Hyaluronic acid: what it really does on your skin

Hyaluronic acid is probably the most famous hydrating ingredient on the shelf. It’s in serums, creams, sheet masks and even cleansers, usually with a claim about holding a huge amount of water. It does hydrate skin, and it’s gentle. But what it actually does depends on the size of the molecule, the weather and what you put on top of it.

What is hyaluronic acid?

Despite the name, hyaluronic acid isn’t an exfoliating acid. It’s a long, chain-like sugar molecule that your body makes. A 2012 review in Dermato-Endocrinology notes that skin holds about half of all the hyaluronic acid in your body, and describes it as the key molecule for skin moisture because of its unique ability to bind and hold water.

In skin, it sits between cells, mostly in the dermis (the deeper, living layer), where it forms a gel that cushions tissue and helps regulate water. It’s also constantly renewed: the same review says hyaluronic acid in skin has a half-life of less than a day. On ingredient lists you’ll often see it as sodium hyaluronate, the salt form, which behaves the same way.

How it hydrates: a humectant

In a moisturizer, hyaluronic acid is a humectant, meaning an ingredient that attracts water and holds on to it. (Our guide to humectants, emollients and occlusives covers the three types of moisturizer ingredients.)

A 2025 review in the journal Biomolecules explains that large hyaluronic acid molecules barely get into the skin. Instead, they form a film on the surface that slows transepidermal water loss, the water that constantly evaporates through your skin, and adds hydration. That film is also why hyaluronic acid serums feel smooth and slightly slippery.

Big molecules, small molecules

Hyaluronic acid comes in very different sizes, measured in kilodaltons (kDa), a unit of molecular weight. Your skin’s own hyaluronic acid is very large. Manufacturers can break it into smaller pieces, often labelled “low molecular weight” or “hydrolyzed”.

Size changes where it goes. The Biomolecules review, which pooled lab studies on human skin samples, concluded that hyaluronic acid below about 100 kDa can penetrate skin, and the smallest pieces can reach the dermis, while very large molecules (1,000 kDa and up) stay on the surface.

What does that mean in real use? Two trials give an idea:

• In a 2011 trial, 76 women applied creams with 0.1 percent hyaluronic acid of one of five sizes around one eye and a plain cream around the other for 60 days. All sizes improved hydration and elasticity compared with the plain cream, but only the two smallest sizes reduced wrinkle depth.
• In a 2024 trial in 36 nursing-home residents aged 60 to 80 with dry skin, a lotion with low molecular weight hyaluronic acid raised skin hydration on the legs more than a high molecular weight lotion or a plain lotion. But water loss through the skin and dryness symptom scores didn’t differ between the three.

Both studies were small, and the differences were modest. The honest summary: smaller pieces may do a bit more, but no size turns hyaluronic acid into a wrinkle treatment. That’s why many products now mix several sizes.

About “1,000 times its weight in water”

This is the claim you’ll see most often, and it’s worth unpacking. The idea is that hyaluronic acid can hold up to a thousand times its own weight in water.

The Biomolecules review gives a more sober number: studies measuring the water tightly bound to hyaluronic acid report about 0.7 to 2 grams of water per gram. A hyaluronic acid gel can trap far more water loosely in its tangled network, which is part of why it’s such a good thickener. But we couldn’t trace the 1,000 figure to a standard measurement, and in any case it describes a molecule in a lab dish, not a serum on your face.

A product with a small percentage of hyaluronic acid isn’t going to pour litres of water into your skin. It helps your outer skin hold on to water, which is useful, but it’s not magic.

The dry-air caveat, and why Canadian winters matter

A humectant needs a water source. A 2016 review of moisturizers in the Indian Journal of Dermatology explains that humectants draw water from two places: from the air, but only in humid conditions, and from the deeper skin up into the outer layer. In dry air, the second source is the only one available, and water brought up to the surface can then evaporate. The review calls humectants a “double-edged weapon” for this reason, and says they’re mostly combined with an occlusive, an ingredient that forms a seal to stop water escaping.

Think of a Canadian January: cold outdoor air holds very little moisture, and indoor heating dries it further. That’s exactly when a hyaluronic acid serum on its own can leave skin feeling tight. It’s not damaging your skin, but it’s not doing its best work either.

The fix is simple:

• Apply it to slightly damp skin, for example right after washing your face.
• Follow with a moisturizer that contains emollients (softening oils) and an occlusive, such as petrolatum or dimethicone, to seal the water in.
• On very dry or cracked skin, a plain moisturizer with glycerin may do as well. The same review calls glycerin the most effective humectant, and it’s in many basic creams.

Is it safe, and who is it for?

Hyaluronic acid is about as low-risk as skin-care ingredients get. The Biomolecules review found it well tolerated across the clinical trials it summarized. One caution about that review: some of its authors worked for a company that makes cosmetic ingredients, and many of the trials it lists tested specific commercial ingredients in small groups of people.

It suits almost everyone who wants extra hydration, including oily and acne-prone skin, since serums are usually light and oil-free. If a hyaluronic acid product stings, look at the rest of the ingredient list. And if your skin stays dry, cracked or itchy despite regular moisturizing, see your doctor or pharmacist, since eczema and other conditions can look like simple dryness.

You can browse the panel’s reviewed serums and treatments or moisturizers.

Hyaluronic acid is a genuinely useful, gentle hydrator, not a miracle sponge. Large molecules form a moisture-holding film on the surface, smaller ones go a little deeper, and the “1,000 times” line oversells it. In dry Canadian air, put it on damp skin and seal it with a moisturizer, and it will do what it’s good at.