What is the "skin barrier", and do ceramide cosmetics work?

This answer is a draft. It stays unpublished until legal review and source verification are complete.

The answerThe skin barrier is the work done by the outermost layer of the skin, the stratum corneum (角層, about 0.02 mm thick): it limits water loss and keeps out irritants, microbes and allergens. About half of the "intercellular lipids" that fill the gaps between its cells are ceramides, and people with dry skin or atopic dermatitis are known to have less ceramide. Ceramides in cosmetics are meant to fill those gaps for a while and reduce water loss, which makes them a reasonable choice of moisturiser. But as a cosmetic their standing is "moisturises"; the claims "repairs the barrier" and "improves atopic dermatitis" are not permitted. The first step in protecting the barrier is not "apply ceramide" but "do not over-wash, do not rub".

Not a day goes by in Japan without a cosmetics advert saying "supports the barrier function" or "contains ceramides" — the words appear on packaging in every drugstore. What do they refer to, and is there any point in applying ceramides? We separate the dermatologist's explanation from what a cosmetic is legally allowed to claim in Japan.

What the barrier is made of

Element	Role	What damages it	How to protect it
Stratum corneum (about 0.02 mm thick, about 15–20 layers of cells)	Limits water loss, blocks external irritants	Over-washing, rubbing, hot water, too much peeling	Wash no more than twice a day, lukewarm water, no rubbing
Intercellular lipids (about 50% ceramides, cholesterol, free fatty acids)	Fill the gaps between cells and hold water in between	Washed out by cleansing agents, age (ceramide falls with age), dry environments	Gentle face wash, moisturiser (ceramides, petroleum jelly etc.)
Natural moisturising factor (NMF)	Holds water inside the cells (amino acids, urea, lactic acid etc.)	Over-washing, UV	As above, plus UV protection
Sebum film	Oily film on the surface; slows water loss	Over-washing, alcohol	Do not over-wash
Mildly acidic (pH 4.5–6)	Keeps the resident bacteria balanced and the enzymes working	Strongly alkaline soap	Mildly acidic to neutral face wash

When the barrier breaks down: transepidermal water loss (TEWL) rises and the skin dries out, and irritants and allergens get in more easily, causing itching and redness. A vicious circle of dry → scratch → more damage.
Types of ceramide: human-type ceramides (listed as Ceramide NP, AP, EOP and so on) have the same structure as the skin's own and are the most studied for moisturising. Plant-derived and pseudo-ceramides have a different structure but work as moisturisers. There is no obligation to declare the amount; the earlier an ingredient appears on the list, the more there is (ingredients at 1% or below may be in any order).
What cosmetics can do: add water and lipids to the stratum corneum and reduce water loss (that is, moisturise). "Improves or repairs the barrier function" and "works on atopic dermatitis" are not permitted claims for cosmetics, and an advert that says so breaches the Pharmaceuticals and Medical Devices Act (Japan's cosmetics and drugs law). Even a quasi-drug (a Japanese category between cosmetics and medicines) may only go as far as "prevents rough skin".
How to use a moisturiser: within 5–10 minutes of bathing, a generous amount (see the FTU guide in our separate article on moisturising the body), press in, do not rub. A cream or lotion containing ceramides is a reasonable choice for dry skin. Petroleum jelly (ワセリン, waserin — sold cheaply at any drugstore) adds no ingredients but strongly limits water loss, at low cost.
If you have atopic dermatitis: skincare (moisturising) is written into the Japanese Dermatological Association guidelines as one of the pillars of care. But that care is a combination of doctor-prescribed drugs (topical steroids, tacrolimus etc.) and moisturiser; ceramide cosmetics alone cannot stand in for it.
A barrier-friendly lifestyle: indoor humidity 40–60%, UV protection, sleep, no nylon wash towel, do not add several new cosmetics at once.

[Verify] The thickness and layer count of the stratum corneum and the ceramide share of the intercellular lipids are standard figures from dermatology. Check the skincare section of the 2021 Atopic Dermatitis Clinical Guidelines.

Misunderstanding 1｜Apply ceramide and a broken barrier goes back to normal

Ceramide in a cosmetic is a moisturiser that fills the gaps in the stratum corneum for a while and reduces water loss. "Repair" is not a permitted cosmetic claim, and unless you stop over-washing and rubbing, what you add washes out again.

Misunderstanding 2｜The barrier is something skincare builds

The barrier is a structure the skin builds itself; the job of cosmetics is "do not damage it, top it up". The biggest measure is not over-washing and not rubbing.

Misunderstanding 3｜Any product with ceramide is the same

The type (human-type, plant-derived, pseudo) and the amount differ, and there is no obligation to declare the amount. Judge by the position on the ingredient list and by how your skin responds.

Misunderstanding 4｜Atopic dermatitis can be fixed with ceramide cosmetics

Care for atopic dermatitis is a combination of dermatologist-prescribed drugs and moisturiser. Moisturising matters, but a cosmetic alone is not medical care.

Misunderstanding 5｜To strengthen the barrier, strip dead skin and speed up turnover

The stratum corneum is the barrier itself; removing it with peels or scrubs weakens the barrier. The drier or more sensitive your skin, the more you should hold back on "stripping" care.

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Sources

Japanese Dermatological Association, "Atopic Dermatitis Clinical Guidelines 2021" [アトピー性皮膚炎診療ガイドライン2021] (skincare and moisturising)

https://www.dermatol.or.jp/uploads/uploads/files/guideline/ADGL2021.pdf

Japanese Dermatological Association, "Dermatology Q&A: dry skin and asteatosis" [皮膚科Q&A：乾燥肌・皮脂欠乏症]

https://www.dermatol.or.jp/qa/

Japan Cosmetic Industry Association, "The scope of cosmetic efficacy claims (56 items) and labelling rules" [化粧品の効能の範囲（56項目）と表示ルール]

https://www.jcia.org/user/business/law/

⚠️ Editorial note: the source URLs above were identified by search. Our working environment cannot reach go.jp domains, so the contents have not been checked against the text. Verify every source before publication and record who checked it and when.

About this article

What we share here are ideas and everyday practices passed down among people living in Japan, together with general information drawn from public sources. They are not official recommendations, and they are not professional instruction. Practices vary between households and regions.

This is general information. Results vary from person to person and depend on your own conditions and surroundings.

Please decide for yourself, using your own judgement, whether any of this suits your situation. We cannot accept responsibility for any damage, loss or injury arising from following this article.

Cosmetics are not medicines and are not meant to diagnose, heal or prevent any condition. If your skin reacts, stop using the product and see a dermatologist.

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What is the "skin barrier", and do ceramide cosmetics work?

What the barrier is made of

Misunderstanding 1｜Apply ceramide and a broken barrier goes back to normal

Misunderstanding 2｜The barrier is something skincare builds

Misunderstanding 3｜Any product with ceramide is the same

Misunderstanding 4｜Atopic dermatitis can be fixed with ceramide cosmetics

Misunderstanding 5｜To strengthen the barrier, strip dead skin and speed up turnover

Sources

About this article

Sources

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