Creative Touch Permanent Makeup and Aesthetics

Sebum is the lipid-rich secretion produced by sebaceous glands and delivered to the skin surface through the follicular canal, where it spreads to form a continuous surface film across the stratum corneum. It is not a single substance but a complex mixture whose composition is distinctive among mammals – and specifically among primates – in ways that matter for understanding both its functions and the limitations of any claim that a topical fat can replicate it. In its glandular form, sebum consists primarily of triglycerides (approximately 57%), wax esters (26%), squalene (12%), cholesterol and cholesterol esters, and smaller amounts of free fatty acids. By the time it reaches the skin surface, the triglyceride fraction has been substantially hydrolysed to free fatty acids by lipases from the cutaneous microbiota – predominantly Cutibacterium acnes – a transformation that is part of normal sebum function rather than microbial disruption of it. ^[3]

Composition and Distinctive Human Fatty Acids

The fatty acid profile of human sebum is defined by the sapienic acid pathway – a metabolic route operating exclusively in human sebaceous glands that produces sapienic acid (C16:1 Δ6) as the dominant fatty acid at approximately 25% of total sebum fatty acids, alongside its elongation product sebaleic acid (C18:2 Δ5,8). Neither molecule is produced by any other mammalian species, and both are absent from all animal fats and plant oils. This uniquely human fatty acid pathway contributes to sebum’s antimicrobial activity against gram-positive skin pathogens including Staphylococcus aureus, and is part of what makes sebum’s surface chemistry compositionally distinctive from any topical fat that might be applied in its place. ^[1]

Squalene, present at approximately 12%, functions as a sacrificial antioxidant at the skin surface, quenching reactive oxygen species generated by UV irradiation before they reach underlying lipids and cellular structures. It is effective under moderate UV load, but under sustained or intense UV exposure it generates peroxidated products – squalene monohydroperoxide and squalene peroxide – that are comedogenic and implicated in the microcomedone formation that initiates acne vulgaris.

Functions of the Sebum Film

The sebum film serves several partially independent functions at the skin surface. As an emollient, it softens the stratum corneum and reduces surface friction. As an occlusant, it contributes to the physical barrier against transepidermal water loss, supplementing the ceramide-based lamellar architecture of the stratum corneum without replacing it. As an antimicrobial film, its free fatty acid fraction – particularly sapienic acid and its downstream metabolites – creates a chemically hostile surface environment for pathogenic bacteria whilst supporting the resident commensal microbiota. It also contributes to the maintenance of the skin’s slightly acidic surface pH (approximately 4.5–5.5), which is itself antimicrobial and supportive of barrier enzyme function. ^[3]

Variation and Ageing

Sebum output varies substantially across body sites, with the face and scalp producing the highest volumes – up to 400–900 sebaceous glands per cm² on the forehead – and the palms, soles, and vermilion border of the lips producing none. Males produce approximately four times more sebum than females, driven by higher DHT concentrations from type 1 5-alpha reductase activity in sebaceous glands. Female sebum output fluctuates with the ovulatory cycle and declines measurably at menopause with oestrogen and androgen withdrawal.

With age, sebum quality declines independently of volume changes. Wax ester content decreases, the sapienic acid pathway shows reduced activity, and squalene peroxidation becomes proportionally higher under equivalent UV exposure as antioxidant capacity diminishes. The surface lipid film of aged skin is therefore less effective as antimicrobial defence and less protective against oxidative damage even when sebum volume appears maintained – a distinction between sebum quantity and sebum function that is clinically relevant for both acne management and anti-ageing treatment planning. ^[2]