Hyaluronic Acid, Explained: What Molecular Weight Changes

One name, a range of molecule sizes

Hyaluronic acid is a glycosaminoglycan — a long chain of repeating sugar units. Skin makes its own, mostly in the dermis, and it holds water. What cosmetic chemists control is how long they cut the chain, reported as molecular weight in kilodaltons (kDa).

The ranges you will see referenced in the literature run roughly like this: very low molecular weight below about 50 kDa, low in the tens to low hundreds, high in the region of 1,000 kDa and above. A single serum often contains several at once, which is why a product can behave like both a surface film and a penetrating humectant.

The practical point is that "contains hyaluronic acid" is not a specification. It is closer to "contains oil" — true, and not yet informative.

What the Raman study actually measured

The most useful piece of direct evidence is Essendoubi and colleagues, published in Skin Research and Technology in 2016. They used confocal Raman micro-imaging on human skin — a method that can tell an applied molecule apart from the skin's own constituents, which is exactly the problem that makes penetration claims hard to check.

The finding: hyaluronic acid in the 20–300 kDa range passed through the stratum corneum, while 1,000–1,400 kDa hyaluronic acid did not. The high molecular weight material stayed where it was applied.

Two things follow. First, "penetrates" is a real property of some hyaluronic acid and not of all of it. Second, a study that measured the stratum corneum is not evidence about the dermis — depth claims that reach further than the measurement did are extrapolation, however often they are repeated.

Staying on top is not the same as doing nothing

High molecular weight hyaluronic acid is sometimes written off because it does not go in. That misreads what it is for. A long-chain polymer left on the surface forms a viscoelastic film, and a film on the surface slows transepidermal water loss and changes how the surface handles light and touch — the immediate smooth, cushioned feel a good hyaluronic acid serum gives on application.

So the two fractions are doing different work:

• High molecular weight — surface film, immediate feel, reduced water loss through that film.
• Low molecular weight — crosses the stratum corneum, distributes within it.

A blend is not a marketing hedge. It is a reasonable formulation choice, because the two effects do not substitute for each other. If you like how a serum feels the moment it goes on, you are probably responding to the high molecular weight fraction.

What the percentage on the bottle is counting

This is the number most often misread. The published topical work sits at fractions of a percent. Pavicic and colleagues, in the Journal of Drugs in Dermatology in 2011, tested cream formulations at 0.1% hyaluronic acid across molecular weights of 50, 130, 300, 800 and 2,000 kDa, and reported that all of the hyaluronic acid creams significantly improved skin hydration against placebo over 60 days.

Set that against a label advertising "2% hyaluronic acid complex." At high concentrations raw hyaluronic acid produces something closer to a gel than a serum — it is a very effective thickener — so a high stated figure usually refers to a pre-diluted solution or a complex, not to that much active polymer. A bigger number on the front is not evidence of a stronger product, and there is no published dose-response curve that would let you treat it as one.

What is worth checking instead is whether the brand states the molecular weights it uses. That is the variable the measurements were about.

The four names you will see on an INCI list

The ingredient list is more informative than the front label, once you know the vocabulary.

• Sodium Hyaluronate — the sodium salt. More stable and more soluble than the acid form, and by far the most common form in finished products. Not a lesser ingredient.
• Hyaluronic Acid — the acid form itself.
• Hydrolyzed Hyaluronic Acid — chains deliberately broken down, so this signals the low molecular weight end.
• Sodium Acetylated Hyaluronate — a modified form with added acetyl groups, formulated for greater substantivity on the skin.

Seeing three or four of these in one list generally means the formulator has built a molecular weight blend on purpose. Position in the list still tells you about quantity, as always — but with an ingredient used at fractions of a percent, everything relevant sits near the bottom, so relative position among the hyaluronates is more informative than absolute position.

The dry-air claim, and what is actually documented

You will read everywhere that in low humidity hyaluronic acid pulls water from the deeper layers of your skin and evaporates it, leaving you drier than before. It is repeated with great confidence and it is worth being precise about its status: we could not find a controlled measurement demonstrating this for hyaluronic acid specifically. The general behaviour of humectants — that they equilibrate with ambient humidity, and that an occlusive step slows water loss — is well established. The specific dehydration story is an inference from that, not a result.

The reason it does not matter much in practice is that the advice you would give either way is identical, and it follows from the measurements we do have. High molecular weight hyaluronic acid reduces water loss through a surface film; that film is thin and easily disturbed. So:

• Apply to damp skin, which gives the humectant water to work with immediately.
• Follow with a moisturiser, which supplies the occlusive layer the serum does not contain.

Believe the mechanism or not — the routine is the same. What you should not do is treat a hyaluronic acid serum as a finished step in dry conditions. It is a humectant, not a moisturiser, and the difference between dry skin and dehydrated skin is exactly the difference between what it can and cannot address.

What hyaluronic acid does not do

Clear boundaries save money.

• It is not an occlusive. It attracts and holds water; it does not seal. Those are different ingredient classes and you need both.
• It does not supply barrier lipids. If the problem is a compromised barrier, the shortage is ceramides, cholesterol and fatty acids in the right proportions — a different repair job, which we covered in what the lipid research found about barrier repair.
• It is not an exfoliant and not an antioxidant. Those are separate steps.
• Topical is not injectable. Dermal fillers are cross-linked hyaluronic acid placed below the skin by a clinician. A serum is not a lesser version of that procedure; it is an unrelated one.

Where it goes in a routine

Hyaluronic acid is unusually easy to place because it has no pH requirement of its own and no meaningful conflicts. It sits comfortably after cleansing and any water-based treatment step, and before your moisturiser.

1. Do not let the skin dry fully after cleansing or after a toner — apply while damp.
2. Apply to skin, not to a dry palm. A few drops pressed in goes further than a layer smoothed on.
3. Seal it. A moisturiser over the top is what keeps the step working past the first ten minutes.
4. Do not stack four humectant serums. Adding more of the same category does not multiply the effect; the occlusive step is what is missing.

It also layers with almost anything, which is why it turns up in so many products. If you want the sequencing logic across a whole routine, we set out which ordering rules have measurement behind them, and niacinamide is the other ingredient people most often ask to pair it with — a combination with no known incompatibility.

Sources and notes. Penetration by molecular weight: M. Essendoubi et al., "Human skin penetration of hyaluronic acid of different molecular weights as probed by Raman spectroscopy," Skin Research and Technology, 2016;22(1):55–62 — confocal Raman micro-imaging on human skin; low molecular weight hyaluronic acid (20–300 kDa) passed through the stratum corneum, in contrast to the impermeability of high molecular weight hyaluronic acid (1,000–1,400 kDa). Concentration and hydration: T. Pavicic et al., "Efficacy of cream-based novel formulations of hyaluronic acid of different molecular weights in anti-wrinkle treatment," Journal of Drugs in Dermatology, 2011;10(9):990–1000 — cream formulations at 0.1% hyaluronic acid at 50, 130, 300, 800 and 2,000 kDa; all hyaluronic acid formulations significantly improved skin hydration versus placebo over 60 days. Checked August 2026. The claim that hyaluronic acid draws water from deeper skin and worsens dryness in low humidity is widely repeated; we were unable to locate a controlled measurement supporting it for hyaluronic acid specifically, and have described it above as an inference rather than a finding. Formulation details, molecular weight blends and concentrations differ between products and are not always disclosed. Patch-test any new product on a small area before applying it to the face, and take persistent, painful or worsening skin concerns to a dermatologist. This article explains ingredient behaviour and is not medical advice.