Exploring the Amino Acid Composition of Human Hair

Introduction Human hair is a remarkable biomarker, offering insights into a person’s genetic makeup, environmental interactions, and even health status. At its core, hair is composed of proteins, primarily keratins, which are themselves made up of various amino acids. The amino acid composition of hair is a subject of great scientific interest, as it holds keys to understanding numerous biological and physiological processes.

Amino Acids in Hair: Analysis and Challenges The analysis of amino acids in hair is typically done through a chemical extraction process called acid hydrolysis. However, this process can destroy certain amino acids, like tryptophan, posing challenges in accurately determining their presence. Despite these hurdles, extensive research has established the average amino acid content in human hair – as well as hair from a few other mammals such as sheep. A lot of research was done on sheep wool and its amino acid content back in the 1970-80s as part of research into improving wool quality for clothing.

Genetic Determination and Ethnic Variability The amino acid sequence in human hair proteins is genetically predetermined, leading to specific protein compositions. Interestingly, no significant differences have been found in the amino acid composition of hair across different ethnic groups, suggesting there is a universal blueprint in human hair protein structure.

Keratins and Cystine: The Backbone of Hair Structure Keratins, the predominant proteins in hair, are known for their high cystine levels. Cystine, a dimer of the amino acid cysteine, is crucial for creating the disulfide bonds that give hair its strength and resilience. The diversity in keratin types, influenced by their location in different hair structures like the cuticle, cortex, or medulla, adds to the complexity of hair’s amino acid composition.

Medulla, Cuticle, and Cortex: Varied Amino Acid Profiles

  • Medulla: The medulla of the hair, is a column of loosely packed keratinized, dead cells in the center of a large terminal hair fiber. Though not always present and difficult to analyze, it has been found to have low cystine and hydroxy amino acids, with higher levels of basic and acidic amino acids.
  • Cuticle: The cuticle is the outermost layer of the hair fiber and can be subdivided into the exocuticle on the very outside of the hair fiber, and the endocuticle just underneath the exocuticle. The hair cuticle shows a stark contrast in amino acid composition between its two main layers. The endocuticle has a low cysteine content, but it is rich in hydrophilic amino acids, while the exocuticle is cysteine-rich, particularly in its outer layers. Cysteine gives hair strength so it makes sense to see high levels of it in the exocuticle layer that is most exposed to environmental factors (water, shampoo, air pollution, etc).
  • Cortex: The cortex of the hair provides the bulk of a hair fiber’s structure and provides strength, hair shape, and incorporates pigment. It consists of various protein classes, each with distinct amino acid profiles. Intermediate Filament (IF) keratins, High Sulfur (HS) Keratin Associated Proteins (KAP), Ultra High Sulfur (UHS) KAP, and High Glycine-Tyrosine (HGT) KAP exhibit varying ratios of amino acids, influencing the physical properties of hair.

The Role of IF Keratins and KAP in Hair Structure IF keratins form the structural backbone of hair. The chemistry is complicated (and beyond my understanding), but IF keratins have a unique amino acid composition with a central domain exhibiting high homology. These proteins, along with the diverse families of KAP, contribute to hair’s structural integrity and physical characteristics like curliness or straightness. If you want to know the chemistry, please check out some of the references in the bibliography below – most of them go into the protein structure and amino acid composition in some detail.

Conclusion The amino acid composition of human hair is a testament to the complex nature of biological proteins. Understanding this composition not only provides insights into the structural and functional aspects of hair but also opens avenues for exploring genetic variations and potential applications in forensics, health diagnostics, and even cosmetic science. As research continues to unravel the intricate details of hair’s proteomic landscape, we stand to gain a deeper appreciation of this ubiquitous yet extraordinary human feature.

Below is a table of the amino acids present in normal human hair in order of their quantity. Note, this table is a synthesis of several different sources and simplifies the composition into a basic table. Different publications give somewhat different numbers, so check the original references listed below if you want to see the details.

Amino AcidAmount in percent (residues extracted per 100 residues)
Glutamic acid11.1
Aspartic acid5.0


Lee LD, Baden HP. Chemistry and composition of the keratins. Int J Dermatol. 1975 Apr;14(3):161–71.
Lindley H. The Chemical Composition and Structure of Wool. In: Asquith RS, editor. Chemistry of Natural Protein Fibers. Boston, MA: Springer US; 1977. p. 147–91.
Baden HP. Biochemistry of hair protein. Clin Dermatol. 1988;6(4):22–5.
Hollfelder B, Blankenburg G, Wolfram LJ, Höcker H. Chemical and physical properties of pigmented and non-pigmented hair ('grey hair’). Int J Cosmet Sci. 1995 Apr;17(2):87–9.
Leroy AF F. Hair Structure, Function, and Physicochemical Properties. In: The Science of Hair Care. Boca Raton: CRC Press - Taylor & Francis Group; 2005. p. 1–74.
Kuzuhara A. Analysis of structural changes in bleached keratin fibers (black and white human hair) using Raman spectroscopy. Biopolymers. 2006 Apr 15;81(6):506–14.
Robbins CR. Chemical Composition of Different Hair Types. In: Robbins CR, editor. Chemical and Physical Behavior of Human Hair. Berlin, Heidelberg: Springer; 2012. p. 105–76.