Moisturizers: What They Are And How They Work

C. W. Lynde, MD, FRCPC
Toronto Hospital, Western Division, Assistant Professor, Department of Dermatology, University of Toronto, Toronto, Ontario, Canada

ABSTRACT

Moisturizers are widely used in various dermatologic and cosmetic skin therapies. Different classes of moisturizers are based on their mechanism of action, including occlusives, humectants, emollients and protein rejuvenators. Commercially available moisturizers often utilize components of each of these classes to provide their beneficial effect. Dry skin (xerosis) is the major indication of use. Others include atopic dermatitis, irritant contact dermatitis, ichthyosis, and dermatoheliosis. Although generally efficacious, moisturizers can cause a number of unwanted side effects, including occlusive folliculitis, irritation, allergic contact dermatitis and contact urticaria.

Key Words:
moisturizers, mechanism of action, side effects, dry skin

“Dry skin” is an extremely common problem. Our society and its advertising suggest that we have a simple solution – just apply a moisturizer. The marketplace has a great number of products to moisturize the skin and billions of dollars are spent yearly on these products.

What Are Moisturizers?

Interestingly, standard textbooks of dermatology devote very little space or discussion to this subject, and no standard definition exists yet dermatologists use and recommend moisturizers many times daily. They are bland oleaginous substances that are applied to the skin by rubbing. They are used to replace natural skin oils, to cover tiny fissures in the skin, and to provide a soothing protective film. They may, thus, slow evaporation of the skin’s moisture, thereby maintaining hydration, and improving the appearance and tactile properties of dry and aging skin.

How Do Moisturizers Work?

Traditionally, moisturization was believed to inhibit transepidermal water loss (TEWL) by occlusion. Water originates in the deeper epidermal layers and moves upward to hydrate cells in the stratum corneum, eventually being lost to evaporation. Occlusive moisturization, then, prevents the dehydration of the stratum corneum.

Much more is now known about the epidermis, and in particular, the stratum corneum. The “bricks and mortar” model suggests that its role is as an active membrane. Loss of intercellular lipids, i.e., the ceramides, cholesterol and fatty acids that form the bilayers, damages the water-barrier function. The stratum corneum then calls into action repair mechanisms.¹

The Natural Moisturizing Factor (NMF), a natural mixture of amino acids, lactates, urea and electrolytes, which help the stratum corneum retain water is also now known.² Dry skin is noted when the moisture content is less than 10%, and there is loss of continuity of the stratum corneum.

Scientifically, the moisturizing treatment involves a 4-step process:

• Repairing the skin barrier
• Increasing water content
• Reducing TEWL
• Restoring the lipid barriers’ ability to attract, hold and redistribute water

Table 1: There are currently several groupings of moisturizing substances that are based on their theoretical mechanism of action.

Occlusives

Occlusives are substances that physically block TEWL in the stratum corneum. Petrolatum in a minimum concentration of 5% is the most effective occlusive followed by lanolin, mineral oil, and silicones such as dimethicone. Petrolatum is widely used as a classic moisturizer. Lanolin, a complex structure of esters, diesters, and hydroxyesters of high molecular weight, lanolin alcohols, and lanolin acids, is also widely used and quite effective.^1,3,4

Humectants

Humectants attract water when applied to the skin and theoretically improve hydration of the stratum corneum. However, the water that is drawn to the skin is trans-epidermal water, not atmospheric water. Continued evaporation from the skin can actually exacerbate dryness. Humectants include glycerin, sortbitol, urea, alpha hydroxy acids (i.e., lactic acid) and other sugars. NMF is made up of a mix of low molecular weight soluble hygroscopic substances including lactic acid, pyrollidone-carboxicilic acid and amino acids. This naturally mixing compound is thought to be a major player, keeping the horny layer hydrated and flexible.² Manufacturers’ attempts to include the above humectants into moisturizers do not always produce a beneficial effect. High concentrations of propylene glycol and urea can be irritating. Pure mixtures of amino acids are useless as moisturizers. Pure solutions of glycerin are ineffective and propylene glycol by itself is irritating.¹ In addition to their humectant properties, urea and lactic acid are keratolytic. Urea is a humectant in lower concentrations (10%), but in higher concentrations (20-30%) it is mildly keratolytic by disrupting hydrogen bonds or epidermal proteins. Alphahydroxy acids, such as lactic acid or glycolic acid, appear to increase cohesion of the stratum corneum cells, thereby reducing roughness and scaling.

Table 2: A summary of some moisturizing products and their active ingredients.

Emollients

Emollients smooth skin by filling spaces between skin flakes with droplets of oil, and are not usually occlusive unless applied heavily. When combined with an emulsifier, they may help hold oil and water in the stratum corneum. Vitamin E is a common additive, which appears to have no effect, except as an emollient. Likewise, other vitamins, e.g., A and D, are also added with questionable effect. Examples of emollients include mineral oil, lanolin, fatty acids, cholesterol, squalene, and structural lipids.

Long chain saturated fatty acids and fatty alcohols are commonly used in topical pharmaceuticals and cosmetic formulations. They exert their benefits through effects on the skin barrier and on permeability. Examples include stearic, linoleic, linolenic, oleic, and lauric, which can be found in palm oil, coconut oil, and wool fat. Fatty acids and fatty alcohols can change the properties of intracellular lipids or the stratum corneum. Medium chain saturated hydrocarbons or longer chain unsaturated hydrocarbons are the most effective.⁵ Essential fatty acids (i.e., linoleic and alpha-linoleic acids) influence skin physiology and pathology via their effects on skin barrier functions, eicosanoid production, membrane fluidity, and cell signaling.

Structural lipids, i.e., intracellular lipids comprising multilamellae, which are located between stratum cornea cells, are also felt to play a considerable role in the water-holding potential of the stratum corneum. Ceramide is a major component of the inner cellular lipids and plays a major role in generating multilamellae architecture. Natural ceramides themselves, or their synthesis, are at present too expensive to make commercially available. Several pseudo ceramides have been synthesized and clinically shown to be effective in preventing and improving dry skin.¹

Moisturizers containing collagen and other proteins, i.e., keratin and elastin, claim to rejuvenate the skin by replenishing its essential proteins. This is unlikely to occur since these protein molecules are too large to penetrate the dermis. Protein additives may provide temporary relief of dry skin by filling irregularities in the stratum corneum. Like emollients, when they dry they shrink slightly, leaving a protein film that appears to smooth the skin and stretch out some of the fine wrinkles.

Indications Of Use

Indications for emollients include dry skin, i.e., xerosis or as a result of metabolic conditions, such as renal insufficiency and diabetes; atopic dermatitis; ichthyosis vulgaris; irritant contact dermatitis and prevention⁶; nummular dermatitis; psoriasis; skin protection, i.e., frequent hand washing; and dermatoheliosis.

Table 3: Adverse effects caused by moisturizers.

What Is The Ideal Moisturizer?

Patients who are confused by media hype often ask this question. The ideal moisturizer should be:^7-10

• An effective moisturizer – hydrating the stratum corneum
  reduces and prevents TEWL
• An emollient – makes skin smooth and supple and reduces TEWL
• An aid in restoring the lipid barrier, i.e., duplicating and
  enhancing the skin’s natural moisture retention mechanisms
• Cosmetically elegant and acceptable
• Moisturizing to sensitive skin – i.e., hypoallergenic, nonsensitizing,
  fragrance free, non-comedogenic
• Offered at an affordable price
• Long-lasting
• Absorbed rapidly providing immediate hydration

Conclusion

As the population ages, the number of people suffering from dry skin will increase. A fundamental understanding of the physiochemical effects of moisturizers on the basic functions of the skin barrier will allow the further development of physiologically effective products for the prevention and treatment of dry skin and its related skin conditions.

References

1. Loden M, Maibach H, Dry Skin and Moisturizers Chemistry and Function. New York:CRC Press 1999.
2. Harding, C. Bartolone, J. Rawlings A. Effects of Natural Moisturizing Factor. In: Loden M, Maibach H, eds. Dry Skin and Moisturizers; Chemistry and Function. New York: CRC Press 1999.
3. Tanner F, Beurbe G. Mineral oil and petrolatum – reliable moisturizers. Cosmetic Toiletries 93:81 (1978).
4. Ghadially R, Halkier-Sorenson L, Elias P. Effects of peterolatum on stratum corneum structure and function. J Am Acad Dermatol 26(3 Pt. 2):387-96 (1992 Mar).
5. Jackson EM. Moisturizers: what’s in them and how do they work. Amer J Contact Dermatitis 3:162-168 (1992).
6. Zhai H, Maibach HI. Moisturizers in preventing irritant contact dermatitis: an overview. Contact Dermatitis 38(5):241-4 (1998).
7. Jackson EM. Moisturizers: adjunct therapy and advising patients. Amer J Conact Dermatitis 7(4):247-50 (1996 Dec).
8. Wehr RF, Krochmal L, Considerations in selection a moisturizer. Cutis 39(6):512-5, 1987.
9. Dry skin, winter itch. Health News. University of Toronto Faculty of Medicine 9(6) (1991 December).
10. Gossel, T. Dry Skin. US Pharmacist 15(1):20-25 (1990).