Amino Acids in Hair Products

Updated: March 2019.

What Are Amino Acids:

Amino acids are very small molecules, they’re not even “proteins” on their own, they’re the constituents of proteins formed from an amine group (nitrogen-containing, and all things proteinaceous contain nitrogen), a carboxylic acid group (carbon, hydroxide or “OH” and oxygen) and a side chain with a varied number of carbons, nitrogens, hydrogens, occasionally sulfur. To have a protein, you put these amino acids together.©Science-y Hair Blog 2013

Where Are They:

In hair, there are many amino acids (one source I have lists 20). The cuticle of hair – it’s multi-layered outer coating of “scales,” contains more amino acids than in the inner parts of the hair fiber - partly because proteins break down into amino acids and there is always protein being broken down at the cuticle. The outermost layer of cuticle (which can be missing at the ends of very long or damaged hair) contains more cysteine forming cross-links with other proteins – creating a strong, resilient outer cuticle. Cysteine is a sulfur-rich amino acid. I imagine this like “cuticle glue.”  Also in high amounts in the cuticle are glutamic acid, proline and valine.

In the epicuticle (a protein-heavy, lipid-rich covering over the cuticle) there are large amounts of glutamic and aspartic acids and lysine.©Science-y Hair Blog 2013

Amino acid content in hair increases with exposure to UV light and to bleaching chemicals or “retexturizing” (permanent waves, chemical relaxers) – as breakdown products from the hair proteins resulting from these processes.

Do Amino Acids Stay on My Hair?©Science-y Hair Blog 2013
You may have heard that amino acids simply rinse away when they are included in a shampoo or conditioner. This is not entirely true. If you apply amino acids to hair mixed in only water, hair will take up as much as it can (there is a point at which the hair will stop adsorbing amino acids when it reaches equilibrium with the solution into which you’ve placed it). But if amino acids are present in a formula (like a conditioner) including cationic surfactants, adsorption of amino acids becomes more difficult because they interact with the other ingredients. In general, studies have shown that the addition of amino acids to a formula helps damaged hair regain it’s hydrophobicity or it’s water-repellent quality which means that amino acids help fill in porosities (gaps in the cuticle). Healthy, undamaged hair is also hydrophobic/water repellent. Amino acids can also strengthen and moisturize the hair because of their water-attracting habit.

Which Amino Acids Work Well (or “should” work well):©Science-y Hair Blog 2013

Of those tested, arginine was strongly attracted to hair. Histidine and phenylalanine improved the strength of hair, but only histidine, phenylalanine, glutamic acid, arginine, alanine, and isoleucine, were studied in the reference I am using. Many were left out!

Usually one finds amino acids listed as “silk amino acids” or "methionine" or “wheat amino acids.” Which should you look for?

Here is what I can find from manufacturers’ descriptions of amino acid additives regarding their constituent amino acids which are present at the highest percentages (they do contain a variety of other amino acids). The italics refer to amino acids which occur in high amounts in the cuticle and epicuticle of hair.©Science-y Hair Blog 2013

Silk amino acids: alanine, glycine and serine

Wheat amino acids: cysteine, glutamic acid

Oat Protein and derivatives: glutamic acid, aspartic acid, proline, alanine

Soy protein and derivatives: phenylalanine, tyrosine, leucine

Keratin amino acids: cysteine, serine, glutamic acid

Cysteine - check out those sulfurs!

Interior or exterior of hair? Amino acids tend to have molecular weights less than 200 g/mol. This is small enough to penetrate into the cortex of hair (and into skin as well). Amino acids can move into the hair's cuticle to help retain moisture for good hydration.

Other notes: If you are allergic to any of these ingredients (like wheat) or have an intolerance which causes severe physical symptoms, you may also need to avoid the hydrolyzed versions of the proteins. 13

Does it really work to try to “replace” amino acids in the hair with amino acids from an outside source? Difficult to say. Whether it is because the amino acid from a hair product takes on a functional role (literally replaces that which was lost and starts playing the same role) or just plays a supporting role, they are effective moisturizers and therefore do have benefits in rinse-off and leave-in products.

Sources:

Journal of Cosmestc. Science, 58, 347-357 , 2007

Hair and amino acids: The interactions and the effects

E. Oshimura, H. Abe, and R. Oota

Chemical and Physical Behavior of Human Hair 

Robbins, 1994. 3^rd Ed. Springer-Verlag, New York