Good evidence
Glycerin
INCI: Glycerin · also glycerol, glycerine
A small three-carbon alcohol that holds water. It is one of the oldest ingredients in dermatology and still one of the few with a mechanism, a receptor-level transport story and a long clinical record all pointing the same direction.
Why it is here
Ancient. Unglamorous. Still undefeated. Your skin makes and moves its own glycerol through aquaporin channels, which is why a cleanser that leaves some behind feels less like a punishment.
Glycerol binds water and holds it in the stratum corneum, so the top layers stay pliable instead of brittle. Skin also makes and moves its own glycerol through aquaporin-3 channels and through sebaceous lipid metabolism, which is why applied glycerol behaves like something the tissue already has a use for. It also slows the lipid phase transition in the stratum corneum and blunts the response to irritants.
The receipt
Reviewed effects include improved stratum corneum hydration, barrier function and mechanical properties, inhibition of the stratum corneum lipid phase transition, protection against irritating stimuli, enhanced desmosomal degradation and accelerated wound healing.
Glycerol and the skin: holistic approach to its origin and functions. British Journal of Dermatology, 2008. review. PMID 18510666
What the study actually did
- Study
- Fluhr JW, Darlenski R, Surber C. Glycerol and the skin: holistic approach to its origin and functions. 2008.
- Journal
- British Journal of Dermatology 159:23-34
- Design
- Review of mechanistic and clinical literature
- Subjects
- Human skin across the reviewed studies
- Concentration
- Varies by study
- Duration
- Varies by study
- Outcome
- Improved hydration, barrier function and mechanical properties; protection against irritants
In practice
- Typical use
- 3 to 10 percent in cleansers and creams; 10 percent and above in humectant serums. In a rinse-off cleanser the point is what survives the rinse, not the number on the sheet.
- Pairs with
- Sodium Cocoyl Isethionate, Ceramides, Panthenol, Niacinamide
- Keep apart from
- Nothing meaningful. Glycerin is compatible with essentially every other ingredient on this list, at every pH they run at.
- Cautions
- None notable. Very high concentrations feel tacky, which is a texture problem, not a safety one. In extremely dry air some people find high-glycerin products less comfortable than occlusive ones.
Questions people ask
Does glycerin pull water out of your skin in dry weather?
This is the internet's favorite glycerin story and the evidence for it is thin. Glycerin is used at meaningful levels in products validated in winter xerosis and atopic dermatitis, conditions defined by dry air and a leaky barrier. If you are in genuinely arid conditions, pair it with an occlusive layer so the water it holds has somewhere to stay.
Why put a humectant in a cleanser that gets rinsed off?
Because rinse-off is not the same as zero deposit. Cleanser formulation is partly the science of what stays behind. Humectants and lipids left on skin during and after the wash reduce the tight, squeaking feeling that follows a stripped barrier, which is the sensation most people misread as clean.
Is glycerin better than hyaluronic acid?
Different jobs, different molecule sizes. Glycerin is small, penetrates the upper stratum corneum and has a receptor and transport story behind it. Hyaluronic acid is large, sits higher up and holds water more like a film. Most well built formulas use both. Glycerin has the longer and deeper clinical record.