The suppleness and overall functionality of the stratum corneum (SC) is closely related to its water content. This is reflected in the signs and symptoms of dry skin. These may manifest as scaling with an associated reduction of mechanical flexibility and an uncomfortable feeling of tightness, in addition to:
Additionally, dry skin is dull and lifeless in appearance.
The role of the NMF
Physiologically, the top of the SC is one of the tissues in the human body with the lowest water content. Naturally, as water is being constantly lost from the skin surface, a water gradient is established within the different layers of the SC. The natural hydration levels in the SC are between 15% at the skin surface and 40 – 45% at its innermost layer compared with 70–80% within the granular layer (stratum granulosum). A selective retention of water in the SC is required and this function is largely dependent on the presence of the so-called Natural Moisturizing Factors (NMF). Dry skin is described to contain less than 10% of water, which relates strongly to a lack of the presence of NMF.
The NMF comprises hygroscopic molecules, produced during the epidermal differentiation process, which are able to bind water at the surface of skin. The NMF may represent up to 15–30% of the corneocyte mass. The NMF is, for the bigger part, produced by the enzymatic breakdown of filaggrin. This process can only take place successfully with optimal water levels in the SC as the activity of these enzymes is strongly dependent on the water level in the SC. This implies that the SC of dry skin not only lacks water but also contains reduced levels of NMF, which is required to bind water in the SC. In this way, a vicious circle is created. The dry skin cycle caused by the impaired NMF production can be disrupted by increasing water levels in the SC by the application of moisturizers, i.e., substances whose application results in increased water levels in the SC.
The prevalence of dry skin, and vitamin D
Over the past decennia the world, in the broadest sense of the word, has gone through tremendous changes. Many of these changes are relevant to our health and our skin in particular. Improved economic situations and urbanization have led to a significant increase in the number of people working and living indoors, mostly in an extremely dry (air-conditioned or heated) environment. Personal hygiene practices have also considerably changed. A growing part of the world population follows stricter and more frequent personal hygiene practices. Sun protection has undergone an increase in acceptance in the world population as well, where significantly more consumers now use sun protective products than a few decades ago. In short, on average our skin is increasingly exposed to influences which promote skin dryness (dry environment, personal hygiene) and decreasingly exposed to sunlight.
Whereas sunlight has many deleterious influences on skin, it has one important positive outcome: it provides our skin with the ability to produce vitamin D. The role of vitamin D in those processes in the skin which are vital for its ability to bind water have been described in the scientific literature. Vitamin D deficiency is common in people suffering from atopic dermatitis, a skin disease which is associated with reduced skin barrier function and skin dryness. Additionally, vitamin D is an important initiator of filaggrin production, which is of particular note in this context, filaggrin being the most important raw material for the production of the NMF.
The importance of the VDR
In more detail, the so-called vitamin D receptor (VDR) is essential for epidermal differentiation. The VDR is the receptor to which vitamin D binds to elicit its biological effects. Interestingly, the presence of vitamin D leads to an increase in the expression of the VDR.
In this sense, vitamin D is able to potentiate its own biological effects, as with more VDR, beneficial biological processes which lie downstream of the activation of the VDR are activated more effectively. An increase in expression of the VDR leads to an improvement of epidermal differentiation, which, as suggested above, includes an increase in filaggrin production and, therefore, more moisturized skin.
Effective skin mositurization
A quality moisturizer formulation should not only reduce skin dryness, but also prevent its return. Today, the amelioration of dry skin symptoms is still the primary benefit of skincare products to the consumer.
Many ordinary humectants (glycerin, sodium pyrrolidone carboxylate, urea, and sodium lactate) are used in cosmetic skincare formulations, but application of aqueous solutions of ordinary humectants has not shown to increase the hygroscopicity of the skin. Moisturization of skin is, therefore, not simple and should not be taken lightly.
It is of eminent importance to consider the core problem of skin dryness. Whereas demographic and sociological changes are impossible to influence, focus can and should be placed on the main downstream physiological consequence of these changes: hypovitaminodosis D, i.e., a lack of the activation of the VDR.