Skin Barrier Function

The stratum corneum is organised as bricks and mortar. Corneocytes are the bricks; the mortar is a lipid bilayer of roughly 50 percent ceramides, 25 percent cholesterol and 15 percent free fatty acids. That composition is not incidental, and the proportions matter more than the absolute quantity, which is why a barrier can fail while total lipid content looks adequate. Three ingredients carry an evidence grade against barrier function here, one at Grade C and two at Grade D, across 10 studies and 560 participants, the thinnest graded set on this site apart from the single-ingredient conditions.

A compromised barrier is a shared upstream feature rather than a disease in itself. It underlies eczema, contributes to psoriasis and rosacea, accompanies intrinsic aging, and defines what is colloquially called sensitive skin. The measurable consequence is elevated transepidermal water loss, and the experienced ones are stinging with products, redness, flaking and greater susceptibility to infection. Filaggrin mutations, over-washing with harsh cleansers, low humidity, topical retinoid use and deficiency of essential fatty acids each degrade it by a different route.

Ceramides hold the highest rating in the set at Grade C on 5 studies and 280 participants. Supplying orally the exact lipid class the barrier lacks is mechanistically appealing and practically indirect, since ingested sphingolipids are hydrolysed and must be resynthesised by keratinocytes rather than deposited intact. Trials generally use plant-derived glucosylceramides and measure transepidermal water loss and corneometry, which are at least the correct endpoints.