Because a barrier is not made by one lipid but by an ordered mixture. The stratum corneum holds ceramides, cholesterol and free fatty acids in an approximate 1:1:1 ratio, and ceramides account for 30% to 40% of those lipids by mass. What lowers permeability is how they organise into lamellae, not how much of any single component is present.
Published 8 September 2026. Last reviewed 9 September 2026.
What the stratum corneum actually is
The stratum corneum is the outermost layer of the epidermis: anucleate corneocytes embedded in a continuous lipid matrix. The usual analogy — bricks and mortar — is useful provided one remembers where the function sits: resistance to the passage of water and of molecules resides in the lipid mortar, not in the bricks.
A ceramide is a lipid formed by a sphingoid base linked to a fatty acid through an amide bond. It is not a single molecule but a family. Kahraman and colleagues (2019) record that twelve types of ceramide have been extracted from human stratum corneum, that they constitute 30% to 40% of that layer's lipids by mass, and that they sit alongside cholesterol and free fatty acids in an approximate 1:1:1 ratio.
That figure needs qualifying. “1:1:1” is a descriptive approximation of average composition, not a formulation prescription. The membrane models that reproduce stratum corneum behaviour do not use equimolar proportions: Roy and colleagues (2023) built their biomimetic liposomes from 40% ceramides, 25% free fatty acids, 25% cholesterol and 10% cholesteryl sulfate, expressed as mole percent. Anyone claiming the barrier requires an equimolar mixture is compressing a statistical description into a rule the literature does not state.
Organisation matters more than inventory
Stratum corneum lipids stack into lamellae with characteristic periodicities, measurable by small-angle X-ray scattering. Roy and colleagues (2023) measured repeat distances of 48.3 A to 83.8 A (4.83-8.38 nm) in models built with phytosphingosine-type ceramides — the short periodicity phase — and of 125.7 A (12.57 nm) in those built with a sphingosine ceramide, corresponding to the long periodicity phase.
The interesting result is what happens when a single component changes. Replacing the saturated fatty acid with oleic acid, which is unsaturated, moved the long-phase repeat distance from 125.7 A to 139.6 A, and the thermal transition temperature fell from 116.5 C to 109.5 C. In the short-phase models the transition dropped from 82.3 C to 77.2 C. The authors describe the effect without ambiguity: an unsaturated fatty acid such as oleic produces a membrane that is more fluid and disordered, with greater leakage in dye-release assays.
In other words: the same lipid family, in the same overall proportion, yields a more or less permeable barrier depending on which chains compose it. That is the argument against reading an ingredient list as though it were a measure of function.
| Model | Fatty acid | Repeat distance | Thermal transition |
|---|---|---|---|
| Sphingosine ceramide (long phase) | Palmitic or stearic (saturated) | 125.7 A (12.57 nm) | 116.5 C |
| Sphingosine ceramide (long phase) | Oleic (unsaturated) | 139.6 A (13.96 nm) | 109.5 C |
| Phytosphingosine ceramide (short phase) | Palmitic or stearic (saturated) | 48.3 A (4.83 nm) | 82.3 C |
| Phytosphingosine ceramide (short phase) | Oleic (unsaturated) | 57.1 A (5.71 nm) | 77.2 C |
Values measured in biomimetic liposomal models, not in human skin. The correspondence between these magnitudes and clinical behaviour is not established by this work.
What has been measured in human skin
Murphy and colleagues (2022, Scientific Reports 12:5223) followed 37 subjects for five weeks, with visual dryness scores between 2.0 and 3.0, applying a lotion. Measurements before and after were:
- Stratum corneum hydration (corneometry): from 22.6 (SD 5.7) to 44.0 (SD 10.2).
- Visual dryness score: from 2.3 (SD 0.4) to 0.2 (SD 0.2).
- Total ceramides across twenty tape strips: from 2.85 (SD 0.70) to 3.80 (SD 0.71) pmol/ug protein, a 33% increase (p <= 0.0001).
- Ceramide classes: nine of the twelve showed statistically significant increases.
Two cautions about how to read those figures. The first concerns design: this is a follow-up with product application, not a randomised comparison against vehicle, so it does not separate the effect of the ingredients from that of occlusion and of elapsed time. The second concerns interpretation: an increase in ceramide content measured on adhesive strips does not by itself distinguish lipid deposited from the formulation from lipid of endogenous origin.
The consequence for formulation
Three conclusions follow, and none of them is a slogan.
- An ingredient list is not a measure of function. The same overall proportion of ceramide, cholesterol and fatty acid yields membranes of differing permeability according to chain length and saturation.
- “With ceramides” identifies nothing. Twelve classes are described in human stratum corneum; naming the family without naming the class leaves the component being supplied undetermined.
- The 1:1:1 ratio is descriptive. It describes the average composition of the tissue, not a requirement of formulations, which work with different proportions justified by their own phase behaviour.
Formulating according to compounding-pharmacy criteria means, in this terrain, precisely that: specifying which class of ceramide, at what chain length, in what proportion and in what vehicle, rather than naming the family and taking the mechanism for granted.
See also our glossary of mechanism terms in medical-grade skincare and the note on molecular weight and lipophilicity in percutaneous penetration.
References
- Kahraman E, Kaykin M, Sahin Bektay H, Gungor S. Recent Advances on Topical Application of Ceramides to Restore Barrier Function of Skin. Cosmetics. 2019;6(3):52. DOI 10.3390/cosmetics6030052
- Roy S, Ho JCS, Teo DLC, Gupta S, Nallani M. Biomimetic Stratum Corneum Liposome Models: Lamellar Organization and Permeability Studies. Membranes. 2023;13(2):135. DOI 10.3390/membranes13020135
- Murphy B, Grimshaw S, Hoptroff M, Paterson S, Arnold D, Cawley A, et al. Alteration of barrier properties, stratum corneum ceramides and microbiome composition in response to lotion application on cosmetic dry skin. Scientific Reports. 2022;12:5223. DOI 10.1038/s41598-022-09231-8
Scope. This article describes findings published in the scientific literature about lipids studied under stated conditions, and does not constitute a claim about any product. Cosmetic products are not intended to diagnose, treat, cure or prevent any disease. Readers with a persistent skin concern should consult a qualified healthcare professional.