Hyaluronic acid

Hyaluronic acid is a humectant. It binds water and holds it, and it is already in your skin and connective tissue doing exactly that.

That is the entire mechanism, and it is a real one. What the marketing has built on top of it is considerably larger.

Molecular weight is the variable

Hyaluronic acid is a chain, and how long the chain is changes what the molecule does.

The expert consensus is direct about the variations: “variations in quality and molecular weight of hyaluronic acid will influence safety and tolerability since low molecular weight hyaluronic acid may penetrate deeper to promote healing and skin barrier repair but may also be more inflammatory”.

So a shorter chain goes further in and may do more, and may also provoke more. A longer chain stays on the surface and holds water there. Neither is the better one in general; they do different things.

And the label almost never says which. Ingredient lists carry “sodium hyaluronate” or “hyaluronic acid” without a weight. Products that state a range, or that name several weights, are giving you information most do not.

That is the same shape as everything else in cosmetics: the variable that decides the outcome is missing from the document that lists the ingredients, which is the point of the guide to reading a cosmetics label.

The dry-air problem

Worth knowing because it turns a good ingredient into an uncomfortable one.

A humectant pulls water towards itself. In humid air it pulls from the air. In very dry air, with nothing above it to seal it, it can pull from deeper in your skin and let that water evaporate, which leaves skin feeling tighter than before.

The fix is not a different serum. It is putting something over it: an occlusive, a moisturiser, Petrolatum in the extreme case. Hyaluronic acid works applied to damp skin and sealed, and works against you applied to dry skin and left bare.

Glycerin is the other humectant in almost every product and behaves the same way, at a fraction of the price and with rather more evidence behind it.

Pregnancy, and the oral version

No clinical body attaches a pregnancy caution to topical hyaluronic acid. It is a humectant rather than an active in the pharmacological sense, and it sits with Niacinamide among the ingredients that need no decision.

Oral hyaluronic acid supplements are a different claim. They are digested like other large molecules, and the evidence that swallowing it raises skin hyaluronic acid is much thinner than the evidence for applying it. That gap between a good topical ingredient and a weak oral one is common, and worth noticing whenever the same name appears in both aisles.