Skin barrier, ceramides and TEWL: how skin retains water

Eclo Science

Barrier · ceramides · water loss

TEWLmeasurement ≠ hydration

In brief. The skin barrier lies mainly in the stratum corneum at the surface of the epidermis. It combines corneocytes with a lipid matrix rich in ceramides, cholesterol and free fatty acids, among other components. TEWL (transepidermal water loss) measures the flux of water that passively escapes through the skin: it is a widely used indicator of barrier function, but it is neither a synonym for “hydration” nor a universal score of skin health.

Evidence level for this page: barrier physiology from scientific reviews, TEWL measurement methodology from systematic reviews, and Céramosides™ data treated separately as studies on a specific ingredient.

What is the skin barrier?

The stratum corneum is often described using a “bricks and mortar” model: corneocytes form the bricks, while intercellular lipids form an organized matrix between them. The image is simplified, but it helps explain why barrier function depends on both cornified cells and lipid organization.

Ceramides, cholesterol and free fatty acids organize into lamellar structures. Their composition and organization contribute to limiting water loss and protecting against the environment. A disrupted barrier can be associated with greater water loss and signs of dryness or irritation, without these concepts being strictly equivalent.

What is the role of ceramides?

Ceramides are a major family of lipids in the stratum corneum. There is not one single “ceramide”: skin contains many ceramide classes and species whose balance and organization contribute to barrier structure.

This complexity matters when discussing oral phytoceramides. A plant extract containing polar lipids is not a reservoir that directly “fills gaps” in the skin. To assess oral supplementation, the exact extract, dose, duration, population and measured endpoints need to be examined.

What is TEWL?

TEWL stands for transepidermal water loss. Instruments measure water-vapor flux at the skin surface, often expressed in g/m²/h. Increased TEWL can be observed when barrier integrity is impaired.

A TEWL value is highly context-dependent: temperature, humidity, airflow, anatomical site, age, acclimatization, instrument and method all influence the measurement. Comparisons are therefore stronger when conditions are standardized and the protocol compares prespecified groups or time points.

Are TEWL and skin hydration the same thing?

No. TEWL measures a flux of water loss. Hydration of the stratum corneum is often assessed by other techniques such as corneometry, which reflects electrical properties related to water content. A study can observe a change in one parameter without an identical change in the other.

Measurement	What it mainly tells us	Interpretation limit
TEWL	Passive water flux through skin	Sensitive to environment, site and instrument
Corneometry	Hydration of the stratum corneum	Superficial and instrument-dependent measurement
Clinical assessment	Appearance, dryness, scaling or comfort depending on the scale	Depends on the scale and evaluator
User questionnaire	Perceived comfort or benefit	Does not replace an objective instrumental measurement

Dry skin, dehydrated skin and impaired barrier: are they synonyms?

Not exactly. “Dry skin” can describe a clinical or sensory state. “Dehydrated” is a cosmetic term often used for reduced water in the superficial layer. “Impaired barrier” more specifically describes disrupted protective function. These situations can overlap, but should not be presented as identical.

Topical ceramides and oral phytoceramides: the same logic?

No. A topical is applied directly to the stratum corneum and, depending on its formulation, may act on hydration, occlusion or surface lipid organization. An oral supplement is digested and absorbed before any potential distant effect. The mechanisms, doses and types of evidence are therefore different.

For oral phytoceramides, the most relevant evidence is a human study conducted with the identified raw material. A randomized, double-blind, placebo-controlled trial published in 2017 evaluated a wheat polar-lipid complex for 60 days, using powder at 30 mg/day and oil at 70 mg/day in 60 women. A 2024 study then randomized 72 women with dry and wrinkled skin between two forms of the complex and placebo for 56 days, measuring hydration, TEWL, elasticity and profilometry. In that study, improvements reported during supplementation were not maintained 56 days after discontinuation.

These data concern the wheat lipid complex studied and its protocols. They should not be turned into a general claim about all oral ceramides.

What does EFSA’s 2012 opinion on wheat polar lipid extract mean?

In 2012, EFSA assessed a dossier concerning a wheat polar-lipid extract and a claim of “protection of the skin against dehydration”. The panel concluded that a cause-and-effect relationship had not been established from the evidence submitted in that dossier.

Clinical trials published later are new scientific data to consider separately; they do not mean the 2012 EFSA opinion was “cancelled” or “overturned”. A regulatory opinion on a given dossier and a later scientific publication operate under different frameworks.

How should a TEWL change in a study be read?

A reduction in TEWL can be compatible with improved barrier function in the studied protocol. The comparator, between-group difference, measurement time and statistical uncertainty still matter. A before/after change in the active group, without a convincing difference against placebo, is weaker evidence than a prespecified intergroup difference.

Conversely, no significant effect on TEWL does not mean no other skin parameter can change. Hydration, roughness, elasticity and perceived comfort are distinct endpoints.

What is the link with Ressource™?

Ressource™ combines Céramosides™, ExceptionHYAL® Star and micronutrients, among other ingredients. The formulation logic connects several dimensions of skin hydration, but the source of each piece of evidence must remain visible: Céramosides™ data belong to Céramosides™, ExceptionHYAL® Star data to the studied hyaluronic acid, and authorized nutritional claims to the vitamins or minerals that meet their conditions of use.

What we know — and what it does not allow us to conclude

Documented: ceramides are major lipids of the stratum corneum and contribute to barrier organization.
Documented: TEWL is a useful measure of transepidermal water flux, but is strongly affected by measurement conditions.
Published ingredient studies: randomized controlled trials have evaluated a wheat polar-lipid complex at the raw-material doses described in their protocols.
Do not extrapolate: an oral phytoceramide does not directly “replace” skin ceramides as though filling a reservoir.
Do not extrapolate: a reduction in TEWL in an ingredient trial is not a study of the finished product containing it.

Frequently asked questions

Does lower TEWL always mean better hydrated skin?

No. TEWL and stratum-corneum hydration are different parameters. Lower TEWL can be compatible with better barrier function in a given protocol, but it does not replace a hydration measurement.

Can TEWL values be compared across different studies?

With caution. Anatomical site, temperature, humidity, acclimatization and instrument can change the values. The strongest comparisons are those planned within the same standardized protocol.

Does an oral phytoceramide supplement replace barrier skincare?

No. Oral and topical routes are different. An oral supplement should be judged on its own trials; a topical product on its formulation and data.

Does Céramosides™ have an EFSA-authorized claim “protects against dehydration”?

No. The 2012 EFSA opinion did not establish a cause-and-effect relationship for the assessed claim. Later clinical studies should be read as separate scientific data, not as authorization of that claim.

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