Understanding the role of water in the skin.

The skin is a vital organ that holds a large proportion of the body’s water. However, water distribution within the skin varies significantly across its layers and reflects distinct biological functions. The dermis, the skin’s middle layer, acts as a true water reservoir. It contains 80% of the skin’s water. This water is termed non-mobilisable because it is tightly bound to the macromolecules of the extracellular matrix, in particular hyaluronic acid, a polysaccharide with high moisture-attracting capacity. Due to its many negative charges, this molecule binds sodium ions and attracts dozens of water molecules, forming a semi-fluid gel. This water is not free to circulate within the body or evaporate, but it plays an important role in tissue pliability and density.

The stratum corneum, the most superficial layer of the epidermis, contains only 10 to 20% water. This superficial water content is the main factor in skin’s hydration state as seen and felt. When skin is dehydrated for example, this reflects a disrupted water balance in the stratum corneum. Unlike water in the dermis, epidermal water is mobilisable. It moves via osmotic or transepidermal gradients. Some of this water reaches the surface, where it evaporates through a process called insensible water loss. This process is continuous, even without visible sweating, and causes skin dehydration when the epidermal barrier is compromised.

The skin’s hydration level depends on the dermis’s capacity to store water and the epidermis’s capacity to distribute and retain it.

An efficient way to reduce transepidermal water loss and maintain skin hydration is to follow a skincare routine adapted to your skin type. It not only compensates for water loss but also restores the biological mechanisms that capture and retain water in the skin’s layers. Two product categories address these needs: hydrating serums and moisturising creams.

Hydrating serums are water-based and contain a high concentration of humectants, compounds that function by attracting and binding water in the stratum corneum. Among the most recognised humectants are hyaluronic acid, which can vary in molecular weight to hydrate the skin at different depths, polyglutamic acid, the spirodela or aloe vera. These actives are used in the first step of the routine. To sustain hydration, it must be sealed with a moisturiser. This moisturiser should combine at least two ingredient types:

• Emollients : The emollients, such as plant oils, esters or waxes, which nourish the skin and strengthen intercellular lipids, contributing to the cohesion of the skin barrier.

• Occlusive agents Occlusive agents, such as butters or silicones, form a water-resistant film on the skin’s surface. This complements the natural hydrolipidic film and reduces transepidermal water loss.

The choice of the moisturiser depends on your skin type. Dry skin has a stratum corneum low in lipids and a fragile hydrolipidic film. We recommend a rich cream with a smooth texture and a high proportion of emollients and occlusives to address the lipid deficit. Combination or oily skin shows shiny areas due to excess sebum production. Lighter textures such as fluids or gel creams are preferable.

Tip : In addition, using hydrating mists or sticks during the day can help support skin hydration.