What hyaluronic acid actually is
Hyaluronic acid is a glycosaminoglycan — a long-chain polysaccharide that occurs naturally in connective tissue, skin, and synovial fluid. It can hold up to 1,000 times its own weight in water. Almost no marketing material tells you that the HA molecule is too large in its native form to penetrate skin. High-molecular-weight HA sits on the surface and forms a film — a good humectant, not a penetrating active.
The molecular weight question
HA comes in a range of molecular weights that behave completely differently. High MW (>1000 kDa) forms a moisture-retaining surface film. Low MW (10–50 kDa) reaches the viable epidermis with longer-lasting hydration effects. Ultra-low MW (<10 kDa) achieves the deepest penetration of the cosmetically-relevant fractions.
When a product lists “Sodium Hyaluronate” once on its INCI, you don’t know which molecular weight you’re getting. Multiple INCI entries — Sodium Hyaluronate, Hydrolyzed Hyaluronic Acid, Sodium Hyaluronate Crosspolymer — tell you there’s a real multi-fraction system.
What the evidence actually says
Surface hydration is well-established across every molecular weight. Deeper penetration of low-MW HA is supported by in vitro and some in vivo studies. Anti-wrinkle effects in long-term studies (8–16 weeks) are real but modest — a hydration-driven effect, not structural rebuilding.
How to use it
Apply to damp skin immediately after cleansing. Always follow with an occlusive or emollient — a HA serum with nothing over it will lose absorbed moisture within an hour. The sweet spot for most formulations is 0.1–1% of each relevant fraction; more than 2% adds tackiness with no additional benefit.