Hyaluronic Acid
Hyaluronic acid (HA) is a sugar-based polymer (a glycosaminoglycan) your body makes naturally — it's the skin's main water-binding molecule. On product labels it usually appears as its salt, Sodium Hyaluronate. As a skincare ingredient it's a humectant: it holds water at the skin's surface.
- Name
- Hyaluronic Acid
- INCI Name
- Hyaluronic Acid
- CAS Number
- 9004-61-9
- Category
- Humectant (glycosaminoglycan)
- Also known as
- Sodium Hyaluronate · Hyaluronan · Hyaluronate
Overview
CosIng lists HA / sodium hyaluronate as a humectant and skin-conditioning agent. Its job is hydration — it binds water and holds it in the upper layers of the skin, temporarily improving suppleness and the look of fine lines. How deep it acts depends entirely on its molecular weight, which the label does not tell you.
How it works
HA is a humectant: it binds a large amount of water (often cited as up to ~1,000x its weight) and holds it at the skin surface and within the stratum corneum. Its effect depends on molecular weight. High-molecular-weight HA (roughly >1 million Da) forms a moisturising film and stays at or near the surface. Low-molecular-weight HA penetrates the upper epidermis. Little to no topically applied HA reaches the dermis, so topical HA is not the same as an injectable filler and does not rebuild the skin’s own HA. In dry air, HA can also draw water outward from deeper skin, so it works best layered under an occlusive moisturiser. Its hydration is real but largely surface-level and reverses once you stop using it.
Water binding (humectant)human
HA's carboxyl and hydroxyl groups bind water, holding moisture at the skin surface and in the stratum corneum — the basis of its hydration and temporary softening of fine lines.
Molecular-weight-dependent penetrationhuman
Penetration depth scales inversely with molecular weight: high-MW HA stays at or near the surface; low-MW HA reaches the upper epidermis. Measured directly in human skin by Raman spectroscopy.
Surface film & barrier supportmechanistic
High-MW HA forms a viscoelastic film that slows water loss and smooths the surface — a physical, temporary effect, not a change to skin structure.
Reported benefits
- Hydration — binds and holds water in the upper skin, improving softness and suppleness; real but surface-level and reverses when you stop.
- Look of fine lines — surface hydration can temporarily soften fine lines; in a controlled trial only low-molecular-weight HA (50-130 kDa) measurably reduced wrinkle depth.
- Elasticity & comfort — topical HA improved measured skin elasticity across molecular weights in vehicle-controlled testing, and helps dry or compromised skin feel more comfortable.
- Layers with almost anything — non-irritating and compatible with retinol, vitamin C and niacinamide; useful for offsetting the dryness those can cause.
Evidence
Moderate
Graded moderate. Topical HA has a vehicle-controlled RCT showing improved hydration and elasticity across molecular weights, with wrinkle-depth reduction limited to low-MW forms (Pavicic 2011), plus direct human-skin evidence that penetration depth depends on molecular weight (Essendoubi 2016), on an established humectant mechanism. Not higher because the effects are largely surface-level and temporary (they reverse on discontinuation), topical HA does not reach the dermis or replace injectable filler, the 'deep dermal plumping' claim is unsupported, low humidity can reverse the benefit, and much of the topical-HA efficacy literature is funded by HA raw-material suppliers.
Forms & derivatives
Related: Hyaluronic Acid (Ultra-High MW) · Sodium Acetylated Hyaluronate
References
- Efficacy of cream-based novel formulations of hyaluronic acid of different molecular weights in anti-wrinkle treatment J Drugs Dermatol. (Corresponding author academic (Department of Dermatology & Allergology, LMU Munich). Other co-author/funding affiliations are not listed in the PubMed record and were not independently verified; given the topical-HA efficacy literature is largely HA-supplier-funded, not asserted independent of the HA-supplier industry.)Vehicle-controlled RCT (76 women, 0.1% HA at five molecular weights, 60 days): all HA formulations improved skin hydration and elasticity vs vehicle; only low-molecular-weight HA (50 and 130 kDa) significantly reduced periocular wrinkle depth.Human StudyPMID: 22052267 →
- Human skin penetration of hyaluronic acid of different molecular weights as probed by Raman spectroscopy Skin Res Technol. (Co-author R. Reynaud is affiliated with Soliance (Pomacle, France), a hyaluronic-acid manufacturer — confirmed in the record; remaining authors academic (University of Reims / CNRS). Not asserted independent.)Raman spectroscopy of human skin showed HA penetration depth depends on molecular weight: lower-MW HA penetrated into the epidermis while high-MW HA remained at or near the surface.Human Studydoi:10.1111/srt.12228 →
Frequently Asked Questions
Does hyaluronic acid plump skin 'from within' or reach the deeper layers?
Mostly no. Topical HA works at the surface and in the upper epidermis; very little reaches the dermis, and high-molecular-weight HA doesn't penetrate at all. The 'plumps from within' idea is largely marketing — real deep filling comes from injected HA, a different medical procedure.
What does 'molecular weight' mean and why does it matter?
HA is a polymer that comes in many sizes, from under 1 kDa to over 2 million Da. Size determines depth: high-MW stays on the surface, low-MW penetrates the upper epidermis. The catch: the label just says 'Sodium Hyaluronate' and tells you nothing about which size.
Is 'Sodium Hyaluronate' different from 'Hyaluronic Acid'?
Functionally no. Sodium hyaluronate is the salt form of hyaluronic acid — more stable in formulas, so it's what you usually see on labels. Both bind water the same way, and neither name tells you the molecular weight. (CAS: 9004-61-9 for the acid, 9067-32-7 for the sodium salt.)
Can hyaluronic acid actually dry out my skin?
It can, in low humidity. With too little water in the air, HA can pull moisture from deeper skin toward the surface where it evaporates. Applying to damp skin and sealing with a moisturiser or occlusive prevents this.
Is hyaluronic acid safe in pregnancy?
Topical HA is generally considered safe, including in pregnancy and breastfeeding — it's a molecule your body already makes and isn't significantly absorbed. Check with your doctor about your specific routine.
High vs low molecular weight — which is better?
Neither is simply better; they do different things. High-MW gives surface hydration and a smoothing film, low-MW reaches slightly deeper, and multi-weight blends aim for both. What matters most is a brand willing to tell you which weights are in the bottle.