Dry skin: Moisturizing is not the same as moisturizing

We hear complaints about dry skin quite often. The basis for such complaints may be the tightness of the skin after washing, roughness, painful microcracks. It would seem that there is nothing simpler - just apply a moisturizer to the skin, and the problem is solved!

Let us recall that one of the fundamental differences between the stratum corneum and other layers of the epidermis is its relatively low water content - about 15%. The stratum corneum (which has no living cells) needs a small amount of water primarily to maintain its plasticity and integrity (this is how the cells of the stratum corneum differ from the cells of the deeper layers of the epidermis, which need moisture to survive). If, for one reason or another, the moisture content in the stratum corneum decreases, its structure is disrupted, which entails a deterioration in its barrier properties. The latter means that the stratum corneum ceases to be an impenetrable barrier for water, and its evaporation increases.

As a result, there is a moisture deficit in the living layers of the skin with all the ensuing consequences - a slowdown in metabolism, the skin does not recover and heal as quickly, its appearance noticeably worsens (it becomes dull, small wrinkles appear over time). In addition, microorganisms can more easily penetrate through the damaged barrier, causing, accordingly, irritation.

How to properly moisturize your skin:

• Occlusion

Water continuously rises from the depths of the skin to its surface and then evaporates. Therefore, if you slow down its evaporation by covering the skin with something gas-impermeable, the water content in the epidermis will increase quite quickly. This method is called occlusive (from the English occlusion - barrier, obstacle). If the film is completely impermeable (for example, polyethylene film), the epidermis will get too wet, which will lead to swelling of the stratum corneum and destruction of the barrier. Rubber gloves and air-impermeable clothing (in such cases they say that "clothes do not breathe", i.e. do not let gas through) also lead to hyperhydration.

A semi-permeable film that only slows down, but does not completely block, the evaporation of water will also eliminate the symptoms of dryness without damaging the skin. Ingredients that slow down the evaporation of water include:

• mineral oil, petroleum jelly, liquid paraffin, ceresin - hydrocarbons, petroleum products;
• liquid silicones (sometimes called silicone oils) are organosilicon compounds;
• Lanolin (from Latin lana - wool, oleum - oil) is an animal wax obtained during the purification of wool wax (it is extracted from sheep's wool with organic solvents);
• animal fats - goose fat, whale fat (spermaceti), pork fat;
• squalene and its derivative squalane (from Latin squalus - shark) - a natural component of human sebum; sources of production are different (for example, shark liver, some plants);
• vegetable oils - mostly solid, such as shea butter;
• natural waxes and their esters - beeswax, vegetable waxes (pine, cane, etc.).

Because Vaseline moisturizes too well, it can slow down the restoration of the epidermal barrier - the cells will not receive a signal in time that the barrier needs repair. Mainly occlusive (i.e. blocking the evaporation of moisture) moisturizing creams quickly eliminate dry skin, reduce inflammation and itching in skin diseases, but they do not act on the cause of skin dehydration. They can be compared to crutches that are necessary for those who cannot move independently, but are completely unnecessary for people with normal legs. If the skin's barrier function cannot be restored, occlusive creams are necessary. If there is a chance of restoration, they should be used only at the initial stage.

• Moisture catchers

The use of substances capable of binding and retaining water molecules (such compounds are called hygroscopic) is a great way to quickly moisturize the skin. In cosmetics, two categories of hygroscopic compounds are used, which act differently on the skin.

The "wet compress" method

Large polymer molecules (more than 3000 Da) are unable to penetrate the stratum corneum. They are fixed on the skin surface and absorb moisture like a sponge, forming a kind of wet compress. This effect is possessed by:

• glycerol;
• sorbitol;
• polyglycols (propylene glycol, ethylene glycol);
• polysaccharides - hyaluronic acid, chitosan, polysaccharides of plant and marine origin (chondroitin sulfate, mucopolysaccharides), pectins;
• protein molecules and their hydrolysates of animal and plant origin (in particular, popular cosmetic ingredients collagen and elastin are included in cosmetics precisely as moisturizing agents);
• polynucleic acids (DNA) and their hydrolysates.

The listed components are found in almost all cosmetic forms, including emulsion (creams). However, they are most abundant in gels and "liquid" products (tonics, lotions, serums, concentrates).

Now, attention: using skin moisturizing products like a "wet compress" is not always justified. For example, in a dry climate, when the relative water content in the environment is lower than in the stratum corneum, the compress begins to "pull" water from the skin - as a result, the stratum corneum becomes even drier. On the contrary, with high air humidity, applying cosmetics with these components really softens and moisturizes the skin. At the same time, the appearance of the skin also improves - it acquires a matte shine, tightens and smoothes a little.

• "Deep" skin hydration

Some cosmetics say that they have a deep skin moisturizing effect. What does this mean? It is a common misconception to think that all skin layers, including the deep ones, are moisturized. In fact, only the stratum corneum is moisturized. The role of natural sponges in the stratum corneum is played by components of the natural moisturizing factor (NMF) - free amino acids, urea, lactic acid, sodium pyroglutamate. They are located throughout the stratum corneum, and only in it.

These compounds are formed as a result of the breakdown of proteins (mainly filaggrins), which provide adhesion of cells lying under the stratum corneum. Having passed into the stratum corneum, the cells not only lose their nucleus, but the connections between them are also gradually destroyed (this is why horny scales that are not bonded to each other are easily exfoliated from the skin surface). NMF molecules are located in close proximity to corneocytes. A significant portion of the water present in the stratum corneum is associated with NMF.

Bound water participates in the gluing of horny scales and, along with sebum, ensures the plasticity and smoothness of the skin surface, but does not prevent the disintegration of scales and their natural removal.

• Osmosis, or dilution effect

Mineral substances (salts) also provide deep hydration of the stratum corneum. The mechanism of action here is completely different. Penetrating the stratum corneum, salts increase its osmotic pressure. In order to restore the natural water-salt balance, water from the underlying layers of the epidermis begins to enter the stratum corneum and linger in it, as if diluting the water phase and trying to bring the level of salt concentration in it into line with the norm. The result is an increase in the hydration of the stratum corneum, i.e. an increase in the water content in it.

• Restoring the barrier

Even if a lipid barrier disorder is not the primary cause of dryness, it still occurs if the skin suffers from a lack of moisture for a long time. Therefore, in addition to using moisturizers that relieve the feeling of dryness and increase the moisture content in the stratum corneum, it is necessary to use products designed to restore the barrier.

First of all, the damage to the barrier should be patched up with something rather quickly. Lipids are used for this purpose both in the form of pure oils and in combination with other ingredients in topical preparations. Lipid molecules penetrate into the intercellular spaces and are incorporated into the lipid barrier. Some of the lipid molecules applied on top gradually move along the intercellular spaces, reach the living layers of the epidermis and are included in cellular metabolism. In particular, they can serve as a substrate for further synthesis of lipids characteristic of the skin barrier.

Natural oils are mixtures of lipids. Therefore, the restorative efficiency and the predominant mechanism of action of oils will depend on their lipid composition. Oils containing essential fatty acids (linoleic and?-linolenic) promote accelerated synthesis of lipid barrier components, delivering the necessary lipid precursors directly to cells (borage, evening primrose, blackcurrant seed oils).

Oils rich in sterols stimulate keratinocytes and have anti-inflammatory properties (rosehip, tamanu, soybean, safflower oils). Oils enriched with saturated and monounsaturated fatty acids have more pronounced occlusive properties and help restore barrier properties by hydrating the epidermis (shea butter, tallow, macadamia, corn, coconut, cocoa, cashew).

Lipid mixtures composed of physiological lipids - ceramides, cholesterol and free fatty acids - are very effective. These lipids are called physiological because they form the natural lipid barrier of the human stratum corneum. It has been experimentally established that the best restorative properties are possessed by their equimolar (i.e. in equal parts) mixture - "ceramides/cholesterol/free fatty acids" in the ratio 1:1:1.

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