Glycerin
The most studied and most reliable humectant in skincare.
Decades of controlled evidence; the benchmark other humectants are measured against.
On the label: Glycerin. Also known as Glycerol, Glycerine.
What it does for you
Draws water into the upper layers of skin and keeps it there.
Why we use it
It is unglamorous and it works. Formulas that skip glycerin to make room for a more marketable humectant are usually worse.
Mechanism
Glycerin attracts water and also supports the channels skin uses to move water between cells.
What the studies say
Ten days of 20% glycerin on normal skin significantly increased corneometer values, indicating increased hydration. The study failed to show any influence of glycerin on transepidermal water loss or on skin sensitivity to an irritant. Hydration is the endpoint glycerin moved; barrier function was not.
- Human in vivo
- n = 17 healthy volunteers
- 10 days
Bilateral, double-blind comparison of a 20% glycerin cream against its own placebo base on normal skin, assessing corneometry, transepidermal water loss and response to an irritating surfactant
The influence of a cream containing 20% glycerin and its vehicle on skin barrier properties Loden M, Wessman W. International Journal of Cosmetic Science, 2001. DOI 10.1046/j.1467-2494.2001.00060.x
Compares glycerin against its own vehicle rather than against no treatment, which is the design that actually isolates the ingredient. The null result on barrier is as informative as the positive one on hydration.
The four-arm design separated the two ingredients: the petrolatum component improved barrier function by reducing transepidermal water loss, while the glycerol component improved skin hydration. The full emollient significantly increased corneometry versus vehicle and versus vehicle plus petrolatum. This is the cleanest available demonstration of what glycerol contributes on its own inside a finished formula, and what it does not.
- Human in vivo
- n = 51 healthy volunteers with dry skin
- 12 hours of serial measurement after application
Randomized, double-blind, four-arm crossover clinical pharmacology study comparing a full emollient, its vehicle alone, the vehicle plus glycerol, and the vehicle plus petrolatum, with transepidermal water loss and corneometry measured serially
Combined effects of glycerol and petrolatum in an emollient cream: A randomized, double-blind, crossover study in healthy volunteers with dry skin Vaillant L, Georgescou G, Rivollier C, Delarue A. Journal of Cosmetic Dermatology, 2020. DOI 10.1111/jocd.13163
The authors state the study was not designed to evaluate therapeutic benefit. Twelve hour window, so it speaks to immediate effects rather than sustained change.
Stratum corneum hydration was significantly improved and epidermal barrier function restored versus the glycerol-free placebo. No significant difference between glycerol and placebo for erythema, SCORAD or local severity score, though both groups improved over time. Tested in atopic dermatitis rather than healthy cosmetic use.
- Human in vivo
- n = 24 patients with atopic dermatitis
- 4 weeks plus a 2 week wash-out
Randomized, double-blind, placebo-controlled; topical 20% glycerol preparation versus its glycerol-free vehicle twice daily, run to the protocol standard of a phase III drug study, with a 2 week wash-out to test persistence
Placebo-controlled, double-blind, randomized, prospective study of a glycerol-based emollient on eczematous skin in atopic dermatitis: biophysical and clinical evaluation Breternitz M, Kowatzki D, Langenauer M, Elsner P, Fluhr JW. Skin Pharmacology and Physiology, 2008. DOI 10.1159/000111134
The authors note that before this trial no controlled study of glycerol on barrier function and hydration in atopic dermatitis had been published. Small sample and a disease population, so it transfers to cosmetic use only indirectly.
Identifies the aquaporin-3 transport channel and lipid metabolism in the pilosebaceous unit as the pathways for endogenous glycerol delivery and metabolism in skin. Catalogues the reported actions of glycerol on the epidermis: improved stratum corneum hydration, barrier function and mechanical properties, inhibition of stratum corneum lipid phase transition, protection against irritating stimuli, enhanced desmosomal degradation and accelerated wound healing. This is the source for the water channel mechanism, not for effect sizes.
- Review
Glycerol and the skin: holistic approach to its origin and functions Fluhr JW, Darlenski R, Surber C. British Journal of Dermatology, 2008. DOI 10.1111/j.1365-2133.2008.08643.x
Narrative review. Use it for mechanism and for the breadth of reported effects, and rely on the controlled trials for what glycerin actually moves in a formula.