How is skin dehydration manifested?

The skin (hypodermis, dermis, epidermis) contains 30% of our body's water and is itself composed of an average of 70% water. The dermis holds 80% water. It is stored in the form of a gel thanks to hyaluronic acid. At the dermis-epidermis junction, this concentration rises to 70%. In the horny layer and the upper layer of the epidermis, it is 10%.

Insensible Water Loss (IWL) refers to a passive phenomenon of water evaporation contained within the skin. The water, originating from the bloodstream, traverses the various layers of the skin and evaporates at its surface. Under normal functioning, this loss amounts to 300 - 400 mL per day across the entire body. The greater the water loss, the more dehydrated the skin becomes. This I.W.L., and therefore skin hydration, is influenced by various factors, over which we may not necessarily have control:

• The state of the hydrolipidic film: When functioning properly, the skin's hydrolipidic film forms a layer that limits the evaporation of water. If it is damaged, for instance due to a lack of lipids on the skin's surface, water evaporates more easily. Therefore, dry skin is often dehydrated. However, dehydrated skin is not necessarily dry.

• The quantity of Natural Moisturising Factors (NMF) in the epidermis: NMFs are a complex blend of hygroscopic substances present in the stratum corneum of the epidermis, which have a strong water retention capacity. They are composed of amino acids, urea, carbohydrates, lactic acid, and mineral ions (chloride, sodium, potassium). They enable the binding of water molecules at the level of the epidermis and thus ensure the hydration of the stratum corneum. Their reduction, therefore, facilitates skin dehydration.

• The quantity of hyaluronic acid in the dermis: This macromolecule captures large amounts of water. However, over the years, its concentration decreases, which impairs the deep hydration of the skin.

• The integrity of the skin barrier: The proper cohesion between the cells of the stratum corneum, thanks to the intercellular cement, also prevents excessive dehydration. This cement consists of three main ingredients: fatty acids, cholesterol, and ceramides. This cement slows down the diffusion of water from the deeper layers of the dermis, thus contributing to the maintenance of skin hydration. Therefore, when the stratum corneum is damaged (burns, friction, micro-cuts, cracks, fissures, etc.), water escapes more easily, leaving the skin dehydrated.