If there is only one key factor for our skins’ health and beauty, it is its moisturization. Any imbalance in this characteristic is a systematic source of discomforts, cosmetic defects, up to skin diseases, all of this being more or less harmful. However, the process of skin moisturization is complex, and the sources of imbalances, varied. A thorough explanation, reported from the 12th Journées d’Echanges Scientifiques et Techniques (JEST: Scientific and Technical Exchanges Convention), held on 12 October 2012, in Montpellier, Southern France.
It is trite to tell it, though it is worthwhile to be reminded. Our human body, and the skin that wraps it, are formidable machines, perfectly designed to assure our health and comfort, day after day. The moisturization processes are no exception, as Stéphane Astie, a PhD in pharmacy and a lecturer in Cosmetology, detailed during the 12th Convention, organized by the French Association Cosmed.
Water, the source of life
Water is the main ingredient of the human body. It comes for about 60% of the body mass of an adult (but 75% of that of a new-born, and 55% of an elderly’s). Just for the record, our blood contains 83% of water, when our brain is made of water for 80%, and our skin, for 70%.
Most of water is inside our cells. Another part is in the inter-cellular space, being a kind of reservoir for cells and blood vessels. The remaining is in our blood and in the lymph, and is constantly running through the entire body.
Water is so badly needed that it is said that, at a moderate ambient temperature, no fluid intake induces death within two to three days. Our needs for water are in the 2.5 liters (0.55 gal [UK]) per day range, though they may vary according to age, ambient temperature, activity, altitude …
Further, water has several functions:
• it is an element of the numerous chemical reactions in our body,
• it transports many dissolved substances essential to our cells,
• it removes metabolic waste from cells,
•it helps maintain a constant temperature in our body.
Hence, the importance to supply our body with water.
The main intakes of water are provided by three sources:
• drinking water: it represents circa 2/3 of our intakes (1 to 1.5 liter),
• water included in food: an average of 0.5 to a liter per day,
•combustion water: this is water produced by the chemical reactions in our body, and is equivalent to circa 300 mL (10.6 fl.oz UK). It is completely linked to the metabolisation of food: the oxidation of a gramme of glucides produces 0.6 mL (0.02 fl.oz) of water.
Water and our skin
Our skin moisturization is intrinsically linked to our body’s. Water and skin have a very strong relationship. But not that simple.
Firstly, the stratum corneum (the most outer layer of the skin) hydration is not the same on all its thickness:
• on the surface, the concentration of water (mainly linked, as trapped) is about 25%,
•a bit deeper, it is in the 70% range; water is essentially free. This allows for exchanges with the lower layers of the skin.
Furthermore, as the epidermis is not vascularized, the water distribution varies.
Natural Loss and Elimination
In fact, water gets out of our body through different ways. Urine is the most important path (about 1.5 liter a day), though our skin lets also this precious liquid leak.
• Perspiration is the first permanent way through which our body water is lost. The natural diffusion of the water flux in our epidermis leads to the evaporation directly from the surface, called TransEpidermal Water Loss (TEWL). Though invisible, it is, nevertheless, important: about 500 ml per day are lost this way every day. Whatever the outside conditions, our skin is constantly dehydrating this way.
• Sweat is the consequence of an exposure to heat or exercise. It is linked to the vasodilation (dilation of blood vessels), to the pores dilation and to the activity of sweat glands.
These glands are present on the entire surface of the skin, but in more important quantities in the areas where the epidermis is thicker (feet, palms) and where the glands are larger (underarms, hairy parts of our faces and necks, groins, etc.). They are about 25 millions. When the body is exposed to heat, which increases the blood flow and dilates vessels, they produce far more sweat. Another source of water loss.
All this is not the only cause for skin dryness.
Skin hydration is controlled by three natural mechanisms, which may be disrupted by external aggressions, and, which are systematically affected by ageing.
• Storage. Water is stored in GlycoAminoGlycanes (GAGs), a kind of “sponges", which are a true water pool and make the volume and the density of our derma. Depending on the needs, the GAGs release water to make it available to all the skin layers. However, trough time, GAGs synthesis diminishes. Less are manufactured, this decreasing the skin water resources.
• Diffusion. Water in our skin is not motionless. It moves between the different layers, after a process called osmotic flow. All the osmotic flows are balanced so that all the skin structures are hydrated the right way. Nevertheless, as time goes by, they tend to become unbalanced: water is not distributed the right way, and some structures may be very low in water.
• Retention. The stratum corneum (the upper layer of the epidermis) is an efficient barrier against de-hydration. It comprises cells, keratinocytes, cross-linked by polar lipids. They block the water flow that tries to go up to the surface: a way to lower its evaporation. However, again due to ageing, keratinocytes synthesize less polar lipids. This adversely modifies this barrier, which prevents water loss.
When controls work poorly
The first role of the cutaneous wrap is to be a barrier against the external aggressions and to prevent water losses. It is both a physico-chemical and a bacteriological barrier. Our skin is not only a factor of control of our water flux; it is also a major factor of our adaptation to temperature's changes: it is our main radiator.
On the surface, a small fraction of the evaporating water combines with the epidermis lipids and the sebum lipids to make a thin emulsion that covers the entire skin. This surface hydrolipidic film (HLF) lubricates our skin, makes it supple and is a part of the barrier function.
When a well moisturized skin is soft and smooth, de-hydration comes with well-known cutaneous signs: dry, cracked, scaling (skin is peeling), irritable, whitish, it feels tight and is not that comfortable. This may lead to real skin diseases, such as the atopic dermatitis.
One may be more sensitive to skin dryness than others. For instance, in new-borns and small children, the body surface/weight ratio is higher than adults’: an explanation to the higher risk of dehydration when exposed to heat. In elderly people, the self-controls mechanisms do not work well; their skin is thinner and slowly dehydrates. Further, there are also different kinds of “dry skins” (go to the Which kind of dry skin is yours? article).
However, for everyone, and especially when ageing, avoiding all the causes of de-hydration and maintaining the skins’s natural hydration should be daily concerns as tooth brushing or showing are.
In addition, after a shower (not too hot, with a not too aggressive cleanser), do not forget milk, cream or moisturizing oil on your body, and a cream on your face. Our skin needs it … and deserves it.