Hyaluronic acid is one of the most widely used ingredients in skincare — and one of the most misunderstood. Most people know it as a hydrating ingredient. Fewer know that not all hyaluronic acid behaves the same way in a formula, and that the difference comes down to something called molecular weight.
Understanding this doesn't require a chemistry degree. Once you know the logic, you'll be able to read a product's claims more critically and choose formulas that actually deliver what they promise.
What Is Hyaluronic Acid?
Hyaluronic acid (HA) is a naturally occurring polysaccharide — a long-chain sugar molecule — found throughout the body, with the highest concentrations in the skin, joints, and eyes. In the skin specifically, it plays a central role in maintaining hydration: a single molecule can hold up to 1,000 times its weight in water.
The skin naturally produces hyaluronic acid, but production declines with age, UV exposure, and environmental stress — contributing to the loss of plumpness and elasticity that characterises ageing skin. Topical HA in skincare is designed to supplement what the skin no longer makes in the same quantities.
What Does "Molecular Weight" Mean?
Molecular weight refers to the size of the hyaluronic acid molecule. It's measured in Daltons (Da) and determines how deeply a molecule can penetrate the skin.
The skin's outer layer — the stratum corneum — acts as a selective barrier. Larger molecules can't pass through it; smaller ones can. This is why molecular weight matters: HA molecules of different sizes interact with the skin at different depths, and therefore produce different effects.
In skincare, hyaluronic acid is typically categorised as:
- High molecular weight HA — large molecules (above ~1,000,000 Da)
- Low molecular weight HA — smaller fragments (below ~50,000 Da)
- Medium molecular weight HA — the range in between
- Multi-weight (or layered) HA — a combination of two or more sizes in a single formula
High Molecular Weight HA: Surface Hydration
High molecular weight HA molecules are too large to penetrate the skin. Instead, they sit on the surface and form a thin, moisture-retaining film.
This isn't as limited as it sounds. Surface hydration has real benefits:
- Immediately reduces the appearance of fine lines and dryness
- Creates a plumping effect that's visible within minutes of application
- Forms a light occlusive layer that slows moisture evaporation throughout the day
- Feels smooth and comfortable on skin
High molecular weight HA is responsible for most of the immediate "dewy" effect people notice after applying a HA serum. It's fast, visible, and effective at what it does — it just doesn't work below the skin surface.
Low Molecular Weight HA: Deeper Penetration
Low molecular weight HA fragments are small enough to pass through the stratum corneum and reach the deeper layers of the epidermis, where they can deliver hydration from within.
This deeper hydration is slower to show visible results but tends to be more durable — less dependent on a surface film that can be disrupted by weather, touch, or evaporation. Low molecular weight HA also has a mild signalling effect, stimulating the skin's own hyaluronic acid production over time with consistent use.
One caveat worth noting: very low molecular weight HA (nano-HA or hydrolysed HA below ~5,000 Da) may cause mild inflammatory signalling in some skin types at high concentrations — a minor consideration, but worth being aware of if you have reactive or sensitised skin.
Multi-Weight HA: The Most Complete Approach
The most effective HA formulas don't choose between surface and deeper hydration — they use both.
A multi-weight formula combines high, medium, and low molecular weight HA to create hydration at multiple skin depths simultaneously:
- High MW → surface plumping and moisture retention
- Medium MW → mid-epidermal hydration
- Low MW → deeper layers and longer-term moisture signalling
This layered approach addresses the fact that skin dehydration doesn't occur at just one level. A formula that only hydrates the surface may feel good immediately but not sustain hydration throughout the day. One that only works deeper may take longer to show results. Multi-weight HA does both.
When reading a product label, multiple forms of HA may appear under different INCI names — all of which refer to hyaluronic acid or its derivatives:
| INCI Name | What It Is |
|---|---|
| Sodium Hyaluronate | Standard HA salt form — penetrates better than the acid form |
| Hyaluronic Acid | The acid form — typically surface-level |
| Hydrolysed Hyaluronic Acid | Broken-down fragments — low molecular weight, deeper penetration |
| Sodium Hyaluronate Crosspolymer | Modified form, extended-release surface hydration |
Seeing more than one of these in a single formula is a good indicator of a multi-weight approach.
Does Hyaluronic Acid Actually Penetrate Skin?
This is a genuinely debated question in cosmetic science, and the honest answer is: it depends on the molecular weight.
Standard high molecular weight HA does not penetrate the skin — this is well-established. The molecules are simply too large. Claims that it does are not supported by the evidence.
Low molecular weight HA and hydrolysed HA do show penetration into the epidermis in studies, with effects on skin hydration beyond what surface-only application would produce. The extent of that penetration — and whether it reaches the dermis — varies by molecular size, formulation, and skin condition.
What this means practically: a product that contains only "hyaluronic acid" without specifying molecular weight, and with no hydrolysed or sodium hyaluronate crosspolymer forms listed, is likely delivering primarily surface hydration. That's not without value, but it's useful to know what you're actually getting.
A Note on Dry Climates
One practical consideration: HA is a humectant, meaning it draws water from its environment into the skin. In humid conditions, it draws moisture from the air. In very dry conditions, it can draw moisture from the deeper layers of the skin instead — potentially increasing surface dehydration.
If you live in or regularly visit a dry climate (or find that HA serums feel comfortable in summer but drying in winter), applying HA to slightly damp skin and sealing it with a moisturiser immediately after reduces this effect significantly. The moisturiser acts as an occlusive layer that prevents the humectant from pulling water back out.
How to Choose the Right HA Product
A few practical questions to ask:
Is the product using multi-weight HA? Check the INCI list for more than one form of hyaluronic acid or sodium hyaluronate. A formula with both Sodium Hyaluronate and Hydrolysed Hyaluronic Acid is working at multiple skin depths.
Where does HA appear on the ingredient list? Hyaluronic acid is typically effective at relatively low concentrations (0.1–2%), so appearing mid-list isn't automatically a concern. But if it's one of the last three ingredients in a long formula, the concentration may be minimal.
What's it paired with? HA works best alongside a sealing ingredient — ceramides, shea butter, plant oils — that locks in the hydration it delivers. A standalone HA serum with no follow-up moisturiser is less effective than one used as part of a layered routine.
FAQ
Does hyaluronic acid actually penetrate skin? High molecular weight HA does not — it works on the surface. Low molecular weight and hydrolysed forms do show epidermal penetration in studies. Multi-weight formulas combine both effects.
Can hyaluronic acid be drying? In very dry conditions, HA can draw moisture upward from deeper skin layers rather than from the air, which can feel drying. Applying to damp skin and sealing with moisturiser prevents this.
How often should I use a HA serum? HA is gentle enough for twice-daily use (AM and PM). It's one of the most universally tolerated skincare ingredients and doesn't require cycling or rest periods.
Is more HA always better? Not necessarily. Higher concentrations don't linearly improve results — formulation matters more than concentration. A well-formulated 1% multi-weight HA product typically outperforms a poorly formulated 5% single-weight one.
Does HA work for all skin types? Yes. Unlike some actives that suit certain skin types better than others, hyaluronic acid is appropriate across the board — dry, oily, sensitive, mature, combination. The format (serum vs gel vs cream) matters more than the ingredient itself for skin type suitability.
The Practical Takeaway
Hyaluronic acid is genuinely effective — but what you're getting varies significantly depending on the molecular weight of the formula you choose. A product using only high molecular weight HA delivers real but surface-level hydration. One using multi-weight forms addresses hydration at multiple depths and provides more durable results.
When evaluating HA products, look for multiple forms in the INCI list, apply to damp skin, and always follow with a moisturiser to seal in what the HA is drawing in. Done consistently, it's one of the most reliable hydration strategies in a routine.
Explore SYLARA's face care collection for hyaluronic acid-formulated products, including the Ceramide Barrier Night Cream which combines sodium hyaluronate with ceramides, shea butter, and jojoba oil for layered, overnight hydration.