Dry skin is not a uniform phenomenon. To truly understand the skin barrier, one must differentiate between two fundamentally different conditions: a lack of water within the stratum corneum – the actual moisture deficiency – and a deficit of lipids, the protective fats that prevent this water from escaping in the first place. Both conditions produce similar symptoms but differ significantly in their cause and treatment logic. Treating one with the approach for the other will lead to lasting disappointment.

For decades, the skincare industry has made the term "moisture" a catch-all that means everything and nothing. A product can be called "intensely moisturizing" and primarily contain glycerin – a humectant that binds water but does not repair a compromised barrier. Conversely, a pure oil treatment can strengthen the barrier without adding a single drop of water. Both approaches are legitimate – but only if they match the actual diagnosis.

This article sheds light on the mechanisms behind skin moisture: from molecular transport channels to natural moisturizing factors and the practical question of which active ingredient classes are useful in what order and at what time of year.

Hydration vs. Lipid Deficiency – Two Fundamentally Different Conditions

The stratum corneum – the outermost layer of the epidermis – is not a dead shell, but a highly organized tissue. Its function is a dual role: protection from the outside and moisture regulation from the inside. This function is only successful if two systems are intact: sufficient water content within the corneocytes (the flattened horny cells) and an intact lipid matrix between these cells.

If the water content of the stratum corneum is too low – measurable by corneometry – it is called transepidermal water loss (TEWL) or simply dehydrated skin. This dehydration often affects oily skin types as well: sebum production says nothing about how much water the stratum corneum actually retains. Oily but dehydrated skin is more common than generally assumed and reacts to aggressive cleansing or air conditioning with a paradoxical effect – the sebaceous glands produce more, while the stratum corneum continues to dry out.

If, on the other hand, the lipid matrix is disturbed, there is structural damage. The skin loses water not because it lacks it, but because the barrier is leaky. The lipids of the stratum corneum – consisting of ceramides, cholesterol, and free fatty acids in a ratio of approximately 1:1:1 – form lamellar structures that control the outward flow of water. If this balance is disturbed, TEWL increases, and the skin feels rough, flaky, and tight – even if plenty of water is supplied.

The practical distinction in everyday life is possible: Dehydrated skin shows fine lines that temporarily disappear after applying a water-based product – a classic sign of water deficiency in the stratum corneum. A compromised lipid barrier, however, manifests as persistent redness, increased sensitivity to products that were previously well-tolerated, and a tightness that does not fully subside even after care.

The Natural Moisturizing Factor – Hydrating Skin from Within

The Natural Moisturizing Factor (NMF) is a mixture of water-soluble compounds found exclusively in the corneocytes of the stratum corneum. It is formed as a breakdown product of filaggrin – a structural protein that, during the terminal differentiation of keratinocytes, breaks down into a cocktail of hygroscopic molecules. This cocktail is responsible for the stratum corneum attracting and retaining water from the environment – even under dry conditions.

The composition of the NMF is complex and precise:

The NMF is significantly influenced by external factors. Frequent washing with alkaline soaps or detergents directly washes NMF components out of the stratum corneum – they are water-soluble and therefore vulnerable. UV exposure reduces filaggrin expression. Genetic variants in the filaggrin gene (FLG) – known from atopy research – can permanently limit NMF synthesis. And aging reduces both filaggrin production and the enzymatic activity that controls its breakdown.

Aquaporins – Water Transport at the Cellular Level

Water movement in biological tissues is not a passive process. It is regulated by specialized transmembrane proteins – aquaporins. These channel proteins, for whose discovery Peter Agre received the Nobel Prize in Chemistry in 2003, enable the selective, high-speed transport of water molecules through cell membranes, while other molecules are excluded.

Several aquaporin isoforms have been identified in human skin. The most important for epidermal moisture regulation are:

Aquaporin-3 (AQP3) is the best-studied isoform in the skin. It is mainly expressed in the basal and suprabasal layers of the epidermis and transports not only water but also glycerol – an important building block for lipid synthesis and moisture retention. Studies show that AQP3-deficient mice exhibit significantly reduced skin hydration, slowed wound healing, and impaired barrier recovery. AQP3 expression decreases with age – which partly explains why mature skin retains water less efficiently, even if sufficient moisture is supplied externally.

Aquaporin-10 (AQP10) has also been detected in the human epidermis, but its exact physiological role is still under investigation. Aquaporin-5 (AQP5) is found in sweat glands and regulates sweat secretion there – relevant for active cooling and thus indirectly for the moisture environment of the skin surface.

Relevant for skincare is the question of whether topical active ingredients can influence AQP3 expression. Current research shows that certain compounds – including hyaluronic acid fragments, niacinamide, and glycerol itself – modulate AQP3 expression in keratinocytes in vitro. In vivo human data are still limited, but the mechanism of action is plausible: improved AQP3 function would optimize internal water distribution without relying on external humectants.

Climatic factors directly influence aquaporin activity. Cold reduces membrane fluidity and thus the transport rate. Dry heating air in winter lowers the osmotic gradient between the dermis and the stratum corneum – the driving force for water transport upwards weakens. This explains why skin in heated indoor spaces dries out despite high water consumption: the problem lies not in water supply, but in disrupted transport and retention.

Ceramides and Hyaluronic Acid – What They Really Do

Ceramides and hyaluronic acid are among the most frequently mentioned ingredients in skincare. Both have proven effects – but on fundamentally different levels. Understanding what each active ingredient can and cannot do allows for more targeted care.

Ceramides: Structural Component of the Lipid Barrier

Ceramides are sphingolipids – made up of a long-chain sphingosine base and a fatty acid. They make up about 50 percent of the total lipids of the stratum corneum and are thus the quantitatively most important component of the lipid matrix. Their main function is structural: Ceramides, together with cholesterol and free fatty acids, arrange themselves in lamellar double layers – an ordered layer structure that limits transepidermal water loss to a physiological minimum.

In a compromised barrier – caused by eczema, psoriasis, excessive cleansing, age-related atrophy, or genetic predisposition – the ceramide composition changes. Certain ceramide subclasses (especially ceramide 1/EOS and ceramide 3/NP) are particularly vulnerable. Studies consistently show that patients with atopic dermatitis have significantly reduced ceramide levels in lesional and often also non-lesional skin.

Topically applied ceramides can support barrier function. Crucial factors are their molecular similarity to endogenous ceramides (skin-identical ceramides are more effective than plant-derived analogues), particle size, and formulation format – ceramides in liposomes or lamellar emulsions penetrate the stratum corneum more efficiently than those in simple oil-in-water emulsions.

Hyaluronic Acid: Humectant with Molecular Deep Action

Hyaluronic acid (HA) is a glycosaminoglycan that naturally occurs in the dermis and epidermis. One molecule of HA can bind up to 1000 times its own weight in water – this exceptional hygroscopic capacity makes it one of the most effective biological moisture reservoirs. In the dermis, HA is crucial for tissue turgor, elasticity, and the support of the extracellular matrix.

Topically, HA is effective as a humectant – but its penetration depth is molecular weight dependent. High molecular weight HA (over 1000 kDa) remains on the surface and forms a moisture-binding film there. It improves immediate feel but, under very dry conditions, tends to draw water out of the skin if the humidity drops below the level of the stratum corneum. Low molecular weight HA fragments (under 50 kDa) penetrate deeper, but in higher concentrations have potentially inflammatory effects – as these fragments act as alarm signals for tissues in vivo.

Correct Layering Order and Seasonal Adjustment

The effectiveness of skincare products depends not only on their ingredients – it largely depends on the order and conditions under which they are applied. So-called layering – the layered application of different product categories – follows a logic based on the physical properties of the formulations and the physiology of the skin barrier.

The Basic Rule: Texture and Depth of Action

The classic layering rule is: thin before thick, water-based before oil-based. This rule of thumb has a physiological background: watery formulations only penetrate the stratum corneum effectively if they are not blocked by an already applied lipid layer. Oils and occlusive substances, on the other hand, develop their protective effect best as a final layer that minimizes water loss to the outside.

A functional layering sequence for dehydrated or barrier-compromised skin could look like this:

1. Cleansing: Micellar water or low-surfactant cleanser at pH 4.5–5.5 – maintains the acidic mantle and minimally washes out NMF components.

2. Moisturizing Serum (Humectants): Hyaluronic acid in various molecular weights, sodium lactate, glycerin, betaine, or panthenol. Applied to slightly damp skin – to improve the adhesion of humectants. The damp surface provides the water molecules that HA and glycerin can attract and bind.

3. Active Ingredient Layer (optional): Niacinamide (ceramide synthesis support, NMF regulation), peptides, vitamin C (at appropriate pH). This layer penetrates most effectively without an occlusive barrier above it.

4. Moisturizer / Emulsion: Ceramide-containing cream that supports the previous layers and supplements the barrier. This is also where cholesterol and fatty acids come into play as a barrier-building complex.

5. Occlusive Cover (if needed): A light facial ointment, squalane, or a plant wax-based finish – especially useful in winter. Petrolatum-containing products are best documented as occlusives in dermatological literature, but many users reject them due to their texture. Squalane and jojoba oil are skin-friendly alternatives with a similar mode of action.

Seasonal Adjustment

Skin is not a static system. Its barrier function, ceramide composition, and NMF content vary measurably with the seasons – due to humidity, temperature, and UV exposure. A skincare routine that is sufficient in summer can be counterproductive in winter – and vice versa.

In summer, humidity increases, which lowers TEWL and increases humectant efficacy. Light, water-based formulations with hyaluronic acid and NMF analogs are often sufficient. Heavy occlusives can clog pores and impair natural temperature regulation. UV protection is the most important single measure – UV radiation damages filaggrin expression and ceramide synthesis.

In winter, outdoor humidity drops, while heating air further dries out indoor air. TEWL increases, and the ceramide synthesis rate decreases at low temperatures. The routine should be extended with a richer emollient layer. Urea (5–10%) in the moisturizer can be particularly effective during this season – it acts both as a humectant and a mild exfoliant, which improves the penetration of other active ingredients.

During transition periods (autumn/spring), it is worthwhile to adjust the routine gradually – not abruptly. Skin reacts to routine changes with an adaptation phase of 2–4 weeks, during which temporary reactions such as mild redness or altered sebum production are normal.

Diagnosis: What type is your skin really?

Before choosing the right skincare, an honest assessment of the skin condition is needed. The following observations can help – they do not replace a dermatological diagnosis, but offer initial guidance.

Dehydrated skin is characterized by:

Fine lines that temporarily smooth out after applying a water-based product. A feeling of tightness that quickly subsides when moisture is added. Lack of radiance and slight dullness of texture. Often combined with an otherwise normal skin appearance – no flaking, no persistent redness.

Lipid-poor / barrier-impaired skin is characterized by:

Persistent flaking that does not completely disappear even after care. Increased reactivity to products that were previously tolerated without problems. Burning or stinging after cleansing. Visible redness, especially around the nostrils, cheeks, and chin. TEWL can be measured with a Tewameter – elevated values almost always indicate barrier dysfunction.

Combined condition (common in winter and with aging skin):

Both factors are affected simultaneously. Here, care is more complex – first strengthen the barrier (ceramides, fatty acids, cholesterol), then add humectants and occlude. The order is not arbitrary: applying humectants to a severely compromised barrier without subsequent occlusion can paradoxically increase water loss.

Frequently Asked Questions