The Science of Hydration
How Hydrating Moisturisers Restore Your Skin Barrier
Your skin is much more than just the surface you see in the mirror—it’s a dynamic, living barrier working around the clock to protect you from environmental aggressors, pollutants, and harmful microorganisms. But beneath its protective prowess lies one fundamental truth: the health and resilience of your skin heavily depend on its hydration levels.
Without adequate hydration, the skin’s barrier weakens, leading to dryness, sensitivity, premature aging, and even inflammatory skin conditions. Enter: hydrating moisturisers. More than just creams that feel good on the surface, these formulations are scientifically designed to restore, repair, and fortify the skin barrier.
In this article, we'll take a deep dive into the science of hydration—exploring how hydrating moisturisers work, why the skin barrier needs them, and the biological mechanisms at play when you apply that bottle of skin-loving goodness.
Understanding Skin Hydration
Hydration vs Moisturization: What's the Difference?
Although the terms "hydration" and "moisturization" are often used interchangeably, they refer to distinct skin needs. Hydration specifically refers to the water content in the skin—how much moisture is present within the cells. Moisturization, on the other hand, focuses on preventing that water from evaporating by creating a barrier or adding lipids.
Think of hydrated skin cells as plump grapes. When they lack water, they shrivel like raisins, leading to dullness, rough texture, and fine lines. A good moisturiser typically hydrates the skin and helps lock that hydration in, addressing both aspects simultaneously.
The Water Content of the Stratum Corneum
The outermost layer of your skin, the stratum corneum, plays a pivotal role in retaining water. Ideally, this layer contains approximately 10-20% water to maintain smoothness and elasticity. Factors like sun exposure, cold weather, harsh cleansers, and aging can reduce this water content, leading to a compromised skin barrier.
Natural Moisturising Factors (NMFs), Ceramides & Lipids
Within the stratum corneum, there are essential molecules known as Natural Moisturising Factors (NMFs)—including amino acids, lactic acid, urea, and PCA—that attract and hold onto water. Alongside these are lipids like ceramides, fatty acids, and cholesterol, which form the skin's "mortar," filling the gaps between skin cells and sealing in hydration.When skin is deficient in NMFs or lipids, it can't hold water effectively, making hydration-focused moisturisers crucial for replenishing these components.
The Skin Barrier: Biology & Function
Your skin is made up of three main layers: the epidermis, dermis, and hypodermis. The epidermis—particularly the outermost part known as the stratum corneum—is the true star when it comes to hydration and barrier function.
The Brick & Mortar Model
To understand how this works, scientists often refer to the brick-and-mortar model. In this model:
When the mortar is intact and well-structured, water stays inside the skin, and harmful irritants are kept out. But when the mortar is degraded—through environmental stress, over-cleansing, harsh products, or dehydration—water escapes via Transepidermal Water Loss (TEWL), leaving the skin dry, sensitive, and vulnerable.
What is Transepidermal Water Loss (TEWL)?
TEWL is a natural process where water passes from the deeper layers of the skin to the surface and evaporates. A healthy skin barrier slows this process to maintain hydration. However, when the barrier is compromised, TEWL increases dramatically, resulting in dehydration, flakiness, and a tight, uncomfortable feeling.Scientific research shows that well-formulated moisturisers, rich in humectants, emollients, and occlusives, play a critical role in reducing TEWL and reinforcing the lipid mortar.
Hydrating Moisturisers: How They Work
Let’s break down the components commonly found in hydrating moisturisers and how each supports the skin barrier.
Humectants
Attracting Water
Humectants are ingredients that draw moisture from the environment or deeper skin layers into the outermost layer. Examples include:Glycerin: A time-tested humectant known to bind water molecules and improve hydration. Hyaluronic Acid (HA): Famous for its ability to hold up to 1000 times its weight in water. Sodium PCA & Aloe Vera: Other natural humectants that increase skin hydration. By increasing water content in the skin, humectants help maintain softness and elasticity.
Emollients
Filling Gaps & Smoothing Skin
Emollients soften and smooth the skin by filling in micro-cracks in the barrier. They mimic the skin’s natural lipids and help improve texture and flexibility. Common emollients include Plant oils (like jojoba, almond, rosehip), Fatty alcohols (like cetyl alcohol) and Esters (like isostearyl isostearate)
Occlusives
Locking in Hydration
Occlusives create a physical barrier over the skin, preventing water from evaporating. They are crucial, especially in dry or cold climates where TEWL can be high. Examples are Shea butter, Beeswax, Squalane and Petrolatum (used sparingly in many formulations)
Barrier-Repair Ingredients
Rebuilding Lipids
Modern moisturisers go beyond basic hydration and contain bio-identical lipids like, Ceramides, Cholesterol, and Fatty acids. These ingredients replenish the skin’s natural lipid matrix, restoring structure and preventing further moisture loss.
Emerging Ingredients
The New Frontier
Scientific advancements have introduced powerful new ingredients to the hydration conversation: Ectoin: An extremolyte molecule that protects cells from dehydration and oxidative stress. Fermented Marine Extracts: Rich in minerals and amino acids, supporting skin resilience. Bio-mimetic Lipids: Lab-created lipids that mirror skin’s natural composition, ensuring compatibility and efficacy.
Hydration & TEWL: The Science
Recent studies have highlighted how specific ingredients impact TEWL reduction and skin barrier repair.A 2018 study published in the Journal of Dermatological Science showed that moisturisers containing ceramides, cholesterol, and free fatty acids in a 3:1:1 ratio significantly improved barrier function compared to moisturisers lacking these components. Similarly, humectants like glycerin have been shown to reduce TEWL by up to 30% within weeks of consistent use, making them key players in long-term hydration strategies.
The pH Connection
Another overlooked aspect is skin pH. The skin’s acid mantle sits at a pH of around 4.7-5.5, crucial for barrier integrity and hydration. Products with a pH far outside this range can impair the barrier, increasing TEWL and inflammation.Hydrating moisturisers often maintain skin-friendly pH levels, further supporting moisture retention and barrier repair.
Dry Skin vs Dehydrated Skin
Understanding the Difference
It’s essential to differentiate:
Interestingly, even oily skin types can be dehydrated, leading to overproduction of oil as a compensatory mechanism. That’s why lightweight, hydrating moisturisers—rich in humectants and lipids—are beneficial for all skin types.
Practical Application
How to Layer Products for Optimal Hydration
Hydration isn’t just about one product—it’s about building a routine that works synergistically:
Cleanse
Step 1
Choose a non-stripping, pH-balanced cleanser to avoid removing essential lipids..
Toner
Step 2
Lightweight, water-based layers with humectants (like hyaluronic acid, glycerin) prepare the skin to absorb hydration.
Serums
Step 3
Hydrating serums penetrate deeper, delivering actives like hyaluronic acid, niacinamide, or peptides.
Face Cream
Step 4
The main event—containing humectants, emollients, and occlusives to attract, hold, and seal in hydration.
Face Oil
Step 6
Particularly for very dry environments or skin types, a face oil or balm provides extra sealing power.
Myth Busting
Common Misconceptions About Skin Hydration
1.
Myth 1: Drinking More Water Directly Hydrates Your Skin
While staying hydrated internally is important for overall health, drinking excess water doesn’t guarantee glowing, hydrated skin. It’s the skin barrier's lipid and water-binding components—and your topical skincare—that dictate how well moisture stays locked in.
2.
Myth 2: Heavy Creams Are Better for Hydration
Not always. For some, especially those with oily or combination skin, heavy creams can clog pores or feel uncomfortable. Lightweight moisturisers rich in humectants + lipids may be more effective at hydrating without heaviness.
3.
Myth 3: Oily Skin Doesn’t Need Hydration
False! Oily skin can often be dehydrated. When water is lacking, the skin produces more sebum to compensate. A proper hydrating moisturiser can help balance this and regulate oil production.
Conclusion
Your skin barrier is the first line of defense against the external world—and hydration is its greatest ally. The right hydrating moisturiser doesn’t just make your skin feel soft for a few hours; it works at a cellular level, replenishing lost water, repairing the lipid matrix, and reducing TEWL to ensure long-term health and resilience.Whether your skin is dry, oily, or somewhere in between, understanding the science of hydration empowers you to choose products and routines that genuinely support your skin barrier—keeping it strong, plump, and glowing.