Lipid Composition of Human Hair: An Overview

Human hair is an intricate filamentous structure that extends from the hair follicle and is composed primarily of proteins (keratins), water, trace elements, and lipids. Although the protein component of hair has historically received the bulk of scientific attention, a growing body of research has demonstrated the critical importance of lipids to hair health and integrity. Lipids within the hair shaft influence moisture retention, tensile strength, elasticity, and surface gloss, acting as both a barrier and a structural component. They also moderate fluid permeability, thus protecting hair from physical and chemical insults.

In recent decades, advances in analytical techniques such as gas chromatography–mass spectrometry (GC–MS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and synchrotron-based Fourier-transform infrared (FTIR) imaging have shed light on the diverse lipid species in human hair. These lipids are often categorized as endogenous (arising within the hair follicle and hair matrix) or exogenous (migrating from the sebaceous glands and scalp surface). The present article summarizes current understanding of the composition and distribution of hair lipids, explores ethnic and disease-related differences, and describes how physical or chemical damage affects the hair’s lipid content.

Overview of Hair Lipids: Human hair lipids can be broadly divided into exogenous and endogenous subclasses:

1. Exogenous Lipids
   • Free Fatty Acids (FFAs)
   • Triglycerides
   • Cholesterol (CH)
   • Wax Esters
   • Squalene (SQ)

These lipids typically originate from sebaceous secretions. They permeate or adsorb onto the hair shaft, especially near the root, and may travel distally along the fiber length over time.

2. Endogenous Lipids
   • Cholesterol (CH)
   • Ceramides (CERs)
   • Cholesterol Sulfate (CS)
   • 18-Methyleicosanoic Acid (18-MEA)
   • Glycosylceramides

Synthesized within the hair follicle itself, endogenous lipids are embedded in the hair matrix and cell membrane complex (CMC). In particular, 18-MEA is covalently bound to the hair’s outermost surface and imparts hydrophobicity to the fiber.

Collectively, these lipid classes comprise anywhere from 1%–6% of the fiber, depending on ethnicity, hair layer, and age. Although modest in amount, hair lipids play a pivotal role in maintaining shaft integrity, shine, flexibility, and moisture balance.

Distribution in Different Hair Layers: From the outside in, the hair shaft is organized into the Cuticle, Cortex, and the Medulla (in larger, terminal hairs)

• Cuticle: The cuticle consists of overlapping, flattened cells that protect the cortex beneath. It has several sublayers, including the epicuticle, exocuticle, and endocuticle. Within the outermost region (the epicuticle), 18-MEA is covalently bonded and forms a hydrophobic monolayer that repels water. As an integral part of the CMC in the cuticle, these lipids help guard the cortex against external chemical and physical stressors. Much research has shown that when 18-MEA is lost (for instance, through bleaching or chemical straightening), hair becomes more prone to tangling, frictional damage, and dryness.
• Cortex: The cortex is the largest component of the hair shaft, composed of keratinized cells tightly packed in a longitudinal orientation. Within and between these keratin-rich cells lie lipids of both endogenous and exogenous origin, including ceramides, cholesterol, and free fatty acids. In some studies, oxidative metabolites of polyunsaturated fatty acids (e.g., linoleic or alpha-linolenic acid) have been detected, especially near melanin granules. Although the cortex has a lower overall lipid concentration than the cuticle or medulla, these internal lipids are essential in influencing the hair’s tensile strength and moisture retention.
• Medulla: The medulla, the innermost layer of thick terminal hairs, is composed of loosely organized cells and air-filled vacuoles, sometimes described as a spongy core. It frequently contains squalene, wax esters, and free fatty acids such as palmitic or oleic acid. Research indicates that the medulla’s lipid content can affect hair’s optical properties, such as gloss. In particular, some findings suggest that unsaturated lipids in the medulla contribute to the reflection and scattering of light, influencing overall luster.

Ethnic Variation in Hair Lipid Content: Human hair can be broadly grouped into three major ethnic categories for scientific comparison: Asian, African, and Caucasian. Variations in lipid profiles are closely linked with differences in hair shape, diameter, and mechanical properties. Several investigations have shown that:

• African hair: Tends to have the highest total lipid content (up to 6%). Fibers are generally elliptical and curly or coiled, with variable cross-sectional diameters. Lipid order in the cuticle is low, leading to relatively high permeability. This can allow for rapid water uptake but also means the hair can be more fragile and prone to moisture-related damage.
• Caucasian hair: Intermediate total lipid content (roughly 3%). Often oval in cross section, with varying degrees of waviness. Generally has highly ordered lipids in the cuticle, which can confer increased hydration and mechanical resilience. Some studies note that Caucasian hair more readily absorbs water without necessarily weakening.
• Asian hair: Lowest total lipid content of the three major groups (around 2%). Typically circular in cross section, resulting in naturally straight, thick fibers. Exhibits a strong, robust cuticle that resists ultraviolet radiation and some chemical treatments better than African or Caucasian hair. Some researchers have observed high levels of integral hair lipids (IHL) that may protect the cortex from oxidative stress.

Despite these broad trends, one should note that individual variation within an ethnic group is common. Nevertheless, these comparisons highlight how lipid composition can influence mechanical strength, moisture retention, and the characteristic shape of ethnic hair types.

External and Chemical Damage to Hair Lipids: Hair is routinely exposed to a range of damaging processes, including chemical treatments (bleaching, dyeing, perming, straightening), surfactant-based shampoos, ultraviolet radiation (UVR), and everyday grooming. Many of these factors accelerate lipid loss.

1. Bleaching and Dyeing: Bleaching agents oxidize cysteine residues in keratin to cysteic acid, resulting in the rapid removal of 18-MEA from the cuticle surface. This leads to higher surface friction, dryness, and decreased manageability. Dyes can also disrupt surface and intracellular lipids, albeit sometimes to a lesser extent, especially with repeated treatments.
2. Permanent Waving (Perming) and Straightening: Permanent waving solutions and chemical relaxers often lift or disrupt the cuticle, facilitating the partial extraction of ceramides, free fatty acids, and 18-MEA. A key outcome is increased porosity in the hair shaft, which contributes to brittleness and breakage, particularly noticeable in African hair if performed frequently.
3. Surfactant Use: Shampoos and cleansing agents rely on surfactants that can remove hydrophobic lipids (like squalene or wax esters) via penetration into the hair shaft and amphipathic lipids (like cholesterol) via “roll-up” mechanisms on the fiber surface. Repeated washing strips away natural lipids, potentially leading to dryness and dullness—especially if not offset by conditioners or lipid-rich hair care products.
4. Ultraviolet Radiation: UV exposure degrades structural lipids, including 18-MEA, and can destabilize the cuticle. Studies using synchrotron-based FTIR imaging show that after a summer’s worth of UV exposure, over 90% of 18-MEA may be removed from certain hair types. This leads to a rougher hair surface and a shift in color or shine.
5. Aging and Graying: As hair ages, both endogenous and exogenous lipid content decreases, including ceramides and dihydroceramide. Gray hair generally has reduced unsaturated lipids, making it less able to retain water and sometimes contributing to its coarser or “stiffer” texture.

Disease Associations: Beyond cosmetic implications, shifts in hair lipid composition have been studied in various dermatologic and systemic diseases:

• Alopecia (especially androgenic alopecia): Observations point to elevated total lipid and lipoprotein content, sometimes making the hair appear greasier.
• Acne: Individuals (particularly women) with acne may have significantly higher cholesterol sulfate (CS) and dehydroepiandrosterone-sulfate (DHEAS) in scalp hair than healthy controls.
• Cancers (Breast, Gastric, Prostate, Lung, Rectal): Multiple researchers have reported an increase in certain phospholipid or wax ester signatures in hair from patients with these cancers. X-ray diffraction (XRD) methods and Raman spectroscopy can detect these lipid “fingerprints.” In breast cancer, a distinct ring in the hair diffraction pattern, thought to arise from disease-associated phospholipids, has been proposed as a potential early screening tool.
• Hypercholesterolemia: One study found a strong positive correlation between high serum cholesterol and hair cholesterol content (r = 0.86), suggesting that systemic lipid disorders may be reflected in the shaft. However, conflicting data from other studies highlights the need for further validation.

Analytical Challenges and Opportunities: Although ongoing efforts have greatly advanced hair lipid research, technical challenges remain:

1. Low Lipid Content: Because total lipid quantity is relatively low, high-sensitivity techniques (e.g., mass spectrometry, advanced chromatography, or synchrotron-based infrared imaging) must be used.
2. Layer-Specific Analysis: Dissecting lipid composition within the cuticle vs. cortex vs. medulla is difficult. Most extraction techniques aggregate lipids from the entire hair shaft, losing data on spatial distribution.
3. Longitudinal Variation: Lipid content can vary considerably from root to tip. Grooming habits, UV exposure, and product use complicate standardization unless hair is sectioned carefully for analysis.
4. Population Heterogeneity: Ethnicity-based groupings provide useful frameworks, but large inter-individual differences exist, implying that results from small cohorts may not be universally generalizable.

Nevertheless, novel approaches—such as confocal Raman microspectroscopy, atomic force microscopy-infrared (AFM-IR) imaging, and advanced x-ray scattering—are increasingly capable of probing hair’s ultrastructure and chemical composition in detail. These methods hold promise for refined diagnostics in both cosmetic science and disease screening.

Conclusion: Lipids are vital yet underappreciated components of human hair, impacting everything from moisture retention and tensile strength to luster and barrier function. Differences in lipid composition and organization partly explain the well-known textural and mechanical variations across African, Asian, and Caucasian hair types. External processes—such as bleaching, perming, dyeing, surfactant washing, and ultraviolet exposure—can degrade or remove these lipids, leading to dryness, breakage, and dullness.

On a broader clinical scale, alterations in hair lipids have been linked to dermatologic issues (e.g., alopecia, acne) and serious systemic diseases (e.g., breast and other cancers), spurring interest in using hair-based assays for non-invasive screening. Despite challenges in layer-specific analysis and potential confounders in patient populations, continuing advances in analytical methods have widened our understanding of hair lipid diversity. Future work on restoring or protecting endogenous lipids, especially in populations with highly fragile hair, will inform the design of hair care products and foster better clinical approaches to hair and scalp disorders. Through collaborative research among dermatologists, cosmetic scientists, and analytical chemists, hair lipids may one day become both key cosmetic targets and powerful diagnostic biomarkers.

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