Hyaluronic acid: the molecular-weight question

What is Hyaluronic acid: the molecular-weight question?

Hyaluronic acid is a glycosaminoglycan found naturally in skin and connective tissue, where it binds water and contributes to volume and mechanical properties. In cosmetics it appears as the acid and as various salts and derivatives, in molecular weights spanning three orders of magnitude — and molecular weight is the single most important variable.

What the penetration research shows

This is unusually well characterised. Raman micro-imaging of human skin found that low-molecular-weight HA in the 20 to 300 kDa range passes through the stratum corneum, while high-molecular-weight HA at 1000 to 1400 kDa is effectively impermeable. Fractions at 5 and 8 kDa penetrate readily. Below about 8 kDa, penetration appears to be assisted by HA's own hydration effect, its interaction with keratin and lipids, and CD44 receptor-mediated uptake.

The practical translation: high-molecular-weight HA works at the surface, holding water in a film that reduces measured water loss and makes skin look and feel plumper. Low-molecular-weight HA can actually get in. Both are useful; they are not doing the same job, and a product that only contains the high-molecular-weight form cannot claim to be acting below the surface.

High-molecular-weight HA can be driven into the stratum corneum with an enhancer such as magnesium chloride, or vectorised with clay, but those are specific formulation strategies rather than a property of HA itself.

The dual-weight approach

Vegalab's Hyaluronic Skin Repair uses a stated dual molecular weight system, high (around 1000 kDa) and low (around 50 kDa), at 75 mg per application. Read against the penetration literature that combination makes sense: the high-weight fraction is a surface humectant and film former, the low-weight fraction sits within the range shown to cross the stratum corneum. Describing both as "penetrating" would be wrong; describing the pairing as covering two different mechanisms is accurate.

Eight forms, and an honest note on levels

The Youlief range lists eight distinct HA forms: sodium hyaluronate, hydrolyzed hyaluronic acid, hyaluronic acid, hydrolyzed sodium hyaluronate, potassium hyaluronate, hydroxypropyltrimonium hyaluronate, sodium hyaluronate crosspolymer and sodium acetylated hyaluronate.

Only two are present at levels that can plausibly do anything: sodium hyaluronate at 0.02% and hydrolyzed hyaluronic acid at 0.006%. The remaining six sit at 1 part per million or below, and the crosspolymer at 0.02 ppm. Those are label-claim quantities.

This is worth stating plainly because the industry practice of listing many HA variants implies a sophistication that the concentrations do not support. A long HA list is not the same as a lot of HA. The two functional forms here are well chosen — one surface humectant, one in the penetrating weight range — and that is the honest story.

What HA does not do

It does not stimulate your own HA synthesis in any established way at topical levels. It does not fill lines the way injected HA does; injectable and topical HA are the same molecule doing entirely different jobs at entirely different depths. And in very dry air a heavy high-weight HA layer with nothing occlusive over it can draw water from skin rather than into it, which is why HA is applied to damp skin and sealed.