Dr. Sophie Brandt, Biochemistry and Ingredients Editor.
Hyaluronic acid is one of the most popular ingredients in modern skincare, present in serums, moisturisers, sheet masks and injectable fillers. Its appeal is easy to understand: it is a molecule the body already produces, it holds up to 1,000 times its weight in water, and products containing it make the skin feel immediately smoother and more hydrated. But the science of how it works — and the circumstances under which it does not — is considerably more complex than the marketing suggests.
- Molecular weight determines behaviour: high-molecular-weight hyaluronic acid sits on the skin’s surface forming a moisture-retaining film; low-molecular-weight fragments penetrate deeper but may trigger inflammation.
- Humidity matters: in very dry environments, topical hyaluronic acid can draw moisture from the deeper layers of your skin rather than from the air, potentially worsening dehydration.
- It hydrates, it does not restructure: hyaluronic acid plumps the skin temporarily by attracting water, but it does not stimulate collagen production or reverse structural ageing.
What Hyaluronic Acid Is
Hyaluronic acid — technically sodium hyaluronate in most topical formulations — is a glycosaminoglycan, a long chain of repeating disaccharide units composed of glucuronic acid and N-acetylglucosamine. It occurs naturally in the extracellular matrix of human skin, where it plays a central role in maintaining hydration, tissue repair and structural integrity. Approximately 50 per cent of the body’s total hyaluronic acid is found in the skin.
The molecule’s water-binding capacity is extraordinary. Each hyaluronic acid molecule can bind hundreds of water molecules along its chain, creating a hydrated gel that fills the spaces between collagen and elastin fibres. This natural reservoir of moisture gives young skin its characteristic plumpness and resilience. As we age, hyaluronic acid production declines: a person over 50 has roughly half the dermal hyaluronic acid they had at 20.
Molecular Weight and Skin Penetration
The single most important variable in a hyaluronic acid product is the molecular weight of the hyaluronic acid it contains. High-molecular-weight hyaluronic acid (above 1,000 kDa) forms a film on the surface of the stratum corneum that reduces transepidermal water loss. It does not penetrate the skin. It works like a breathable moisture blanket, and it works well for this purpose.
Low-molecular-weight hyaluronic acid (below 50 kDa) can penetrate the outer layers of the epidermis, reaching the viable cell layers beneath the stratum corneum. A 2011 study in the Journal of Drugs in Dermatology demonstrated that nano-hyaluronic acid (5 kDa) penetrated to a depth of approximately 200 micrometres in excised human skin, well into the living epidermis.
However, a 2014 study published in Experimental Dermatology found that very low-molecular-weight hyaluronic acid fragments (below 50 kDa) can trigger pro-inflammatory signalling through Toll-like receptor 2. This is not inherently harmful — mild inflammation can stimulate repair processes — but it complicates the assumption that deeper penetration is always better.
The Humidity Problem
Hyaluronic acid is a humectant: it attracts water from its surroundings. In a humid environment — a bathroom after a shower, a coastal city in summer — it draws moisture from the air into the film sitting on your skin. This is the ideal scenario and the one demonstrated in most product testing.
In a dry environment — heated indoor air in winter, arid climates, air-conditioned offices — the air does not offer enough moisture. The hyaluronic acid film still seeks water, and the nearest available source is the deeper layers of your own skin. In these conditions, topical hyaluronic acid can theoretically increase transepidermal water loss, leaving the skin drier than before application.
The practical solution, recommended by dermatologists, is simple: apply hyaluronic acid to damp skin, then seal it with an occlusive moisturiser — a cream containing petrolatum, dimethicone, squalane or a similar barrier-forming ingredient. This traps the water the hyaluronic acid has attracted and prevents it from evaporating or drawing moisture from below.
What Hyaluronic Acid Does Not Do
Topical hyaluronic acid does not stimulate collagen production. It does not repair sun damage. It does not prevent wrinkles from forming. It provides temporary hydration and a visible plumping effect that lasts as long as the molecule remains on the skin — typically hours, not days.
This is not a criticism. Effective hydration is a legitimate and valuable function. Skin that is properly hydrated looks better, feels more comfortable, and has a more resilient barrier. But the clinical evidence does not support the implication, common in marketing, that hyaluronic acid serums produce lasting structural changes in the skin comparable to retinoids or vitamin C.
Choosing a Hyaluronic Acid Product
The best evidence supports formulations that combine multiple molecular weights of hyaluronic acid — a high-weight fraction for surface film formation and a medium-weight fraction for hydration of the upper epidermis — in a base that includes occlusive ingredients to prevent moisture loss. Products that contain only low-molecular-weight hyaluronic acid without an occlusive layer risk the humidity-dependent drawback described above.
Concentration is less important than formulation. Studies showing clinical benefits have used concentrations ranging from 0.1 to 2 per cent. Above 2 per cent, the product becomes sticky without proportionally improving hydration. The molecule’s water-binding capacity has a practical ceiling, and exceeding it adds texture problems without functional benefit.