What are the benefits of glycerin for the skin?

Being among the most effective and widely used humectant polyols (hygroscopic) as moisturizing ingredients in the cosmetic industry, the glycerin is known to increase skin hydration and decrease uncontrolled evaporation of water from the epidermis, thus preventing dehydration and the resulting physical damage.

Compared to hyaluronic acid, urea, sorbitol, or even propylene glycol, glycerin is themost effective humectant available according to a 2016 study.

Various mechanisms of action of the glycerol have been reported to understand its moisturizing properties:

• Increasing Epidermal Hydration: To assist the skin in retaining its moisture, glycerol has the ability to permeate the stratum corneum and draw water towards the epidermis from the dermis and/or the environment under humid conditions. This is due to its three hydrophilic hydroxyl groups (-OH) that bind and retain water.

  In fact, glycerol will create a "reservoir" in the deep layers of the stratum corneum within the lipid bilayers. Furthermore, glycerin has excellent water solubility. According to a study, the higher the solubility of a polyol, the greater its ability to absorb water.

  Note : Pure glycerol (92 g/mol) can absorb its own weight in water in 3 days.

• Enhancing the skin's protective barrier function: glycerol plasticizes the stratum corneum by forming a "pseudo-occlusive" film, which helps to limit transepidermal water loss and improve the smoothness of the skin's surface.

• Inhibiting the crystallization of lipids in the stratum corneum: FRIBERG S. E. & al. have demonstrated with in vitro models that glycerol is capable of influencing the crystalline arrangement of lipids in the stratum corneum, by inhibiting the transformation of lipids from the liquid crystalline form to the solid crystalline form, thereby preventing water loss and improving the properties of the skin barrier.

  Note : In dry skin, the proportion of lipids in a solid state can be high.

• Optimizing Desmosomal Degradation: The desmolytic effect of glycerol, demonstrated by SABIN R. & al., can induce a decrease in intra-epidermal pressure on intercellular lipids and, consequently, indirectly cause an increase in lipids in the liquid crystalline state.

In over-the-counter skincare products, the glycerin is often combined with various other types of compounds to optimize its moisturizing effects (hyaluronic acid, urea, xylitol, sorbitol, etc.).

It's important to understand that the degradation of corneodesmosomes, structural proteins that hold together the dead cells of the stratum corneum (corneocytes), is a phenomenon dependent on the water content in the stratum corneum. Indeed, it is significantly reduced when the ambient humidity is low (44%) compared to high humidity (80%), leading to defective desquamation associated with a thickening of the stratum corneum.

Indeed, SABIN R. and his colleagues have demonstrated in a study in vitro the ability of glycerol (5%) to increase the degradation of these protein bonds. Measurements of the mechanical resistance of the corneal layer lamellae indicated a reduction in intercorneocyte forces after the use of glycerol.

According to initial data, glycerol appears to influence the activity of enzymes responsible for the breakdown of intercorneocyte attachments ("Stratum Corneum Tryptic Enzyme" and "Stratum Corneum Chymotryptic Enzyme"), which are desquamating proteases that are heavily influenced by the activity of water in the stratum corneum.

Furthermore, other studies have shown that glycerin could modulate the behavior of intercellular lipids in the stratum corneum and prevent the crystallization of their lamellar structures in vitro at low humidity. Thus, based on the results obtained, glycerin would have allowed to "normalize" the enzymatic digestion of corneodesmosomes thanks to its humectant properties, thereby restoring the mechanisms of corneocyte elimination on the skin surface.