The Controversial Influence of Moisturizers on Skin Water Barrier

The outermost layer of human skin, the stratum corneum (SC), is composed of corneocytes that provide protection against desiccation and environmental stress.¹ Protection is partially achieved by retaining skin hydration, which is affiliated with the phenotype of the corneocytes, their unique positioning and their composition.¹ As is described here, moisturizers can influence the properties and structure of corneocytes by influencing the SC and consequently, the skin water barrier function. This hydrative influence of moisturizers could be beneficial as well as destructive toward skin barrier function.

Moisturizers and Irritant Contact Dermatitis

Irritant contact dermatitis develops due to constant exposure to irritants such as soaps, water, detergents and friction.² In some cases, it may not be practical to avoid contact with some irritants, especially at work. Hence, the prophylactic application of moisturizers may reduce the chance of developing irritant dermatitis.³ The protective benefits of moisturizers on skin hydration have been documented in cases of cleaners⁴ and kitchen assistants⁵ who are continuously exposed to water and detergents. Moisturizers, especially those containing urea, have demonstrated an increase in skin healing rates and reduction in skin susceptibility to sodium lauryl sulphate (SLS).⁶ Also, Ramsing et al. showed that besides protective capabilities against irritant contact dermatitis, moisturizers can enhance skin barrier repair. ⁷

Furthermore, the continuous application of moisturizers has been shown to act as a protective barrier against repeated contact with irritants.² Studies suggest that regular application of moisturizers could provide preventive benefits against the development of dermatitis in health care settings,² and that workers who develop the habit of using hand lotions and creams could enhance skin hydration, thus resolving the contact dermatitis induced by hand hygiene products.⁸

Conversely, the literature also demonstrates the role of moisturizers in increasing the incidence of irritant contact dermatitis.⁹ The long term application of moisturizers on normal skin has been shown to cause a statistically significant increase in measured transepidermal water loss (TEWL) values. Hence, the observed reduction in skin barrier function may also increase skin susceptibility to irritants and enhance the penetration of noxious substances with increased ease,² which is discussed in more detail in this article. And while Williams et al.² expressed that most studies agreed on the positive influence of moisturizers against skin irritants, these studies only focused on a single product alongside a single irritant. Finally, the application of a 24-hr patch test may not truly represent real life situations, where skin exposures occur briefly in an open, cumulative manner.²

Moisturizers and Susceptibility to Irritants

While TEWL and electrical capacitance measurements have been crucial in assessing the impact of moisturizers, especially on contact dermatitis, insufficient research has been conducted regarding how moisturizers affect the susceptibility of normal skin to irritants.^{10, 11} As previously stated, it has been demonstrated that if skin is hydrated with moisturizers, its permeability to hazardous substances may increase; TEWL measurements have demonstrated that exposure of skin to lipid-rich moisturizers may intensify the reaction to SLS, compared with low lipid content moisturizers or untreated skin.¹² This observation was speculated due to the enhanced penetration of hydrophilic substances into the skin layers and possible dysfunction at the skin lipid bilayers.

Within the limited studies published on skin barrier function and susceptibility to irritants, it also has been revealed that the use of daily moisturizers may enhance the average water content of SC artificially.¹² These increased hydration levels would reduce barrier properties; TEWL would be increased, thereby reducing the protective properties of the SC.¹³ On the other hand, Loden et al. demonstrated that compared with untreated skin, skin treated with moisturizers containing urea showed less contact irritation with SLS exposure.¹⁴ Further, bioengineering means and electrical capacitance measurements have shown that urea-containing moisturizers do not enhance skin hydration levels.¹⁵ This phenomenon of daily moisturizers enhancing water content in the SC should therefore be further investigated because it has important applications for wet work employees who are advised to apply moisturizers concurrent with work-related water and detergent exposures.¹²

Based on the findings presented here, it could be deduced that treatment with moisturizers may or may not provide any skin protection, and could impart different influences on the skin’s susceptibility to irritants.¹² In general, however, all the findings support the idea that skin hydration levels may be directly correlated with the degree of skin susceptibility to irritants.¹²

Moisturizers and Atopic Dermatitis

Atopic dermatitis (AD), a chronic disease that exacerbates or flares over the years,¹⁶ affects patients through the induction of skin damage, soreness, sleep loss and the social stigma of a visible skin disease.¹⁷ AD has been considered a primary disorder associated with SC barrier dysfunction, which exposes patients to possible eczema flares.^{18, 19} Conventionally, topical gluco- corticosteroids or topical calcineurin inhibitors in conjunction with moisturizers are used for the treatment of AD.²⁰ This may in part be because moisturizers can reduce dry skin symptoms, lowering the need for corticosteroids.²¹ In fact, moisturizers have not only been shown to reduce the levels of corticosteroids required and improve the appearance of eczema, ^{22, 23} they also have been shown to extend clinical improvements after treatment termination.²⁴

However, due to a lack of conclusive data, recommendations to use moisturizers to treat AD do not enjoy universal support,²⁰ especially since some studies demonstrate the noted controversial influence of moisturizers.^{24, 25} For example, Wiren et al.¹⁶ demonstrated that moisturizers reduce the relative and absolute risk of eczema relapse by 53% and 36%, respectively. However, they also compared treatment outcomes using moisturizers with results using calcineurin inhibitors or corticosteroids on atopic eczema, and results did not demonstrate significant differences.^26–28 Similar outcomes regarding the delay in eczema relapse in patients using moisturizers compared with patients using anti-inflammatory treatments indicate that barrier-strengthening therapies could be considered a strong alternative in the long-term management of eczema.^{19, 29}

Based on most research findings presented in this section, the application of barrier-improving urea moisturizers on corticosteroid-healed sites in AD patients decreases the possibility of eczema relapse among the patients; however, further investigations would be required to determine the effective long-term management of eczema.¹⁶

Long-term Use of Moisturizers

The controversy regarding the long-term use of moisturizers is described briefly above, and knowledge about the effects of long-term exposure of skin to moisturizers remains scarce.³⁰ Studies on damaged as well as healthy skin have indicated that TEWL and consequently skin barrier function could be weakened or strengthened due to exposure with moisturizers, depending upon the composition of the products.^{31, 32} Specifically, lipids and natural moisturizing factors, especially urea, have been suggested as the most important factors.³⁰

Further, it has been demonstrated that both the susceptibility of skin to irritants and at the same time skin recovery could be influenced by moisturizers.³¹ Buraczewska et al. experimented with the application of different moisturizers on volunteers for seven weeks.²⁴ They demonstrated that those containing hydrocarbon cream, canola cream, canola/urea cream and polymer gel increased TEWL measurements and consequently reduced skin water barrier function, in comparisons with untreated skin. However, ordinary creams were found to improve skin barrier function and reduce TEWL measurements. Hydrocarbon creams were the only products that demonstrated a reduction in skin water capacity.

A similar pattern was reported regarding skin exposed to SLS after undergoing treatment with moisturizers. SLS-exposed skin treated with hydro- carbon cream, canola cream, canola/urea cream and polymer gel demonstrated higher TEWL measurements compared with SLS-exposed untreated skin. Skin treated with ordinary cream demonstrated lower TEWL values than the control.6 Creams that increased TEWL after application to skin contained a higher percentage of lipids, i.e., 40%, compared with cream containing 20% lipids.⁶ In the creams containing 40% lipids, the structures of the emulsifiers used resemble intercellular bilayer lipids, which could affect the organization of skin bilayers.¹ The presence of urea in ordinary creams also contributes to the lower TEWL values.³¹

Conclusion

The application of moisturizers to the skin influences the hydration levels of the SC. Based on a moisturizer’s contents, especially lipid percentage and urea, it can induce a wide range of effects on skin barrier function. For instance, the use of moisturizers can provide prophylactic benefits against irritant contact dermatitis for wet work employees who are in continuous contact with irritants. This protection had been demonstrated by a decline in TEWL measurements, which represents an increase in skin barrier function.

However, the enhanced hydration levels of skin, especially through the long-term use of moisturizers with high lipid contents, could cause disadvantages for consumers. For instance, enhanced skin reactions due to SLS exposure after long-term treatment with high lipid content moisturizers indicate a reduced skin barrier function. This reduction makes skin more susceptible to the penetration of noxious substances, thereby exposing skin to tissue damage.

The proper use of moisturizers has been indicated in patients afflicted by AD to reduce the eczema relapse. In addition, the use of moisturizers could be a strong alternative for corticosteroid treatments that are commonly prescribed for patients diagnosed with AD. Based on the literature, moisturizers have a great potential to act as prophylactic and protective means for skin. At the same time, due to concerning outcomes of few studies, more research could provide beneficial information to assist in resolving the indicated controversy and in establishing thorough guidelines regarding the proper use of moisturizers. In relation, as the presented findings suggest, cosmetic companies that produce moisturizers could provide better protection for consumers by properly instructing them regarding the duration and frequency of using such products. In addition, through further investigations, manufacturers could optimize the protective properties of products by adjusting the lipid and other chemical contents of moisturizers to reduce the possible destructive effects of the moisturizers and provide consumers with the greatest benefits.

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