Moisturizers: What They Are and a Practical Approach to Product Selection

J. N. Kraft, BSc (Hons)¹ and C. W. Lynde, MD, FRCPC^2
1Faculty of Medicine, University of Toronto, Toronto, Ontario, Canada
²University Health Network (Western Division) and Department of Dermatology, University of Toronto, Toronto, Ontario, Canada

ABSTRACT
Moisturizers are widely used products that are important in many dermatologic and cosmetic skin therapies. They contain varying combinations of emollients, occlusives, and humectants to achieve their beneficial effects, and there is an overwhelming number of formulations available. To develop a rational approach for prescribing moisturizers, commercially available products can be categorized on the basis of application site.

Key Words:
dry skin, emollients, humectants, moisturizers, occlusives

There is a vast array of moisturizers available on the market today and consumer demand for these products is growing. These products range from value brands that provide basic moisturization to luxury therapeutics with claims of anti-aging benefits. A recent US study found that moisturizers are the third most commonly recommended OTC topical skin product (13.4%) behind hydrocortisone (27.6%) and anti-infectives (23.4%).¹

What Are Moisturizers?

The term moisturizer is a marketing term with little or no scientific meaning. Consumers see moisturizers as actively increasing the water content of the skin. Dermatologists see moisturizers as bland oleaginous substances that are applied to the skin by rubbing.² The term “moisturizer” does not necessarily imply that moisture or water is being added to the skin. Moisturizers are a key component of basic skin care especially when there is alteration of the epidermal barrier and reduced water content in the epidermis.³ They are used to restore the barrier function of the epidermis, to cover tiny fissures in the skin, provide a soothing protective film, and increase the water-content of the epidermis. 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. Newer products claim to have other properties such as anti-aging, skin-firming, anticellulite, and sun-protectant effects.

How Do Moisturizers Work?

For many years, epidermal water content has been known to be crucial for skin plasticity and the prevention of “dry skin”.4 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 (SC), eventually being lost to evaporation.

The SC architecture is the most important factor in water flux and retention in the skin, and in overall level of moisturization.⁵ The four key processes for the formation and functioning of the SC are the corneocyte process, SC lipid process, natural moisturizing factor (NMF) process, and desquamation process.6 Corneocytes are the physical barrier of the SC and, when hydrated, contribute to elasticity. The lipid bilayers of the SC function as a moisture barrier and although they prevent the entry of many chemicals, they are the means of entry for most topically applied substances. The NMF is found within corneocytes and is a mix of hygroscopic molecules that, by helping maintain hydration in the corneocyte, keep the SC hydrated. Half of the NMF is amino acids derived from the protein filaggrin in keratinocytes, and the other half is salts, including lactates, urea, and electrolytes. Production of NMF is directly related to external humidity. In desquamation, corneodesmosomes are degraded by water-dependent hydrolytic agents. When there is low moisture in the SC, these enzymes do not work efficiently. Corneocytes accumulate on the skin surface producing the signs of dry skin, e.g., when the moisture content is less than 10%, and when there is loss of continuity of the SC.²

The moisturizing treatment involves repairing the skin barrier, retaining/increasing water content, reducing TEWL, restoring the lipid barriers’ ability to attract, hold and redistribute water, and maintaining skin integrity and appearance. Moisturizers perform these functions by acting as humectants, emollients, and occlusives.⁷ Moisturizers containing collagen and other proteins, i.e., keratin and elastin, claim to rejuvenate the skin by replenishing its essential proteins but whether or not they have any effect on skin hydration is questionable.² Moisturizers also act to reduce skin friction and increase skin hydration by providing water directly to the skin from their water phase and by increasing occlusion, as measured as a decrease in TEWL.⁸ Loden suggests that skin care products not only form an inert, epicutaneous layer, but that they also penetrate and influence the structure and function of the skin.⁹

Moisturizers have little effect on the mechanical properties (i.e., distensibility, hysteresis, and elasticity) of the skin but do increase skin hydration significantly, as shown by an increased skin capacitance.¹⁰ When moisturizers are used to improve skin plasticity it is suggested that lipid-rich formulations be used.¹¹

Emollients

Emollients, which are mainly lipids and oils (see Table 1), hydrate and improve the appearance of the skin by contributing to skin softness, enhanced flexibility, and smoothness. The “skin slip” or lubricity of some moisturizers, contributes to consumer satisfaction and product preference.⁵ Consumers desire smooth skin following moisturizer application.³ Emollients serve to fill the cracks between clusters of desquamating corneocytes and are not usually occlusive unless applied heavily.

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, partially through improved repair, and on permeability.¹² Examples include stearic, linoleic, linolenic, oleic, and lauric, which can be found in palm oil, coconut oil, and wool fat. A sterol-enriched fraction from canola oil reduced clinical signs of sodium lauryl sulphate (SLS)-induced irritation.¹³ Other lipids (e.g., fish oil, petrolatum, shea butter, and sunflower seed oil) had no effect on the degree of irritation. Loden and Andersson suggested that canola oil assisted the skin in supplying the damaged barrier with adequate lipids. 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.²

Occlusives

Occlusives reduce TEWL by creating a hydrophobic barrier over the skin and contributing to the matrix between corneocytes, and have the most pronounced effect when applied to slightly dampened skin. There is a wide range of agents with occlusive properties (see Table 2). Their main limitations include odor, potential allergenicity, and the greasy feel associated with most occlusives.

Petroleum jelly, in a minimum concentration of 5%, reduces TEWL by more than 98% and is the most effective occlusive, followed by lanolin, mineral oil, and silicones (e.g., dimethicone), which only reduce TEWL by 20%-30%.^2,14 Occlusives are thought to diffuse into the intercellular lipid domains, thus contributing to their efficacy. 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.^14,15

Humectants

Humectants (see Table 3) are able to attract water from two sources: they enhance water absorption from the dermis into the epidermis, and in humid conditions they also help the SC to absorb water from the external environment. Many humectants also have emollient properties.³ The most effective humectant is the trihydroxylated molecule, glycerol.16 Immature corneocytes are fragile but mature into more resilient and protective cells as they migrate through the SC.^7,17 Glycerol hastens the maturity of corneocytes through the activation of residual transglutaminase activity in the SC.¹⁸ Also, by facilitating the digestion of desmosomes and subsequently enhancing desquamation, glycerol reduces the scaling associated with xerosis.¹⁹

Found in the NMF, pyrrolidine carboxylic acid hydrates the skin, and has been shown to improve xerosis.20 Urea is another important humectant. In double-blind studies moisturizers with urea have been shown to reduce TEWL in atopic and ichthyotic patients,^21,22 and reduce SLS-induced skin irritation.⁸

Alpha hydroxy acids (e.g., lactate) are effective agents for the treatment of dry skin; following treatment with lotions containing D-, L-lactic acid, the SC prevents xerosis more effectively.²³ Lactic acid, particularly the L-isomer, stimulates ceramide biosynthesis leading to higher SC ceramide levels that result in a superior lipid barrier and more effective resistance against xerosis.

One major drawback of humectants is that some of them can increase TEWL3 by enhancing water absorption from the dermis into the epidermis where it can then be lost into the environment. For this reason, they are almost always combined with an occlusive agent. Occlusive and humectant ingredients work together to enhance epidermal hydration and barrier function.

Where Are They Used?

Moisturizers are often used in a variety of conditions including xerosis that is due to a genetic tendency (e.g., ichthyosis) or is secondary to an underlying disease (e.g., diabetes, hypothyroidism, or atopic dermatitis) (see Table 4).

They are also used following epidermal barrier damage from harsh cleansers, topical medications or astringents.

What is the Ideal Moisturizer?

Patients who are confused by media hype often ask this question. The ideal moisturizer should be:²

• Effective-hydrating the SC 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., hypo-allergenic, nonsensitizing, fragrance free, noncomedogenic
• Affordable
• Long-lasting
• Absorbed rapidly providing immediate hydration.

Formulation Characteristics

Nearly all contain a combination of emollients, occlusives, and humectants. Combining occlusives and humectants enhances the water-holding capacity of the skin. Also, the esthetic properties of the moisturizer and the stability of the active ingredients can be influenced by the addition of certain emollients.³⁶ When glycerol, a humectant, is combined with occlusive agents, there is a synergistic alleviation of dry skin.³⁷ The predominant form of delivery is the cosmetic emulsion. The process of emulsification combines the phases containing the ingredients. The majority are lotions (oil-in-water emulsions) or creams (water-in-oil emulsions). More complicated emulsions (e.g., oil-in-water-in-oil, oleaginous mixtures, serums, gels, sprays, and milks) are used to deliver and stabilize some active ingredients. The esthetics vary in accordance with consumer preferences and the desired attributes. Compliance will likely be poor if the patients are not satisfied with their prescribed moisturizer.²² Low pH and sensory reactions, e.g., from lactic acid and urea, can cause burning on application and may reduce patient acceptance.
The precise nature of these formulations is not disclosed and the ingredients are not always listed on the product.³⁸

Lotions tend to be thinner and are commonly preferred for daytime facial use. The typical components include propylene glycol, mineral oil, and water. Creams are generally made with heavier lipids, are often applied at night, and are typically composed of petrolatum, lanolin, mineral oil, and water. Industry adjustment of the oil-water ratio, occlusives, and emollients provides the basis of formulations for different skin types (oily, normal, dry complexions) and sites of application. Ideally, dermatologists should recommend therapeutic moisturizers that are noncomedogenic, devoid of irritant ingredients, and compatible with many therapeutic regimens.³¹

Moisturizers are generally marketed in two categories: face care, and hand and body care.5 Within each category are specialized products geared for certain areas such as the lips, eyes, and feet. Common moisturizers available over-the-counter can be classified according to application site (see Table 5). The face is particularly prone to effects of the environment (e.g., drying in cold, arid conditions, and aging from sun exposure). Moisturizers designed for the face are typically non-greasy, noncomedogenic emollients, with an emphasis on skin feel and aesthetics with maximal skin benefits. Silicone derivatives in particular are targeted for consumers with oily skin. Other ingredients are added to reduce the appearance of excess shine such as oil-absorbent compounds (e.g., kaolin, talc).

Antiaging technology is the fastest growing segment of facial moisturizer market.⁵ Moisturizers play a role in treating and augmenting therapy for the aging face.39 Certain agents are especially useful for photoaged skin and include sun protectants, alpha hydroxy acids (e.g., glycolic acid), and retinol and its derivatives.^36,40

Hand and body care is mainly aimed at the prevention and treatment of dry skin. Some specialized products’ aims include the reduction of cellulite, firming, bronzing, and minimizing the signs of aging. There are a wide variety of products ranging from those for everyday use and good value to more expensive products for cosmetic and therapeutic use.

Conclusion

As noted in Table 5, the skin care marketplace offers a wide array of moisturizers targeted for face, body, hands, or feet, providing the consumer with good, effective moisturization. Even more clinically effective and cosmetically appealing formulations will occur with improved emulsion technologies, better delivery of active ingredients, and further combinations.

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