Hyaluronic Acid: How a Hydrator Can Leave Your Skin Drier

Hyaluronic acid appears in more British skincare products than any other single active ingredient. Walk into any high-street chemist and you’ll find it in serums, moisturisers, sheet masks, eye creams, lip balms, and SPF formulations. The marketing pitch is consistent: deep hydration, plumped skin, reduced wrinkles. It sounds definitive. The chemistry is rather less so.

The molecule problem

Hyaluronic acid is a glycosaminoglycan — a large sugar molecule that occurs naturally in skin, joints, and connective tissue. In its native form, it holds roughly a thousand times its weight in water. That property is real and well-documented. What’s less well-documented, at least in the adverts, is that the same property doesn’t automatically transfer when you smear it across your face from a dropper bottle.

The reason is molecular weight. Native hyaluronic acid has a molecular weight somewhere between 1,000 and 8,000 kilodaltons. The skin’s outermost barrier — the stratum corneum — is designed to keep large molecules out. Research published in the Journal of Investigative Dermatology has shown that molecules above roughly 500 daltons struggle to penetrate intact skin. Standard-weight hyaluronic acid sits at thousands of times that threshold. It sits on the surface, forms a film, and draws moisture from the atmosphere to the skin’s outer layer. That is a useful function. It is not the function being advertised.

The molecular weight marketing that pervades skincare extends to hyaluronic acid in a particularly creative way: brands now offer “multi-weight” formulations, claiming that smaller fragments penetrate deeper layers while larger ones hydrate the surface. The idea is appealing. The evidence is thin.

When hydration works in reverse

Here’s the part the adverts leave out entirely. Hyaluronic acid is a humectant — it attracts water from wherever water is available. In a humid bathroom after a shower, that water comes from the surrounding air. In a centrally heated flat in January, or an air-conditioned office in August, the air holds very little moisture. So the hyaluronic acid on your skin draws water from the next available source: the deeper layers of your own epidermis.

This is not a theoretical concern. Dermatologists have been flagging the problem for years, and clinical observation supports the mechanism. If you apply a hyaluronic acid serum in a dry environment without immediately sealing it with an occlusive moisturiser, you can end up with skin that feels tighter and drier than before you started. The product designed to hydrate is doing precisely the opposite.

The standard industry response is to tell consumers to apply it to damp skin and follow with a heavier cream. That’s reasonable advice. It’s also an admission that the ingredient doesn’t work the way it’s sold — as a standalone hydrating solution that you pat on and walk away from.

The evidence question

When you look at the clinical evidence for topical hyaluronic acid, a pattern emerges that will be familiar to anyone who’s read about who funds skincare research. A 2023 systematic review in Clinical, Cosmetic and Investigational Dermatology pooled results from multiple trials and found modest improvements in skin hydration and elasticity. Modest, not transformative — and the majority of included studies were funded by manufacturers of hyaluronic acid products.

Independent reviews are less enthusiastic. Harvard Medical School published a piece noting that while the ingredient has genuine utility as a humectant, the marketing has far outstripped the evidence. The molecule’s popularity, they observed, owes more to its satisfying texture and consumer perception than to clinical superiority over simpler moisturising ingredients.

This matters because hyaluronic acid serums routinely cost three to five times more than a basic glycerin-and-ceramide moisturiser that achieves comparable surface hydration through a less glamorous mechanism. Glycerin is a smaller molecule, penetrates more readily, and has decades of robust evidence behind it. But glycerin doesn’t photograph well in a dropper bottle, and nobody is filming application videos of a glycerin cream for social media.

What it actually does — and what it cannot

Strip away the marketing and hyaluronic acid is a perfectly competent surface humectant. Applied to damp skin, sealed with an occlusive, it helps the outer layer of the epidermis retain water. For people with genuinely dehydrated skin who understand the application conditions, it can improve comfort and the temporary appearance of fine lines.

What it cannot do is penetrate into the dermis in any standard topical formulation, rebuild collagen, or provide lasting structural change. The claims about hydrating skin from within belong to injectable hyaluronic acid — a medical procedure with a genuinely different mechanism — not to a serum bought off a shelf.

The distinction between a hero ingredient and a useful one is worth keeping in mind. Hyaluronic acid is useful. The hero status — the idea that this one molecule is doing something no moisturiser can — doesn’t survive contact with the research. A well-formulated, unglamorous moisturiser applied to damp skin achieves the same surface hydration at a fraction of the cost, without the risk of the reverse-draw problem in dry environments.

The skincare industry has built a category worth billions around a molecule that, when you strip the branding away, is a decent humectant being sold as a revolution. That gap between what’s claimed on the box and what’s happening on your skin is wider than most people suspect — and it’s the kind of gap that only benefits the companies filling the bottles.