Moisturizers

Function

The stratum corneum contains high concentrations of osmotically active molecules, including amino acids and their derivatives, lactic acid, urea, and electrolytes. These molecules form from the breakdown of filaggrin and are referred to as the natural moisturizing factor (NMF). The molecules that make up NMF are hygroscopic and absorb atmospheric water at concentrations as low as 50%.

The formation of NMF from filaggrin is highly correlated with the water content of the stratum corneum and only occurs within a specific range of water concentrations. When the water content is high, filaggrin is stable, and conversion to NMF does not occur.[4] Conversely, when the water content is too low, hydrolytic enzymes needed for this reaction cannot function. The water content of the stratum corneum ranges from 10% to 30% in healthy skin, compared with 75% to 85% water content of stratum basale.[5] This gradient is a key feature in its function as a barrier.

The stratum corneum comprises various proteins and lipids, organized in a “bricks and mortar” structure.[6] Corneocytes have a tight organization in an interlocking and overlapping fashion, surrounded by lipid lamella.[4] The major lipids in this structure are ceramides, cholesterol, and free fatty acids, which are synthesized by the keratinocytes and stored in secretory granules.[2][4][7] Under normal conditions, these granules get released slowly, but upon insult to the barrier, they undergo rapid secretion. The acute barrier disruption results in the loss of the calcium gradient, an important regulator in the secretion of these granules.[8]

Moisturizers work by hydrating and restoring the natural barrier function of the skin. Moisturizers contain a combination of humectants, occlusive agents, and emollients that work together to improve skin hydration and prevent water loss. Humectants, such as glycerin and hyaluronic acid, attract water to the skin and help to increase its water content. Occlusive agents, such as petrolatum and mineral oil, form a physical barrier over the skin that reduces water evaporation.[9] Emollients, such as shea butter and ceramides, help to smooth and soften the skin by filling in gaps between skin cells.[10] 

Issues of Concern

Dry skin has many causes, including frequent hand washing, exposure to harsh chemicals, low ambient humidity, and medical conditions like atopic dermatitis, ichthyosis, and psoriasis. The goal of moisturizing the skin is to improve its appearance and function. In patients with medical conditions associated with impaired skin barrier function, like atopic dermatitis, the diligent use of moisturizers is a fundamental component of their treatment.[11]

The choice of moisturizing agents can be adjusted to best-fit patients' needs. Factors like acne, flaking, hyperkeratosis, itching, or sensitive skin merit consideration when recommending a moisturizing agent. 

Clinical Significance

Types of Moisturizers

Moisturizers can be divided into several categories; their functions include preventing water loss through the skin, adding water to the skin, and reducing visible flakes. In recent years, with a fuller understanding of the structure and function of the skin barrier, the science of moisturizer development has advanced significantly. As a result, modern moisturizers frequently combine ingredients with different mechanisms of action to achieve specific results.

Occlusive moisturizers

Occlusive moisturizers prevent water evaporation or reduce transepidermal water loss (TEWL) from the skin by forming a barrier. This barrier allows replenishment of the water content of the stratum corneum by the deeper layers of the epidermis and dermis.

Petrolatum is a classic example of an occlusive moisturizer and the most effective, reducing water loss through the epidermis by nearly 99%.[12] Many other substances can serve as occlusive moisturizers:

• Hydrocarbons: Petrolatum, mineral oil, paraffin, squalene
• Silicones: Dimethicone
• Vegetable oils: Coconut oil, castor oil, olive oil, grape seed oil, soybean oil
• Animal Fats: Mink oil, emu oil, lanolin
• Wax Esters: Lanolin, beeswax
• Sterols: Cholesterol, ceramides

Humectants

Humectants are compounds that attract and bind water. Some of these substances, like urea, are endogenously present and comprise the NMF. Humectants can hydrate the skin if the humidity is more than 70%; more commonly, however, they draw water from the deeper epidermis and dermis. Some humectants can increase TEWL and exacerbate skin dryness without concomitant use of an occlusive moisturizer.  

Alpha-hydroxy acids (AHAs) are an important group of humectant molecules often used as moisturizing agents and in cosmetics. AHAs include glycolic and lactic acids. These agents impact the water content of the stratum corneum by several mechanisms; in addition to being effective humectants, AHAs promote desquamation by reducing corneocyte cohesion and also improve barrier function by increasing levels of ceramides.[4][13] Lactic acid has been used topically at concentrations up to 12% in treating ichthyosis and dry skin for many years.[4]

Urea is a component of NMF. The addition of urea to moisturizers helps with the absorption of water into the stratum corneum. It promotes the desquamation of corneocytes and increases penetration of other topically applied medications.[13] It also improves the skin's barrier function and reduces TEWL.[4] Urea improves itching and is useful in treating ichthyosis, disorders associated with hyperkeratosis, and atopic dermatitis.[11]

Glycerin is another commonly used humectant and is present in many skincare products. In addition to attracting and binding water, it has beneficial effects in barrier repair.[4][13] Other humectants include honey, propylene glycol, sorbitol, and hyaluronic acid.

Emulsifiers

An emulsifier is another classification of ingredients commonly found in moisturizers. It is a molecule that allows 2 immiscible substances to exist as a single phase. The molecules are typically long carbon chains with a polar group. Commonly used emulsifiers are fatty acids, like stearic acid.[14] 

Formulations

When a moisturizer contains humectants, the skin's water content increases more rapidly; however, this does not necessarily reduce TEWL. Similarly, when using an occlusive agent, the skin's water content takes a long time to increase, as the water must be drawn from deeper levels of the skin before an improvement occurs. "Cream" traditionally refers to a product containing more occlusive ingredients, whereas a "lotion" primarily contains humectants.[14]

Modern moisturizers often contain both occlusives and humectants that contribute to their efficacy. Understanding the physiology of the skin barrier and how a disease state or circumstance may contribute to dry skin, impaired barrier function, or skin flaking can help us choose the best ingredients for a patient. The specific balance and combination of ingredients will help achieve various outcomes depending on the consumer's desire.

Other Issues

Moisturizer Choice Based on Skin Type

Since moisturizers are used not only by patients with dermatological diseases but also by healthy individuals, the choice of moisturizer is crucial. Choosing the right moisturizer according to skin type can help ensure the skin remains hydrated, healthy, and balanced.

Dry Skin

People with dry skin need to look for a moisturizer that contains rich, nourishing ingredients such as shea butter, ceramides, and glycerin. They should avoid using lightweight or gel-based moisturizers, as they may not provide enough hydration for very dry skin. A moisturizer with a thicker consistency, such as a cream or ointment, helps seal in moisture.[15]

Oily Skin

Those with oily skin should choose a lightweight and oil-free moisturizer, as heavy or greasy products may clog pores and cause breakouts. They should look for a moisturizer that contains ingredients such as hyaluronic acid or glycerin, which are hydrating but not greasy. They should consider using a gel-based moisturizer, which tends to be lighter and more easily absorbed.[16]

Combination Skin

People with combination skin should choose a lightweight, oil-free moisturizer that is still hydrating enough to address any dryness. They need to look for a moisturizer that combines humectants and occlusive agents, such as glycerin and dimethicone, to provide balanced hydration without clogging pores.

Sensitive Skin

People with sensitive skin need to look for a fragrance-free and hypoallergenic moisturizer, as these are less likely to irritate. They should choose a moisturizer that contains soothing ingredients such as aloe vera or chamomile and avoid products that contain harsh chemicals or preservatives.

Mature Skin

As we age, our skin tends to become drier and thinner. Moisturizers that contain ingredients such as retinol or peptides can help improve skin texture and reduce fine lines and wrinkles. Older people should choose a moisturizer with a richer consistency, such as a cream or ointment, to provide deeper hydration.[17]

Enhancing Healthcare Team Outcomes

The liberal use of moisturizers is a fundamental component of treatment for managing chronic medical conditions like atopic dermatitis.[11][18] As documented in numerous randomized controlled trials, the consistent use of moisturizers in atopic dermatitis can reduce symptoms and frequency of disease flares. However, applying moisturizers can be time-intensive for patients and caregivers and, as a result, is often neglected. Discussing the importance of consistent moisturizer use with patients is something that should be done by all members of the care team, including dermatologists, pediatricians, and nurses,

Dry skin affects millions of people and can seriously affect their quality of life. The use of moisturizers is the simplest way to obtain symptom relief. Hence, nurses, pharmacists, and primary care providers should encourage patients to use moisturizers liberally. However, patients should be advised to select fragrance-free moisturizers, as moisturizers with more chemicals increase the likelihood of allergic reactions. 

As with any intervention, using moisturizers is most effective with an interprofessional team approach.

• Clinicians: Need to include moisturizers in their prescribing and care instructions
• Pharmacists: Can recommend the most appropriate types of agents to prescribers and nurses
• Nurses: Often in charge of moisturizer application in the inpatient setting and can assess its effectiveness; can determine compliance in the outpatient setting and communicate treatment efficacy to the interprofessional team
• Pharmacists: Can use moisturizing agents in the compounding process if making a topical preparation to deliver medication; should consult with clinicians on such matters on a case-by-case basis