Understanding the Role of Water in the Skin.

When we drink a glass of water, the H2O molecules flow through the digestive tract and are absorbed by the intestinal wall into the bloodstream. The water, enriched with nutrients, then crosses the vessel walls to reach the various tissues and organs. At the cellular level, water enters the cells through aquaporins: tiny channels that let water in and out according to the cell's needs. The organs then reject this water loaded with waste into the bloodstream. Water is then lost in different ways: in the urine, during breathing, but also through the skin. Indeed, the vascular fraction is filtered by the kidney and is found in the urine. Another part leaves the body through the lungs during breathing, forming mist in winter.

Two types of water are present in the skin:

• The water known as "free", which can be mobilized and is likely to evaporate;

• The so-called "non-mobilizable" water, which is firmly attached to the structures of the dermis and therefore cannot escape.

It is therefore the "free" water that will influence the skin's hydration level. When the ambient temperature rises or during a major physical effort, the perspiration mechanism is activated in order to cool the body. But there is also a more passive phenomenon, during which water diffuses through the different skin layers, from the deepest to the most superficial, until it reaches the surface and evaporates passively: this phenomenon is known as insensible water loss. In order to avoid dehydration, the goal is to limit this loss.

In the skin, water is present at two levels: the dermis and the stratum corneum. The dermis is a true water reservoir, containing the skin's "non-mobilizable" water. This water cannot evaporate because it is firmly attached to the molecules of the dermis, such as hyaluronic acid. The stratum corneum, on the other hand, is built like a "brick wall". It is composed of cells, the "bricks", called corneocytes, and are held together by a "cement" made up of lipids (fatty acids, ceramides, cholesterol, etc.). These two elements (bricks + cement) prevent the evaporation of the water contained in the various layers of the skin. Corneocytes are loaded with keratin, a very resistant fibrous protein. The N.H.F. (Natural Hydration Factors) allows the connection between water and keratin by forming a bridge between both molecules. The water molecules are then inserted between the keratin fibers, bringing suppleness to the horny layer. Without water, thus, the skin is less flexible and feels uncomfortable: it pulls. Moreover, the hydrolipidic film, composed of an emulsion of sweat and sebum, opposes the evaporation of the "free" water contained in the skin thanks to the occlusive effect of sebum. Dry skin, which does not produce enough sebum, is therefore often correlated with skin dehydration.

In conclusion, a well-hydrated skin should be :

• Deeply moisturized, at the level of the dermis. This stage of the skin is not affected by the use of cosmetic products but by good oral hydration.

• Hydrated on the surface, at the level of the epidermis and in particular the horny layer.