The Science of Skin Hydration — Beyond Moisturisers

Last updated: 5 March 2026

Medically reviewed by Dr Julia Thornton, PhD, BSc (Hons) Pharmacology, Cosmetic Scientist specialising in hydration science and barrier function with 16 years of research experience.

Skin hydration is one of the most fundamental aspects of skin health and appearance, yet it is also one of the most misunderstood. Simply applying a moisturiser is not the complete picture — true skin hydration involves complex biological processes, multiple skin layers, and a sophisticated interplay between water, lipids, and naturally occurring moisturising factors. Understanding this science can transform your approach to skincare.

How Skin Stays Hydrated

The skin’s hydration system operates on multiple levels, each with distinct mechanisms:

The Skin Barrier (Stratum Corneum)

The outermost layer of skin forms the primary barrier against water loss. Its “bricks and mortar” structure — corneocytes (dead skin cells) surrounded by a lipid matrix of ceramides, cholesterol, and fatty acids — creates a waterproof seal. When this barrier is compromised, transepidermal water loss (TEWL) increases dramatically, leading to dehydration regardless of how much water you drink or how many hydrating products you apply.

Natural Moisturising Factor (NMF)

Within the corneocytes themselves, a complex mixture of hygroscopic (water-attracting) molecules called Natural Moisturising Factor helps maintain hydration. NMF components include amino acids, urea, lactate, pyrrolidone carboxylic acid (PCA), and various minerals. NMF can absorb water from the atmosphere, keeping the stratum corneum plump and flexible.

Research published in the British Journal of Dermatology (2018) showed that NMF content decreases with age and UV exposure, partially explaining the dry, rough skin texture associated with ageing and sun damage.

Hyaluronic Acid in the Dermis

Hyaluronic acid (HA) is the skin’s primary water reservoir, capable of binding up to 1,000 times its weight in water. It is predominantly found in the dermis, where it creates a hydrated gel that supports the collagen and elastin network and maintains skin volume and turgor. Dermal HA content declines significantly with age — by 50, HA levels may be half of what they were at 20.

Aquaporins

Aquaporins are water channel proteins in cell membranes that facilitate the rapid transport of water between cells. Research has identified aquaporin-3 (AQP3) as particularly important in skin hydration, facilitating water and glycerol transport in the epidermis. AQP3 expression decreases with age, contributing to reduced skin hydration capacity.

Hydration vs Moisturisation: Understanding the Difference

“Many patients use the terms interchangeably, but they describe different processes,” explains Dr Thornton. “Hydration refers to increasing the water content within skin cells and the extracellular space. Moisturisation refers to preventing that water from evaporating. Effective skincare addresses both.”

Humectants (Hydration)

These ingredients attract and bind water, pulling moisture from the deeper dermis and from the atmosphere into the stratum corneum:

• Hyaluronic acid: The most popular humectant in skincare. Low molecular weight HA (less than 50kDa) penetrates more effectively but may cause inflammation in some skin types. High molecular weight HA (above 1,000kDa) sits on the surface providing a moisture-binding film
• Glycerin: One of the most effective and well-studied humectants, enhancing barrier function and skin hydration at concentrations of 5-20%
• Urea: A component of NMF that attracts water and at higher concentrations (10%+) also provides mild exfoliation
• Panthenol: Provitamin B5, which penetrates the stratum corneum and acts as both a humectant and barrier enhancer

Occlusives (Moisture Retention)

These ingredients form a physical barrier on the skin surface, reducing TEWL:

• Petrolatum: Reduces TEWL by up to 99% — the gold standard occlusive
• Dimethicone: A silicone-based occlusive with a lighter feel
• Shea butter: Natural occlusive with additional fatty acid content
• Squalane: A lightweight occlusive that mimics the skin’s natural sebum

Emollients (Surface Smoothing)

These fill in the gaps between desquamating (shedding) corneocytes, creating a smoother surface and improving skin feel. Many ingredients serve dual functions — ceramides are both emollients and barrier-repair agents, whilst squalane is both emollient and occlusive.

Professional Hydration Treatments

Profhilo

Profhilo delivers ultra-pure hyaluronic acid directly into the dermis, providing deep tissue hydration and stimulating the production of collagen, elastin, and the patient’s own hyaluronic acid. Its unique cooperative complex of high and low molecular weight HA makes it one of the most effective professional hydration treatments available.

Skin Boosters

Micro-injections of non-cross-linked or lightly cross-linked hyaluronic acid improve skin hydration, elasticity, and luminosity over a course of treatments. Products like Restylane Skinboosters and Juvederm Volite are specifically designed for this purpose.

Mesotherapy

Custom cocktails of hyaluronic acid, vitamins, minerals, and amino acids injected into the superficial dermis can provide targeted hydration and skin quality improvement.

Common Hydration Mistakes

• Using hyaluronic acid without an occlusive: In dry environments, topical HA can actually draw moisture out of the skin if not sealed with a moisturiser
• Over-cleansing: Stripping the skin barrier with harsh cleansers destroys the lipid matrix, dramatically increasing TEWL
• Assuming oily skin does not need hydration: Oily skin can be dehydrated — water content and oil production are independent
• Relying solely on drinking water: Whilst adequate water intake supports overall health, drinking more water does not directly increase skin hydration in well-hydrated individuals

Frequently Asked Questions

Does drinking more water improve skin hydration?

Severe dehydration will affect skin appearance, but for individuals who maintain normal hydration, drinking extra water has minimal direct impact on skin moisture levels. A study in Clinical, Cosmetic and Investigational Dermatology (2015) found no significant correlation between water intake and skin hydration measurements in adequately hydrated individuals. Topical hydration and barrier maintenance are far more effective strategies for skin hydration.

Is hyaluronic acid the best hydrating ingredient?

Hyaluronic acid is an excellent humectant, but it is not necessarily superior to glycerin for topical hydration. A comparative study found that glycerin at equivalent concentrations provided similar or superior stratum corneum hydration. The most effective approach combines multiple humectant types with occlusives and barrier-repair ingredients for comprehensive hydration.

Why does my skin feel dry even though I moisturise regularly?

Persistent dryness despite moisturising usually indicates barrier dysfunction rather than insufficient product application. If the lipid matrix between skin cells is compromised, water escapes faster than your moisturiser can replace it. Switching to a barrier-repair product containing ceramides, cholesterol, and fatty acids may be more effective than simply applying more conventional moisturiser.

Can professional hydration treatments replace my daily skincare routine?

No. Professional treatments like Profhilo and skin boosters provide a deep hydration boost and stimulate the skin’s own hydration mechanisms, but they are not a substitute for daily skincare. Think of professional treatments as resetting your hydration baseline, whilst daily skincare maintains and protects that baseline between appointments.