The skin's outer layer has the vital function of forming a barrier against the environment to regulate water loss and to protect against pathogens, toxic substances and UV-light. This water barrier is constantly challenged by factors in our environment. Sun, wind, cold and heat as well as air pollution, makeup and daily cleaning procedures are especially harsh on the barrier. If the barrier is disrupted, the result is a loss of water and water-retaining molecules and dry, red, flaky skin. Get to know your skin better and understand how to protect the water barrier from environmental challenges and you will be rewarded with a glowing, younger-looking skin.
The Epidermis Water Barrier
The epidermis barrier, also called the Stratum corneum forms a water-tight barrier on the skin surface. It is composed of non-living cells and a special oil mixture enriched in ceramides and cholesterol, organized as a brick wall with the oil as mortar.
The cells in the barrier are renewed every 3-4 weeks from one layer of dividing keratinocyte cells situated on the border between the skin's inner and outer layer. Once new cells are formed, they are pushed outward towards the skin surface, whereafter they undergo radical changes and get rid of most of their normal cellular content. Instead, they produce huge amounts of specialized proteins, mainly keratin and filaggrin, and finish up as non-living flat hexagonal bricks.
The keratin protein produced by the cells are the same hard material that makes up our hair and nails. The keratin is organized into a skeleton structure supporting the shape of the cells as well as connecting them to each other into long sheets, ensuring a strong rigid structure even after the cells are dead. The filaggrin protein works together with keratin to create this skeleton structure.
To rejuvenate the skin and to avoid that the skin does not get too thick and hard, there is a natural exfoliation process, Desquamation, where the oldest cells are shed from the skin surface. This peeling process is an active breakdown of the special connections between the cells in stratum corneum and requires water to function properly. When the skin is not hydrated enough, the shedding does not work correctly and the old cells fall off in big chunky flakes instead of one by one.
Natural Moisturizing Factor (NMF)
The epidermis water barrier is jam-packed with water-retaining molecules called Natural moisturizing factor (NMF). NMF is not one substance but a mix of small hygroscopic molecules that keep the outer layer of the skin moist and supple. You may recognize many of them from the ingredient lists of skin moisturizers. Some, like glycerin and urea, have been used in skin care since the 1940s for their excellent properties to rehydrate the skin.
The components of NMF are extremely important for skin health. They hydrate the skin and provide elasticity and suppleness to avoid that the skin surface cracks and tears. They are crucial for the natural shedding of dead cells to function correctly. They are involved in the regulation of stratum corneum and in maintaining the skin's acidity referred to as the Acid mantle. An important protection against bacteria.
Free amino acids are the most abundant among the NMF and compose around 40% of all NMF. The free amino acids have water-retaining properties but their main function is to limit the friction between keratin filaments to increase skin suppleness. They are also the raw material for some of the other NMF. All proteins are made up of strings of amino acids. Dry skin triggers the breakdown of the filaggrin protein to release the amino acids and rehydrate the stratum corneum. But for the breakdown of filaggrin to work, water is required. When the skin is too dry, the process of breaking down proteins does not work, making it more difficult to rehydrate severely dry skin with the result of brittle skin that cracks and tears easily. Several serious skin diseases are caused either by problems of producing the filaggrin protein or by problems of breaking it down into amino acids. The filaggrin protein decreases in the skin with age, which might be a reason why elderly often have problems with dry skin and the anti-aging science is searching for substances that stimulate its production.There is also a genetic link between low amounts of filaggrin and dry and eczema-prone skin.
Pyrrolidone carboxylic acid (Sodium PCA / sodium L-pyroglutamate) makes up a whole of 12% of NMF and is formed from the free amino acids. PCA is extremely hygroscopic and the moisture retention provided by PCA is essential for a healthy skin. In sensitive skin the levels of PCA are lower than normal and in atopic dermatitis, PCA might be as low as half of its normal concentration. When added to skin creams, PCA is easily taken up by the skin and a rehydrates the straum corneum very effectively.
Glycerin / Glycerol is formed from the breakdown of oils and other fat molecules and found in very small amounts in the stratum corneum. Glycerin is one of the oldest and most wetll-known humectants added to skin care products. Glycerin changes the behavior of the stratum corneum oils in a way that keeps the skin supple even in very dry conditions. The barrier in skin treated with glycerin for longer periods recovers quicker from acute perturbations.
Urea is not only hygroscopic but also mildly keratolytic meaning it is taking part in the natural peeling process by cleaving keratin molecules, thus reducing the thickness of the stratum corneum. Urea is a very efficient humectant when added to moisturizers, sometimes under the name Carbamide, referring to its chemical composition. Urea-containing creams have been shown to improve barrier function and reduce skin water loss. Urea is often added to foot creams to soften hard soles.
Lactic acid or Sodium lactate is an alpha-hydroxy acid (AHA) and as all AHA it rejuvenates the stratum corneum by accelerating epidermal turnover and increase exfoliation by breaking the connections between the cells in the stratum corneum. Unlike other AHAs that are not part of NMF, lactate also adds moisture to the stratum corneum and has been shown to increase ceramide production, which improves barrier function. Addition of Sodium lactate has also a proven restorative effect on photo-damaged skin.
Hyaluronic acid (HA) is only found in very small amounts but it is extremely hygroscopic and can hold between 500 and 1000 times its own weight in water. Unlike the other NMF that are very small molecules, hyaluronic acid is a large sugar polymer and hyaluronic acid added to skin care products does not penetrate the epidermis but stays on the surface as a film. This can be problematic in dry climates as it might actually draw moisture out of instead of into the skin. Broken hyaluronic acid, often called low weight HA (LMW-HA) is sometimes marketed as a superior alternative, small enough to penetrate into the skin. However, broken HA is not natural and even though some tests using LMW-HA seem to show increased skin humidification and even wrinkle improvement others show that it triggers the skin's immune system and cause inflammation and scarring. In either case, topical application of hyaluronic acid, broken or not, will not have the same behavior as hyaluronic acid produced in the skin.
Some other and minor components of NMF are Urocanic acid, Glucosamine, Creatine, Citrate and various Salt ions.
Be Kind to Your Skin and Protect it from Harsh Environments
Contrary to what many believe, most of the moisture in the epidermis comes from the inside of our body and not from the environment. Dry climate though, increases the loss of water from the epidermis, usually referred to as transepidermal water loss (TEWL). Thus, everything that disrupts the highly organized stratum corneum barrier results in an increased loss of water to the environment. Washing the skin with surfactants, alcoholic facial toners or even staying too long in the bath dissolves the oils in the stratum corneum and creates holes in the barrier. Even free fatty acids present in small amounts in vegetable oils or added as thickeners to skin creams compromises the barrier by disrupting the organization of the oils. What is worse, disruption of the barrier, for whatever reason leads to, not only a loss of water but a loss of NMF. Most of the NMF are small and water-soluble and are easily rinsed away through cleaning procedures.
Harsh surfactants, like anionic sulfate surfactants (i.e. SLS and SLES, not only removes the protective oils from the barrier but also has the ability to destroy the 3D-structure of skin proteins. The first reaction from the skin to surfactants is actually a temporary swelling that gives the false impression of hydration only to quickly de-swell with the water evaporating minutes after you stepped out of the shower, leaving the skin feeling tight and dry. If harsh surfactants are left longer on the skin, their negative charge disrupts the structure and function of skin proteins and they can penetrate deep into the Epidermis and break the cells on the basal lamina, triggering an inflammatory response with dry, red and flaky skin as a result. The stronger the surfactant, the quicker the holes in the barrier are created. The weather has also an enormous impact on this. Cold and dry winter climate, as well as hot and dry summer days, makes the skin more sensitive to harsh cleaning procedures and the barrier breaks more easily. Though the barrier function has been shown to be generally better in the summer than in the winter. Breaking the Epidermis water barrier does not only result in moisture and NMF streaming of the skin but also that any toxic substances that you expose your skin to afterward are taken up by the skin more easily and can penetrate into its deeper layers, from environmental pollution to questionable substances in your facial cream.
To keep a healthy skin it is important to take care not to disrupt the epidermal water barrier. Don't wash your skin with stronger surfactants than necessary. Rinse quickly and don't leave shampoo and washing products on your skin for longer periods. And don't forget to moisturize. Add back the NMFs to your skin. Water in a skin cream only gives a transient increase in skin hydration while PCA, lactate, urea and glycerin have been scientifically proven to rehydrate the skin. For them to be effective though, they must be added in combination with oils and lipids that limit water evaporation. Skin creams and lotions today are combinations of humectants, emollients and occlusives. Humectants bind water and help to keep the moisture inside the skin. The most common natural humectants are of course the hygroscopic NMF components. Other natural humectants are aloe vera, panthenol, sorbitol and hydrolyzed proteins. Emollients fill the crevices between the cells in stratum corneum and smooths flaky skin. Emollients give an instant feel of lubrication and increased skin flexibility and might improve the skin barrier but are not that effective in preventing water loss. Natural emollients are many vegetable oils, lecithin and squalane. Occlusives on the other hand, form a thin film on the skin surface, which limits the evaporation of water. Among the natural occlusives you find vegetable butters such as shea and cacao butter, lanolin and waxes like beeswax, carnauba, candelilla and to some extent jojoba oil, a liquid wax with both emollient and occlusive properties. When the climate is challenging it might be a good idea to use creams with a higher proportion of occlusives to minimize water loss. To use pure vegetable oils and body butters instead of water/oil emulsions can also be effective in improving the skin barrier and minimize water loss but works best on damp skin. Use immediately after the shower before the skin is completely dry to lock in the moisture and protect against after shower-dehydration.