Hyaluronic Acid, Explained: Molecular Weight, Sodium Hyaluronate and What a Serum Can Really Do

Hyaluronic acid shows up in serums, injectable fillers and supplements, and the same name is used for very different things. Here is what it actually is, why its size matters, and what the evidence supports.

What It Is

Hyaluronic acid (HA) is a long-chain sugar molecule that skin makes naturally and that can hold a lot of water. It is a widely distributed glycosaminoglycan with a high water-retaining ability, present in the epidermis, and it plays a central role in skin hydration and elasticity^[1]. Youthful skin keeps its turgor and pliability partly because of its high water content, and HA is the key molecule involved in skin moisture^[2].

It Declines With Age

HA is an important part of the skin's supporting matrix, and the loss starts at around 25 years old^[3]. In aging skin, the HA chains also become progressively smaller^[2]. That is part of why skin loses the plump, springy feel it had when younger.

Why Size Matters

The HA used in skincare comes in different molecular weights, and size affects how far it travels. In a skin-section imaging study, low-molecular-weight HA (20 to 300 kDa) passed through the outer layer of skin, while high-molecular-weight HA (1,000 to 1,400 kDa) did not^[4]. In a trial of 76 women aged 30 to 60, creams with 0.1% HA at sizes from 50 to 2,000 kDa were applied around one eye twice a day for 60 days, with a plain cream around the other. All of them improved skin hydration and elasticity compared with the plain cream, but wrinkle depth improved significantly only with the 50 and 130 kDa versions^[5]. Labels list the ingredient name, not the size, so you usually cannot tell which type a product uses.

Hyaluronic Acid vs Sodium Hyaluronate

On an ingredient list you will most often see sodium hyaluronate. Safety reviewers treat hyaluronic acid, sodium hyaluronate and potassium hyaluronate together: they work in cosmetics as skin-conditioning agents at concentrations up to 2%, and the panel judged them safe for use in cosmetics^[6]. Sodium hyaluronate is the sodium salt of hyaluronic acid, so a label that lists either one is naming the same family of ingredient.

What a Serum Can and Cannot Do

A topical HA product is a humectant: it draws water in and helps hold it near the skin's surface. That is why the trials above measured better hydration and elasticity, and, for some sizes, shallower wrinkles over about two months. It does not replace the HA your skin makes, and it is not the same as an injectable filler, which is placed under the skin by a clinician. Think of a serum as a way to keep skin comfortable and plump-looking, not as a way to rebuild what age takes away.

Does Taking It as a Supplement Work?

In a randomized, double-blind, placebo-controlled trial in 150 healthy adults, 120 mg a day of oral sodium hyaluronate for 12 weeks improved skin hydration and elasticity and reduced water loss and the depth of wrinkles around the eyes compared with placebo. A 60 mg dose showed similar but smaller effects^[7]. It is one trial, in Caucasian adults, so we would call it promising, not settled.

A note on the sources: the review of topical HA was written by L'Oréal researchers, the supplement trial was run by researchers at Contipro, a hyaluronan manufacturer, and one author of the penetration study works at Soliance, a cosmetic ingredient company. All were published in peer-reviewed journals, but you should know who was involved.

How to Use It Well

• Apply to slightly damp skin, so there is water for the HA to hold.
• Seal it with a moisturizer. A humectant alone can only hold the water that is available.
• Be patient. The trials ran for about two months before measuring results.
• Do not expect filler results. A serum hydrates and softens; it does not change facial volume.
• Keep sunscreen as the last morning step. Hydration does not protect against UV.