Our skin barrier plays a vital role in protecting our body from harmful elements, pathogens, and moisture loss. When this barrier is compromised it can lead to dryness, irritation, and increased risk of infection. One of the most effective ways to support and repair the skin barrier is through the use of moisturizers. But how exactly do moisturizers work to improve the health of our skin?
This week we're breaking down the science behind why and how moisturizers impact the skin barrier and why they’re so important for skin health.
The skin barrier is made up of several layers, each with a specific function. The outermost layer, known as the stratum corneum, is composed of skin cells embedded in a matrix of lipids. This layer acts as the first line of defense, preventing water loss and blocking harmful substances from entering the skin.
The barrier is also governed by hydration, which is maintained by various compounds like natural moisturizing factors (NMFs)—a mix of amino acids, fatty acids, and other hygroscopic substances that draw and hold moisture within the skin.
When this barrier becomes damaged or weakened, the skin becomes vulnerable to environmental stressors and infections. This is where moisturizers come in, as they help restore and strengthen the barrier’s function by improving hydration, restoring lipids, and balancing the skin’s pH.
How Moisturizers Work
Humectants: Drawing Water Into the Skin
Humectants are compounds that draw water from deeper layers of the skin (the dermis) into the stratum corneum. Common humectants include ingredients like glycerin, hyaluronic acid, and urea, many of which are also naturally found in NMF. These compounds help keep the skin hydrated by increasing water content in the SC and preventing transepidermal water loss (TEWL).
Humectants don’t just hydrate the skin—they also support the function of key proteins in the barrier. For example, aquaporins, claudins, and filaggrin are upregulated by hydrating compounds, contributing to stronger, more resilient skin. A 2023 study found that moisturizers containing humectants significantly improved skin moisture content compared to controls over just 2 weeks.
Occlusives and Ceramides: Strengthening the Barrier
Moisturizers that contain occlusive agents (like petrolatum or dimethicone) form a hydrophobic layer on the skin’s surface, helping to trap moisture and reduce water loss. On the other hand, moisturizers containing ceramides incorporate barrier lipids into the skin, enhancing the lipid matrix in the SC and improving skin’s ability to retain moisture.
Both types of moisturizers were found to significantly reduce TEWL after 2 weeks of use, highlighting how their hydrophobic properties help seal in hydration.
Urea: A Multi-Tasking Ingredient
Urea is a well-known NMF component and an effective hydrator. When included in moisturizers, urea not only boosts moisture retention but also plays a role in stabilizing the skin's lipid matrix. By incorporating into the hydrophilic spaces between lipids, urea helps strengthen the SC, reducing moisture loss and improving skin resilience. In fact, a 2015 meta-analysis found that moisturizers containing urea provided some of the strongest evidence for improving hydration and reducing TEWL, especially when used at concentrations of less than 10%.
But urea’s benefits go beyond just hydration. At higher concentrations, urea can have a keratinolytic effect, helping to break down excess keratin in the skin, which in turn allows more water to bind to the skin’s surface.
pH: A Crucial Factor in Barrier Health
The pH of the skin plays a crucial role in its barrier function. The skin’s slightly acidic pH (around 4-5) supports a healthy microbiome, keeps enzymes active for lipid synthesis, and inhibits the growth of pathogens like S. aureus. Conditions like eczema, however, often result in a neutral pH, impairing barrier function and increasing the risk of infection.
Moisturizers that maintain or restore the acidic pH of the skin can significantly improve barrier function. Studies have shown that moisturizers containing lactic acid or sodium lactate, which lower the skin’s pH, can reduce dryness and flakiness, improve ceramide content, and increase the skin's resilience to environmental stress.
A healthy skin microbiome is crucial for maintaining barrier integrity. The balance of microorganisms on the skin helps protect against pathogenic bacteria and supports the skin's immune function. When the skin barrier is damaged, like in conditions such as atopic dermatitis, harmful bacteria can proliferate, further aggravating skin conditions.
Emollient therapy (the use of moisturizers) has been shown to shift bacterial populations, reducing the abundance of S. aureus and promoting a more diverse microbiome. Interestingly, some studies have suggested that moisturizers containing ingredients like shea butter, niacinamide, and thermal spring water can promote healthier microbial diversity.
One study found that after 84 days of twice-daily emollient therapy, patients with atopic dermatitis saw reduced S. aureus and an increase in microbial diversity, alongside improvements in their condition. This suggests that moisturizers may not only support the skin’s hydration but also help protect and restore the microbiome.
Moisturizers + Immune System
The immune barrier of the skin is another key factor in its overall health. The skin’s immune system works closely with the microbiome, responding to pathogens and inflammation. Regular use of moisturizers has been shown to influence immune responses, helping to reduce inflammation and support immune defense.
For example, petrolatum, one of the most effective (yet controversial) occlusive agents, has been shown to reduce levels of T cells in both lesional and non-lesional skin of individuals with atopic dermatitis, while simultaneously increasing levels of antimicrobial peptides (AMPs) like LL-37, which boost the skin's innate immune function. These changes may help prevent infections and reduce inflammation.
The research is clear: moisturizers are not just about cosmetic hydration—they are a critical component of maintaining and repairing the skin barrier. By incorporating key ingredients like humectants, occlusives, ceramides, and urea, moisturizers help improve skin hydration, reduce TEWL, and support the skin's microbiome and immune function.
As our understanding of the skin barrier deepens, it is likely that future moisturizers will be formulated to target specific skin conditions and individual needs, offering even more personalized solutions for skin care.
