The skin does absorb oil, but the process is highly selective and limited. Oil penetration focuses on lipids, primarily triglycerides, fatty acids, and sterols. The skin selectively allows certain lipid-soluble molecules to penetrate its outermost layer. This absorption is regulated by the oil’s chemical structure and the physical condition of the skin.
The Skin Barrier: Understanding the Entry Point
The skin’s main defense is the Stratum Corneum (SC), its outermost layer. The SC is often described using the “brick and mortar” model, functioning as a robust physical barrier. The “bricks” are flattened, dead skin cells called corneocytes, packed with keratin and proteins.
The “mortar” is a continuous, multi-layered lipid matrix surrounding the corneocytes. This matrix is composed of ceramides, cholesterol, and free fatty acids in an organized structure. Since this lipid matrix is inherently hydrophobic, the skin is highly resistant to water-soluble substances. Conversely, this lipid-rich environment makes the SC receptive to penetration by lipid-soluble substances, such as cosmetic and natural oils.
Mechanisms of Oil Penetration
Oil components primarily penetrate the Stratum Corneum via passive diffusion. This slow movement is driven by the concentration gradient, where oil molecules move from the high concentration on the skin surface into the lower concentration within the SC. Diffusion occurs mainly through two routes, both challenging for external molecules.
Intercellular Route
The most common pathway for lipid-soluble molecules is the intercellular route. This involves oil components maneuvering through the tortuous, lipid-filled channels between the corneocytes, integrating into the barrier’s “mortar.” Highly lipophilic molecules tend to accumulate and diffuse laterally within this lipid matrix.
Transcellular Route
The secondary and less common route is the transcellular pathway, where molecules pass directly through the corneocytes themselves. This route is more difficult, requiring the substance to pass through the lipid structures of the cell envelope and the protein-rich cytoplasm. Since oils are chemically similar to the SC’s lipid matrix, the intercellular pathway is the primary route.
Factors Influencing How Much Oil is Absorbed
The extent and rate of oil absorption depend on the oil’s specific physical and chemical properties.
Molecular Characteristics
Molecular size and weight are primary determinants, as smaller molecules navigate the narrow intercellular spaces more easily. For example, the triglycerides and fatty acids in coconut oil penetrate further into the SC than the large hydrocarbons in mineral oil.
Fatty Acid Profile
The fatty acid profile is another factor, particularly the ratio of oleic acid to linoleic acid. Oils rich in monounsaturated oleic acid, like olive oil, can act as penetration enhancers by temporarily disrupting the organized SC lipid structure. Linoleic acid, conversely, helps maintain and restore the integrity of the water permeability barrier.
Skin Condition and Application
The condition of the skin and the application site also influence absorption. Application to thinner skin or compromised skin (barrier dysfunction) results in greater penetration. Occlusive environments, such as covering the area with a bandage, increase skin hydration. This slightly loosens the SC structure and enhances passive diffusion.
The Functional Role of Absorbed Oils
Once oil components successfully penetrate the Stratum Corneum, they perform several functions within the epidermis.
Barrier Repair
A primary function is barrier repair. Free fatty acids from the oil integrate into the SC lipid matrix, filling gaps and supporting the lamellar structure. This action helps restore the skin’s natural defensive function, particularly when the barrier has been compromised.
Moisturizing and Delivery
Absorbed oils contribute to moisturization by reducing Transepidermal Water Loss (TEWL). By creating a semi-occlusive layer within the upper SC layers, the oils trap existing moisture and slow down evaporation. Oils also function as a delivery vehicle for beneficial compounds, known as bioactives. When absorbed, the oil carries fat-soluble vitamins and antioxidants, such as tocopherols, deeper into the epidermis.