The top layer of your skin, the stratum corneum, is often described as bricks and mortar. The bricks are flat cells. The mortar is a mix of lipids in the gaps between them, and ceramides are its largest part. It is generally accepted that the mortar is about equal parts ceramides, cholesterol and free fatty acids1.
Why the ratio matters
In a 1996 study, mostly on mouse skin with some human skin, applying one or two of the three lipids delayed barrier recovery, while an equal mix of all three allowed normal recovery. Recovery was faster still when any one of them was raised up to three times2. That is the idea behind barrier creams that supply all three, rather than ceramides alone.
Most conditions with a weak barrier show less ceramide in the skin, but the link is hard to prove because there are so many variables. Reviews say formulations with skin-identical lipids could improve disturbed skin1.
What else keeps skin hydrated
A review of moisturisation names three things: the lipid mortar, the long winding path around the cells, and the natural moisturising factor made from a protein called filaggrin3. DermNet describes a moisturiser as a mix of three ingredient types4.
| Type | What it does | Examples |
|---|---|---|
| Humectant | Attracts and holds water | Glycerin, urea, hyaluronic acid |
| Emollient | Smooths the surface | Squalane, plant oils |
| Occlusive | Sits on top and slows water loss | Petrolatum, dimethicone |
Cleansing and pH
Skin is slightly acidic: the stratum corneum measures about 4.1 to 5.85, and a 330-person study put the natural forearm pH at around 4.7. It found skin with a pH below 5 was in better condition, and that soap and tap water raise skin pH for a few hours6. That is why a low-foam cleanser near pH 5.5, rather than a soap, makes sense for most people. Panthenol, a form of vitamin B5, sped barrier repair after an irritation test in a 2002 study7.
What a moisturiser cannot do
DermNet says large molecules such as peptides and collagen cannot get through the stratum corneum, and that marketing claims about them reducing the signs of ageing can be misleading4. A moisturiser helps skin hold water and feel comfortable. It does not cure eczema and it does not reverse ageing. If you have a skin condition, see a doctor.
