Today I do not want to talk about the threat to the skin from heated indoor environments while it's cold outside (at least in most of the northern hemisphere), but rather I want to highlight the importance of the skin's main function, which we tend to forget. The skin's primary function is to protect the body from external substances and excessive water loss. Water is an essential element for skin well-being, which is why cleansing and moisturizing are essential steps to keep it in good condition. Water follows a well-defined trajectory within the different layers of the skin: from the bloodstream, it reaches the dermis, and then spreads regularly and constantly to the upper layers of the epidermis, where its task is to preserve skin hydration. The aqueous film covering the epidermis is often defined as an "acid mantle," based on its acidic pH. Its composition comes from the secretion products of the sweat and sebaceous glands, from peptides derived from the breakdown of corneocytes (dead cells), and from some key epidermal lipids. In other words, it is a mixture of oils and water-soluble substances, whose task is to protect the skin. This protective film is divided into two parts: the upper part, where sebum and epidermal lipids are found; below it is the NMF (natural moisturizing factor) and it is composed of water-based molecules present on the epidermal surface. There is a constant and imperceptible evaporation of water from the outer layer of the epidermis. This mechanism is called Transepidermal Water Loss (TEWL). TEWL reflects the integrity of the skin barrier and is therefore used as a benchmark to assess skin health. Water, along with proteins and lipids, provides the skin's top layer with softness, flexibility, and elasticity. The state of hydration is regulated by substances present in corneocytes (dead cells), capable of retaining water, and the quality of the lipids present. The most important proteins here are represented by keratin, involucrin, and filaggrin: they have the ability to bind water molecules (remember when I tell you please do not over-exfoliate because you are killing all these valuable proteins?). The lipids of the outermost layer are essential for retaining the proper amount of water in the skin and for regulating TEWL (barrier effect). In particular, linoleic acid (omega 6) plays a key role in the synthesis of barrier lipids. The main dehydration factors are chemical (e.g., the dissolving and solid-removing action linked to the repeated application of surfactants - cleansing agents - which is why I always mention that cleansing is the most important step in beauty routines) or they can be linked to environmental aggressions: wind, cold, and humidity related to the environment, when they intervene separately or together, cause dehydration with the formation of dry, rough, flaky, chapped, and cracked skin. Dehydrated skin favors the penetration of foreign substances and the appearance of inflammatory problems such as eczema, dermatitis, and causes loss of elasticity and plasticity. The hydration strategies are basically two:
- Regulate skin water content
- Adjust skin lipid content
A good moisturizer should act on both levels, to restore the skin's hydrolipidic film.