The Scientific Principles of Protecting the Skin's Moisture Barrier

As interest in skin hydration and barrier function continues to grow, the roles of hyaluronic acid and natural moisturizing factors (NMF)—and their interactions in preserving the skin's moisture barrier—are becoming increasingly significant. This article explores, from a dermatological perspective, the physiological and molecular characteristics of these two components, how they interact within the skin, and how this understanding informs cosmetic applications.

Structure of the Skin Barrier and the Core of Moisture Retention

The outermost layer of the skin, the stratum corneum, has a "brick and mortar" structure, where corneocytes serve as the "bricks" and the intercellular lipids as the "mortar." This architecture prevents the infiltration of external harmful substances and minimizes transepidermal water loss. However, the moisture retention ability of the stratum corneum is not determined by its physical structure alone. NMF located within the corneocytes, and hyaluronic acid distributed throughout the epidermis and dermis, play critical roles in maintaining hydration.

Physiological Functions of Natural Moisturizing Factors (NMF)

NMF are complexes of water-soluble, low-molecular-weight substances within corneocytes. Key constituents include amino acids, pyrrolidone carboxylic acid (PCA), lactic acid, urea, inorganic salts, and sugars. NMF are derived from the breakdown of the filaggrin protein and exhibit hygroscopic properties, allowing them to attract and retain water within the corneocytes. NMF constitute approximately 10% of the stratum corneum and function like sponges to prevent dryness of corneocytes.

A notable feature of NMF is their ability to absorb moisture from the atmosphere and retain it in the skin, even in dry environments. This maintains skin elasticity and flexibility and prevents scaling or fissuring of the stratum corneum. When NMF are deficient, the skin becomes dry, more sensitive, and prone to irritation. Additionally, NMF, in conjunction with intercellular lipids, stabilize the skin barrier and protect against external irritants and microbial invasion.

Biological Functions of Hyaluronic Acid in the Skin

Hyaluronic acid is a naturally synthesized high-molecular-weight polysaccharide found in the dermis, epidermis, joints, cartilage, and other tissues. It is highly hydrophilic and viscous, capable of binding water up to several thousand times its own weight. In the skin, hyaluronic acid acts as a water reservoir and fills the spaces between collagen and elastin fibers in the dermis, maintaining skin volume and elasticity.

Hyaluronic acid is directly related to skin aging. As the body ages, its levels of hyaluronic acid decrease, leading to reduced hydration and visible signs of aging such as dryness, loss of elasticity, and wrinkles. In both cosmetics and dermatologic procedures, hyaluronic acid is used in various molecular weights. High-molecular-weight hyaluronic acid forms a hydrating film on the skin's surface to prevent water evaporation, while low-molecular-weight hyaluronic acid penetrates the epidermis to deliver deep hydration.

Mechanisms of Interaction Between NMF and Hyaluronic Acid

NMF and hyaluronic acid interact in a complementary and synergistic manner to maintain the skin's moisture barrier. Hyaluronic acid stores water in the dermis and epidermis and draws it upward to the epidermis, while NMF within the corneocytes captures and retains that moisture, preventing evaporation. In this way, hyaluronic acid acts as a "moisture reservoir," and NMF as a "moisture magnet."

Together with intercellular lipids, these two elements form a triple hydration system. When water supplied by hyaluronic acid reaches the stratum corneum, NMF absorbs it and retains it within corneocytes, while lipids prevent further evaporation. If the balance between NMF and hyaluronic acid is disrupted, the skin barrier weakens and water loss accelerates.

Skin Barrier Damage and Changes in NMF and Hyaluronic Acid

External aggressors (such as UV rays, fine dust, pollution, and over-cleansing) and skin disorders (such as atopic dermatitis and psoriasis) reduce the levels of NMF and hyaluronic acid. In particular, individuals with atopic dermatitis often have filaggrin gene mutations that reduce NMF synthesis, leading to severely dry skin. Hyaluronic acid is also degraded or synthesized less efficiently with aging, UV exposure, and inflammation.

As a result, the skin loses moisture, the stratum corneum thickens or cracks, and the barrier function weakens, making the skin more sensitive to external stimuli. Therefore, dermatological treatment and skincare regimens that restore NMF and hyaluronic acid are emphasized for skin barrier repair.

Cosmetic Applications and Emerging Trends

The cosmetics industry is increasingly developing moisturizers that combine NMF-derived ingredients (such as amino acids, PCA, and urea) with hyaluronic acid. Low-molecular-weight hyaluronic acid effectively penetrates the epidermis and stratum corneum, delivering hydration deep into the skin, while high-molecular-weight hyaluronic acid forms a protective film on the surface to reduce transepidermal water loss.

In clinical dermatology, treatments such as hyaluronic acid fillers and skin booster injections are widely used. These directly increase the hyaluronic acid concentration within the skin to improve hydration and elasticity and support barrier restoration. A notable trend involves combining professional moisturizers containing NMF with hyaluronic acid treatments to achieve an integrated approach to skin barrier management.

Practical Strategies for Maintaining the Skin Moisture Barrier

To maintain optimal levels of NMF and hyaluronic acid, it is essential to avoid harsh cleansing, frequent exfoliation, and alcohol-based products. Moisturizers should be selected according to skin type and seasonal conditions, and a personalized approach guided by dermatological diagnosis is recommended.

In particular, during dry seasons or in low-humidity environments, it is effective to apply ample moisturizers containing NMF and hyaluronic acid, and to supplement with dermatological treatments (e.g., hyaluronic acid injections, topical treatments) if needed. Replenishing intercellular lipids such as ceramides, cholesterol, and fatty acids also aids in barrier repair.

Future Directions in Dermatological Cosmetics

The future of cosmetic formulation will focus on personalized ingredient combinations tailored to skin physiology—especially maximizing the synergy between NMF and hyaluronic acid. Skincare strategies based on the scientific principles of the skin barrier will become essential in maintaining healthy, beautiful skin. Understanding the interaction between NMF and hyaluronic acid, and applying this knowledge in both dermatological treatments and cosmetic development, will redefine the standard of skin health.

Ongoing research into skin hydration and barrier function will continue to deepen our understanding of the roles of NMF and hyaluronic acid and how to optimize their interaction. An integrated approach to managing hydration balance and barrier function will remain a cornerstone of both dermatology and cosmetic science moving forward.