicles, ex vivo hair growth studies, and experiments on mice.

These results indicate a mechanistic direction for research on GLYYF.

However, an important distinction must be made:

human-derived cells ≠ clinical trial in humans.

Similarly:

human hair follicles maintained ex vivo ≠ people using available scalp serums.

Human hair follicles studied ex vivo are a valuable model, but they are still not an in vivo study

Ex vivo human hair follicle models can provide information that simpler cell cultures cannot. They retain a larger portion of the follicle architecture and allow, among other things, the observation of hair elongation.

However, the hair follicle has been removed from the living environment of the scalp. It is no longer subject to the same influences from the circulatory system, the immune system, the hormonal environment, the skin barrier, or the physiology of the entire body.

Ex vivo data can therefore reinforce mechanistic studies, but they do not thereby become the equivalent of a human clinical trial.

Research on AP collagen peptides is another example of preclinical data

Another study on AP collagen peptides involved cell experiments, cultured human hair follicles, and animal models. [7]

Researchers described changes related to growth pathways and hair-related results in these models.

These are data relevant from the perspective of biological research, but they should remain designated as preclinical and ex vivo evidence until they are confirmed in appropriate clinical trials in humans.

The result regarding the biological pathway does not prove a clinical effect

Terms such as Wnt, beta-catenin, growth factor, adiponectin receptor, dermal papilla, or anagen can make the result sound very clinically advanced.

However, these are the names of biological pathways, structures, or stages.

They do not automatically prove a visible benefit in humans.

The peptide can alter the signaling marker in cells without causing a significant increase in hair density on the human scalp. Similarly, the hair cycle response in mice does not necessarily correspond to human hair loss disease.

The route of administration must remain part of the interpretation

Research on hydrolyzed keratin clearly shows why the route of exposure matters.

The work analyzed material administered orally to mice, as well as the results of cellular studies. [8]

It has not demonstrated clinical hair regrowth following the use of keratin shampoo, topical keratin serum, or any other topical peptide product.

Therefore, the route of administration, model, and endpoint should always be provided together.

Omitting the route of administration can make the data look more relevant to the topical product than they actually are.

User reviews do not allow the peptide's effects to be isolated

A consumer review may describe reduced greasiness, easier styling, less tangled hair, a better scalp feel, or the impression of greater volume.

Such observations can be useful as user experience information, but they do not independently confirm the peptide's efficacy.

The result may be influenced by, among other things, lighting, styling, hairstyle, hair length and color, frequency of washing, simultaneous use of other products, natural fluctuations, seasonal hair loss, or user expectations.

Similar restrictions apply to „before and after” photos if the conditions under which they were taken have not been standardized.

Photos are more informative when standardized

Photography becomes more useful when the camera, lighting, angle, distance, hair arrangement, parting placement, styling, and image analysis method are controlled.

Even then, a defined endpoint and an appropriate comparison group are needed.

An impressive photograph on its own does not indicate which ingredient is responsible for the observed change.

Multi-ingredient serums require data for the entire formulation.

A multi-peptide serum can contain many ingredients that affect the hair or scalp.

Therefore, one cannot simply add up studies on individual ingredients and consider them proof of the effectiveness of the entire mixture.

If Study A shows that peptide X binds to keratin, Study B indicates that peptide Y affects cells, and Study C demonstrates a change in a laboratory marker under the influence of plant extract Z, this does not automatically mean that a product containing X, Y, and Z increases hair density in humans.

The ready formulation requires data regarding this exact formulation.

Shampoos and hair loss research

Studies of shampoos may concern the deposition of ingredients rather than the growth of hair

Shampoos are rinse-off products. Their effectiveness depends, among other things, on surfactants, dilution, contact time, water, friction, rinsing, and the system for depositing the ingredients.

The peptide contained in the shampoo can therefore be studied for retention, conditioning, film formation, or interaction with existing fibers.

It doesn't have to be an examination of the hair follicles.

The study of the keratin hydrolysate of feathers included shampoo and conditioner formulations and an evaluation of the fiber properties.[1] The studies of modified wheat protein also concerned the system associated with the shampoo.[3]

These are data relevant to formulation technology and hair properties, and not clinical studies confirming the treatment of hair loss.

„Peptide shampoo” does not define the level of evidence

The term „peptide shampoo” describes a category of product.

It does not tell us which peptide is used, in what concentration, whether it settles on the hair, or reaches the scalp or follicles, nor whether it affects hair growth.

The decision on whether a claim is justified is determined by the actual composition and the examination.

If the shampoo study was evaluated solely on surface deposition, the evidence relates specifically to deposition.

The word „peptide” does not change this endpoint into evidence of hair regrowth.

Products that are rinsed off and remain on the skin are not equivalent.

The serum left on the skin usually provides a different exposure profile than the shampoo rinsed off.

However, this does not mean that:

longer contact = automatically greater effectiveness.

The concentration of the peptide, the solvent system, chemical stability, deposition, penetration through the scalp, the possibility of irritation, and the intended target of action may also be important.

A reliable comparison of formats would require testing the relevant products using comparable endpoints.

Contact time is only one element of the exhibition

The study of collagen hydrolysate retention showed that adsorption depended, among other things, on the time of application and the properties of the other components of the formulation.[4]

This illustrates an important principle: exposure depends on more than just time.

Short contact with strong fixation may differ from longer exposure with poor retention.

The full context of the exhibition is determined by the formulation technology.

Oral study does not confirm the activity of the topical product

The clinical study may be important for the study of hair, but at the same time it is not suitable as evidence for a topical product.

The route of administration is part of the intervention.

In a randomized, placebo-controlled study from 2026, the oral consumption of low-molecular-weight collagen peptides was evaluated for 24 weeks in Korean adults with damaged hair but no hair loss.[9]

Compared to placebo, improvements were noted in several parameters related to the condition of the hair.[9]

This is a study in humans, but it concerns a specific oral intervention.

It does not confirm the effectiveness of topical, shampoo or scalp serum action, nor of hair loss treatment.

The population is just as important as the delivery route

The oral collagen peptide study involved people with damaged hair who did not have alopecia.[9]

This is an important restriction.

A positive result in such a population cannot be automatically converted into evidence regarding the treatment of androgenic alopecia, plaque alopecia, telogen hair loss, or other types of hair loss.

The population being studied must correspond to the claim we want to formulate.

Damage to the hair, brittleness, loss and baldness are not the same thing

These terms describe different problems.

Hair damage primarily affects the fiber itself. Brittle hair means physical breakage of the stem. Hair loss involves the shedding of hair from the follicles, while baldness encompasses various forms and causes of hair loss.

The study can be very important for one of these issues, while at the same time providing little information about the other.

A broad definition of „hair health” can blur these differences.

Claims regarding hair loss require appropriate endpoints

Depending on the research question, the appropriate results may include standardized hair counts, the number of hairs per specific area of the scalp, the ratio of terminal to medullary hairs, the diameter of the shaft, standardized photography, the number of hair loss episodes, and measurements of the anagen and telogen phases.

The claim of increased density should not be based solely on gloss or a subjective impression of greater volume.

Similarly, the result regarding less fragility should not be presented as a treatment for hair loss associated with ingrown hairs.

How can evidence regarding different product formats be compared?

Compare the material, goal and endpoint

A good comparison starts with three questions:

1. Was the same material studied?
2. Were they targeting the same biological or physical target?
3. Was the same result measured?

If the peptide sequences are different, equivalence cannot be assumed automatically.

If one experiment concerns isolated hair fibers, and the other cellular structures, they investigate different targets.

If one study measures shine and the other measures hair density, the percentage results cannot be placed on a single scale of effectiveness.

Consider the route of exposure

In the case of many claims regarding hair, one more question must be asked:

Was the route of the exhibition the same?

Oral administration, rinsing the product, serum remaining on the skin, direct exposure of cells, and ex vivo hair follicle testing are different interventions.

Changing the route of exposure can significantly limit the ability to transfer applications from one study to another product.

Before evaluating the percentage result, check the comparison group.

The percentage alone says little about the quality of the evidence.

The result compared to untreated fiber differs from the result obtained with the appropriately matched base formulation. The change from the initial value is not the same as the difference in the randomized placebo-controlled study.

The comparative group helps determine which alternative explanations remain possible.

A large change in a study without control does not have to be stronger evidence than a smaller one, but it is a well-controlled effect.

Check what exactly the percentage refers to

The statement „the condition of the hair improved by 20%” is incomplete.

Twenty percent of what?

It could be about tensile strength, shine, combing strength, brittleness, diameter, density, number of hairs, or the participants' assessment.

The significance of the result is determined by the unit, the measurement method, the denominator and the comparison group.

Applying a cosmetic product to the hair is not the same as producing thicker hair

The measured increase in fiber diameter can have various causes.

If a coating remains on the existing hair, it may appear thicker. This is a different phenomenon than the formation of a new, naturally thicker hair shaft by the follicle.

A reliable description should indicate where the diameter was measured, whether product remained on the hair, whether a newly formed hair was analyzed, and when the measurement was performed.

Visual volume is not the same as hair density

„Fuller” or „thicker looking” hair is often a visual or subjective description.

The density of the scalp usually requires a standardized measurement in a specific area.

The product can make the hair look fuller without increasing the number of hairs growing from the follicles.

Both results may be significant, but they are not equivalent.

Consider durability

The temporary conditioning effect may still be useful.

However, the effects of temporary and permanent ones should not be described in the same way.

Important questions include:

• Does the effect remain after washing?
• Does it require regular application?
• Does it disappear after removing the product?
• Does it persist after use is discontinued?

The answers require separate data.

Tolerance and effectiveness are two different concepts

The material, which is well-suited to insulated hair fibers, was not automatically considered suitable for long-term use on the scalp.

Similarly, good short-term skin tolerance does not confirm effectiveness in the area of hair growth.

In a 24-week study of oral collagen peptides, no adverse events related to the intervention under study were reported in the analyzed population [9].

This observation applies, however, to a specific oral intervention and a specific population. It does not confirm the safety of a topical product containing another peptide.

Practical scheme for evaluating peptide products for hair

1. Determine what material was used

Is it a defined peptide, a mixture of peptides, hydrolyzed keratin, hydrolyzed collagen, hydrolyzed wheat protein, or a multi-component formulation?

2. Determine the route of the exposure

Was the material applied to isolated hair, used in shampoo or conditioner, applied as a product left on the scalp, taken orally, administered directly to cells, or tested on mice ex vivo?

3. Set a goal

Was the surface of the hair, the internal structure of the fiber, the scalp, the cells of the epidermis, the tissue of the scalp, or the appearance of the hair on the entire person examined?

4. Determine the endpoint

Was the placement measured, the strength of the combing, mechanical properties, brittleness, gloss, diameter, number of hairs, density, signaling trails, lengthening of the comb, or the evaluation of the participants?

5. Check the comparison group

Were the hair treatment being compared with a non-administered treatment, a baseline formulation, a placebo, an initial value, another active product, or no control?

6. Check the model or population

Do the data come from isolated fibers, cells, human ex vivo hair follicles, mice, healthy volunteers, people with damaged hair, or people with a specific type of hair loss?

7. Check the duration

A short laboratory exposure and multi-month clinical observation answer other questions.

8. Evaluate durability

Was the result measured only immediately after the procedure, or also after subsequent washes and the observation period?

9. Evaluate tolerance

Was the material under investigation evaluated under conditions corresponding to the declared method of exposure?

10. Do not go beyond the limits of the research findings

Do not confuse:

binding with keratin → for hair growth,

conditioning → in the treatment of hair loss,

the result in mice → in the effectiveness in humans,

I

oral studies → to prove the effectiveness of the topical product.

Frequently asked questions

1. Do thicker or fuller-looking hair mean new growth?

No. Hair that is thicker or fuller in appearance may result from changes in existing fibers rather than the production of additional hair. Conditioning, depositing ingredients on the surface, reducing brittleness, increasing diameter due to a coating, better styling or styling can change the appearance of the hairstyle. Research on hydrolates on hair fibers primarily concerned material and surface properties rather than the formation of new hair from follicles.[1] The claim regarding growth requires appropriate measurements of hair follicles or measurements of hair on the scalp.

2. Can the results of the shampoo test be applied to the serum for the scalp?

Not automatically. Shampoo and serum can differ in the base, concentration, contact time, rinsing, deposition, site of action, and type of exposure. Studies of collagen hydrolysate retention showed that formulation composition and application conditions influenced adsorption.[4] An experiment with shampoo on fibers does not therefore confirm the delivery of ingredients from the serum remaining on the skin to the follicles, nor its clinical effect.

3. Is hydrolyzed keratin the same as a defined peptide?

No. Hydrolysed keratin usually consists of a mixture of fragments whose size and sequence depend on the source and the manufacturing process. A defined peptide has a specific amino acid sequence. In the study of feather keratin hydrolysis, the degradation of the peptide sizes present in the material was characterized.[1] The results regarding this hydrolysis cannot be automatically attributed to each component labeled as hydrolyzed keratin.

4. Do all peptides bind to hair equally well?

No. The sequence, loading, hydrophobicity, size, and other physicochemical properties can influence the interaction with keratin. A study of 1235 decapeptids derived from hair proteins showed differences in keratin binding depending on the properties of the individual peptides.[2] However, this study was conducted using isolated keratin and does not confirm the efficacy of the finished cosmetic for each of these peptides.

5. Does a smoother image of the hair cuticle indicate that the hair has become mechanically stronger?

No. Microscopy and mechanical tests answer other questions. Microscopy can show surface features and deposits, whereas strength requires proper testing using force. Studies of hydrolysates provided surface observations relevant to conditioning. [1] [3] A rougher shell does not indicate specific tensile strength or fracture resistance if these parameters are not measured.

6. Does the term „bond repair” mean the repair of all damaged connections?

No. „Bond repair” requires defining what type of bond is being discussed. The peptide backbone and the cross-linking disaccharides of keratin are different chemical structures. Studies on a single interaction do not demonstrate the repair of all damaged bonds. In the PepACS study, the specific interaction of a designed peptide with keratin and selected fiber properties were analyzed. [5] This does not constitute validation of any product advertised as „bond repair”.

7. Does less fragility mean less hair loss?

No. Fragility means the breaking of the stem along its length, whereas shedding occurs when the hair is released from the follicle. The conditioner can reduce fragility without changing the hair cycle. Research on keratin hydrolysate primarily concerns the properties of the fiber, not the clinical hair loss. [1]

8. Does a multi-peptid serum have to work better than a product with a single peptide?

No. The sheer number of peptide components does not necessarily indicate greater effectiveness. What matters are the identity and sequence of the peptide, its concentration, stability, interactions with other components, the vehicle, and the site of action. Peptide binding studies with keratin show that different structures may behave differently.[2] A mixture of many peptides therefore requires data regarding that particular mixture.

9. Is the result obtained in human skin wart cells a clinical study in humans?

No. Cells of human origin tested in the laboratory are not the same as those used in a ready-made product. Direct exposure of the cells bypasses many barriers associated with the local delivery of the ingredient through the scalp. The GLYYF study included cell models, ex vivo mice, and not a clinical study of scalp serum. [6]

10. Does hair regrowth in mice prove efficacy in androgenic alopecia in humans?

No. The mouse hair cycle biology, exposure conditions, physiology, and disease models may differ from human hair loss. AP collagen peptide studies have shown growth-related results in cultured cells, germ cells, and animal models.[7] They may warrant further research, but do not confirm clinical efficacy in individuals with androgenic alopecia or other specific conditions.

11. Can an oral collagen test confirm the effect of a peptide shampoo?

No. Oral administration and topical use of shampoo are different interventions. In a 24-week collagen peptide study, Korean adults with damaged hair but no hair loss participated. [9] Neither shampoo nor scalp serum was studied, and no hair loss treatment was shown.

12. Does a product that soothes the scalp also need to stimulate hair growth?

No. Head skin comfort and growth are different endpoints. The product can reduce the sensation of dryness, oiliness, or discomfort without increasing hair production. GLYYF studies concerned biological processes associated with growth in experimental systems.[6] They do not constitute evidence for unrelated soothing products.

13. Does the serum left on the skin always work better than the rinsed-off shampoo?

No. Longer contact time does not automatically mean greater effectiveness. Concentration, stability, base, deposition, delivery to the site of action, and tolerance are also important factors. Studies on the retention of hydrolysates have shown that exposure conditions and other components of the formulation influence adsorption.[4] This does not mean that every leave-on product is more effective than every shampoo.

14. Does greater shine mean more hair on the scalp?

No. Gloss is primarily an optical property. The hair surface, its smoothness, coating, and arrangement can affect light reflection without changing the hair follicle density. In a study of oral collagen peptides, gloss was one of the parameters of hair condition in people without hair loss.[9] The gloss result should remain the gloss result.

15. Is there a proven method for using all multi-peptidic serums?

No. The available data do not define a single universal scheme for different formulations. Products differ in the application site, concentration, method of administration, vehicle, stability, and exposure. Simulation studies of application show that the formulation and contact time can influence retention. [4] However, this does not create a universal protocol for the use of all hair peptides.

16. Can a hair fiber test confirm scalp tolerance?

No. Isolated hair fibers do not reflect the living scalp. The material can have a beneficial effect on the fibers without being evaluated for long-term contact with the skin. The PepACS and hydrolizate studies provide information about interactions with the hair and material properties, not about universal safety for the scalp.[1][5]

17. How best to evaluate claims regarding hair peptides?

First, it is necessary to determine which material was studied, by what route it was administered, what the target, model or population, comparator group and endpoint were. Then it is necessary to check whether the conclusion remains within these limits. The peptide binding assay, the mouse experiment, the in vivo study and the oral clinical study answer different questions.[2][7][9]

18. Can the product make the hair look thicker without producing thicker new hair?

Yes. The coating, the deposition of material, the surface modification, the arrangement of the fibers, or the reduction of fragility can all affect the visual thickness of existing hair. This is not equivalent to indicating that the follicles have started producing new hair of a larger diameter. To make such a conclusion, it is necessary to properly measure the newly formed hair.

19. Does the keratin-binding peptide have to penetrate into the hair’s interior?

No. Bonding to keratin and penetration into the inner part of the hair fiber are separate phenomena. The material can remain on the surface or in the outer layers of the hair. The study of decapeptids concerned binding to isolated keratin. [2] It did not automatically show deep penetration of these peptides into intact hair.

20. Does the implantation of a peptide on the hair mean that it reaches the scalp?

No. The hair and follicle surfaces are different places. The substance can settle on the existing follicle without reaching the living epithelial cells. The claim regarding action on the follicle requires data regarding the appropriate exposure site.

21. Is the ex vivo result stronger than a conventional cell study?

The ex vivo model can retain a greater portion of tissue architecture than isolated cell culture, and therefore can provide additional information. However, it still does not replicate the entire living organism. Human ex vivo cultures do not have the same circulation, hormonal environment, immunology, or exposure barrier as the ones found in the human scalp.

22. Does the positive result regarding the Wnt pathway or beta-catenin mean hair growth in humans?

No. The result regarding the biological pathway is primarily a mechanistic result. It can indicate a potential direction of action and justify further research, but it does not replace clinical measurement of hair growth or density in humans.

23. Does „hydrolysed protein” always mean the same peptide composition?

No. The hydrolysis process, the source of the protein, and subsequent modifications affect the breakdown of the size of the fragments, their sequences, charge, and properties. Therefore, two components with similar names may not be chemically or functionally equivalent.

24. Do the studies on hair peptides prove that peptides treat baldness?

No. The literature discussed includes many different types of studies: fiber conditioning, keratin binding, deposition, mechanical properties, cellular models, ex vivo scaffolds, animal models, and specific human studies. They do not constitute a single coherent evidence for the treatment of hair loss by peptides as a whole category.

Limitations of available evidence

Research on peptides and hair has several recurring limitations.

First of all, „peptides” do not constitute a single, uniform component. Hydrolysates, defined sequences, chemically modified peptides, and multi-peptide mixtures can have completely different properties.

Secondly, the research models are very diverse. Isolated hair fibers, isolated keratin, cells, ex vivo scaffolds, mice, and humans respond to different questions.

Thirdly, the route of exposure is essential. Data regarding oral administration do not confirm local action, and direct exposure of cells does not replicate the use of the cosmetic on the scalp.

Fourthly, the effects regarding the fiber are not the same as the effects regarding the hair. Less breakage, greater shine, a smoother surface or greater visual volume can improve the appearance of the hair without increasing the number of follicles.

Fifthly, a positive binding or attachment result does not prove clinical efficacy. Each subsequent step of the evidence chain requires its own data.

Sixthly, populations with damaged hair should not be equated with people with baldness. A positive result in people with damaged hair does not automatically constitute evidence of hair loss treatment.

Seventhly, multi-component formulations make it difficult to attribute an effect to a single component. If a serum contains several peptides and other functional substances, improvements observed without proper control cannot be automatically attributed to a specific peptide.

After eight, laboratory tests on the fibers do not confirm the tolerance of the living scalp.

Ninthly, the result immediately after application does not yet tell how long the effect will last. Repeated washing and longer observation may be necessary to assess its durability.

Finally, the studies discussed represent different parts of a broad field, rather than a standardized system of evidence. Each publication should be interpreted separately in relation to the material being studied, the route of exposure, the purpose, the model, the comparator group, and the endpoint.

Summary

Peptides play several distinct roles in hair research and hair care products.

Protein hydrolysates can contain mixtures of peptides that interact with existing fibers. Defined sequences can be tested for binding to keratin or designed to interact with specific hair structures. Other peptides are studied in cells, ex vivo tissues, and animal models to assess their potential impact on biological pathways related to growth.

These categories should remain separate.

The hair shaft ≠ the hair follicle.

Conditioning ≠ hair growth.

Fragility ≠ falling out of the nest.

Visual volume ≠ confirmed increase in hair density on the scalp.

Bonding with keratin ≠ clinical effectiveness.

Placement on the surface ≠ delivery to the residence.

Cell signaling ≠ clinical outcome in humans.

Ex vivo human tissues ≠ in vivo human testing.

The growth of hair in mice ≠ evidence of efficacy in human baldness.

The study of oral peptides ≠ confirmation of the effectiveness of serum or shampoo.

Multipeptide composition ≠ automatically greater effectiveness.

Selected publications, however, show that studies of peptides and hydrolizates can provide valuable information about interactions with hair fiber, deposition on the surface, conditioning, mechanical properties, designed interactions with keratin, resident signaling, ex vivo hair follicle behavior, hair biology in animal models, and selected results regarding hair condition in humans.

The most reliable interpretation should always take into account:

material + exhibition route + goal + model or population + comparison group + result + duration + limitations.

This approach allows us to discuss research on hair peptides without overlooking promising results, but also without attributing them greater significance than actually results from the data.

Disclaimer

The article is purely educational and scientifically informative. It discusses the chemistry of peptides, conditioning of hair fibers, cosmetic formulations, research on products for the scalp, hair follicle models, preclinical studies, and published studies involving humans.

It does not constitute medical, dermatological or hair loss treatment advice, supplementation or the use of specific products. It is also not a recommendation regarding any peptide, shampoo, conditioner, scalp serum, oral collagen supplement or any other intervention.

Results obtained from isolated hair fibers, laboratory cells, ex vivo tissues, animal models, when administered orally or in a single formulation study, should not be automatically transferred to another route of administration, product, population, or type of hair loss.

Individuals who experience persistent or unexplained hair loss, scalp diseases, or other problems that may have a medical basis should consult with a suitably qualified specialist.

References

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2. Cruz, C. F., Azoia, N. G., Matamá, T., & Cavaco-Paulo, A. (2017). Peptide-protein interactions within human hair keratins. International Journal of Biological Macromolecules, 101, 805–814. https://doi.org/10.1016/j.ijbiomac.2017.03.052
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