The Natural Moisturizing Factor (NMF) is a natural skin protector against dehydration; it also plays a major role in the physical properties of the outermost layer of skin, the horny layer. Good moisturizing of this layer is essential for skin smoothness. These body natural substances, which fixate the water in the horny layer, are produced during keratinocyte cornification (e.g. pyrrilidocarbonic acid), sweat and secretion by sebum glands (including urea, salts and organic acids).

The cytoplasm of deepest korneocytes includes dense masses of proteins (keratin and filagrinin) with marked hydrophobicity. It is accepted that the skin barrier function is based on these horny cells since, in intermediate and superficial korneocytes, part of the filagrin is separated from keratin fibers and hydrolyzed into its constituent amino acids; furthermore, some metabolites appear (pyrrolidone carboxylic acid, urocanic acid); all of this implies that those horny cells are expressing hydrophilic osmolites, which are more permeable to water.

Thus, NMF is a mixed complex of water soluble low molecular weight substances, produced within korneocytes through the degradation of filagrin

The main cosmetic use of Moisturizing Factor is related to moisturizing products for every kind of skin, especially for dry skin, or to products to be used in areas with low environmental humidity.

• Amino acids

Because of their small size, amino acids have the ability to penetrate to the deepest horny layers, where they exert their moisturizing action.

• Urea

Urea is considered the best natural moisturizer because of its ability to capture water (hygroscopic action). Furthermore, it has mild keratolytic effects, since it cleaves keratin molecules, thus reducing the thickness of the horny layer and restoring the hydration-evaporation process to normal. As the urea concentration decreases, its keratolytic action also decreases, but the moisturizing action is maintained. A raise in urea concentration consequently produces a raise in the keratolytic action.

• α-hydroxy acids and their salts (lactates)

In 1972, a researcher observed that lactate salts – one of the NMF components – had noticeable moisturizing ability. In 1974, a researcher reported that lactate salts were suitable to treat skin dryness and scaling. The sodium salt is the most hygroscopic one and it plasticizes the horny layer, over 50% humidity, although it can be readily absorbed as a free acid. These compounds act by adsorbing on the polar groups that adhere to keratinocytes, thus reducing the interactions among them (ionic bonds) and promoting desquamation.

• Pyrrolidone carboxylic acid (PCA)

This is a moisturizing molecule that increases the water fixation capacity. It enhances skin moisture, smoothness and elasticity without producing irritation.