What Glycerin Actually Is
Glycerin (also called glycerol) is a simple polyol — a small, water-soluble molecule with three hydroxyl groups that give it a strong affinity for water. It occurs naturally in the skin as a byproduct of triglyceride metabolism in sebocytes and keratinocytes. When the skin barrier is functioning well, glycerin is one of the principal components of the natural moisturising factor (NMF) — the collection of molecules in the stratum corneum that maintains hydration from within.
In compromised skin, NMF levels are reduced. Studies in eczema patients consistently show lower glycerin concentrations in the stratum corneum compared to controls, which correlates with increased TEWL and reduced barrier function. This means topical glycerin supplementation has a physiological rationale for barrier-compromised skin that goes beyond simple surface hydration — it is partly restoring what the barrier is failing to produce endogenously.
How Glycerin Works in the Barrier
Glycerin's role in skin is more mechanistically significant than most humectants. It works through at least three distinct pathways:
1. Humectancy — drawing and holding water
As a humectant, glycerin attracts and binds water molecules through hydrogen bonding. Applied to the skin surface, it draws moisture from the environment (in humid conditions) and from deeper layers of the dermis toward the stratum corneum. This is the mechanism most people are familiar with — but it is the least interesting of the three in the context of barrier repair.
2. Aquaporin-3 regulation
Aquaporin-3 (AQP3) is a membrane protein that forms water channels in keratinocytes, regulating the movement of both water and glycerin through the epidermis. Research has established that glycerin is a preferred substrate for AQP3 — it is actively transported by this channel, not merely absorbed passively. Skin with reduced AQP3 expression (which occurs in barrier-compromised and photoaged skin) shows significantly lower glycerin levels and higher TEWL. Topical glycerin application has been shown to partially compensate for AQP3 deficiency by providing additional substrate for transport.
3. Filaggrin expression
Filaggrin is a structural protein that is critical for both corneocyte formation and the production of NMF components (filaggrin breaks down into the amino acids and urocanic acid that form NMF). Mutations in the filaggrin gene (FLG) are the strongest known genetic risk factor for eczema and are found in a significant proportion of people with chronic barrier dysfunction. Glycerin has been shown to upregulate filaggrin expression in keratinocytes — a mechanism with direct relevance to barrier repair in FLG-mutation carriers and in stress-induced filaggrin deficiency.
Glycerin vs Hyaluronic Acid: The Comparison That Matters
Hyaluronic acid (HA) has dominated the skincare hydration conversation for the past decade. Glycerin, despite having a larger and older evidence base for barrier function specifically, receives considerably less marketing attention. The clinical comparison is instructive.
Hyaluronic acid is an outstanding humectant — it can hold up to 1,000 times its weight in water, far exceeding glycerin's water-holding capacity. For pure surface hydration, high molecular weight HA is highly effective. However, HA's mechanism is almost entirely humectantic. It has no documented role in filaggrin expression, AQP3 regulation, or ceramide synthesis. Its barrier-repair credentials are limited to the indirect benefit of keeping the stratum corneum hydrated.
Glycerin's water-holding capacity is lower than HA's, but its mechanisms are more directly tied to barrier biology. For skin that needs surface hydration, HA is effective. For skin that needs to repair compromised barrier function at a structural level, glycerin's multi-pathway action makes it more relevant. Most well-formulated moisturisers use both. See our humectant guide for a full comparison of the category.
How to Use Glycerin Effectively
Glycerin performs best as part of a layered moisturisation system rather than as a standalone product. The humectant role requires an occlusive layer applied on top to prevent the drawn moisture from evaporating. Without an occlusive, particularly in dry climates, glycerin's water-drawing effect can increase surface evaporation as much as it increases surface hydration.
- Apply to damp skin: Apply glycerin-containing products immediately after cleansing, while skin is still slightly damp. This gives the humectant access to surface water to bind and hold rather than drawing entirely from deeper layers.
- Follow with an emollient or occlusive: A ceramide moisturiser, facial oil, or balm applied on top locks in the moisture drawn by glycerin. The combination produces significantly better hydration outcomes than either alone.
- Standalone high-concentration glycerin serums: These can be effective but require the above-mentioned occlusive step. Pure glycerin applied undiluted (100%) can feel sticky and, in very dry climates, may draw moisture from below rather than from the environment — a reason to use formulated products rather than cosmetic-grade pure glycerin.
