What are the benefits of glycerine for the skin?

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

In comparison 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 moisturising properties:

• Enhancing epidermal hydration: to assist the skin in retaining its moisture, glycerol is able 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, glycerine 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 protective barrier function of the skin: glycerol plasticises the stratum corneum by forming a "pseudo-occlusive" film, which helps to maintain the transepidermal water flow and improve the smoothness of the skin surface.

• Inhibiting the crystallisation of lipids in the stratum corneum: FRIBERG S. E. & co. 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, thus 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.

• Optimising 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 glycerine is often combined with various other types of compounds to optimise its moisturising effects (hyaluronic acid, urea, xylitol, sorbitol, etc.).

It is important to understand that the degradation of corneodesmosomes, structural proteins that hold together the dead cells of the corneal layer (corneocytes), is a phenomenon dependent on the water content in the corneal layer. Indeed, it is significantly reduced when the ambient humidity is low (44%) compared to high humidity (80%), thus leading to a 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 significantly influenced by the activity of water within the stratum corneum.

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