Hyaluronic acid: the universal hydration ingredient

Hyaluronic acid (HA) is a glycosaminoglycan, a naturally-occurring molecule that the body uses extensively for tissue hydration. A single HA molecule can bind up to 1,000 times its weight in water. Topical HA pulls water from the air (when humidity is above 70%) and from deeper skin layers (always) to hydrate the surface.

The plumping effect is mostly mechanical: hydrated cells take up more volume, smoothing visible fine lines from the inside. The visible effect appears within minutes of application and lasts as long as the HA is on the skin. There is no cumulative anti-ageing benefit beyond the immediate hydration, despite marketing claims to the contrary.

HA molecules come in different sizes, and the size determines how the ingredient behaves on the skin. High-molecular-weight HA (1,000,000+ Da) sits on the surface and provides immediate plumping and film-forming hydration. Low-molecular-weight HA (50,000-300,000 Da) penetrates slightly deeper and supports hydration in the upper dermal layers. Multi-weight formulations stack the two for both surface and slightly-deeper hydration.

Marketing language often emphasises the molecular-weight innovation, but for most users, any reasonable HA formulation works well enough. The bigger variable is whether the surrounding climate has enough humidity for HA to draw water from the air rather than pull it from deeper skin layers (which is the opposite of what you want in dry climates).

Apply to slightly damp skin after cleansing, before moisturiser. The water on the skin gives the HA a substrate to bind, which is more effective than applying to dry skin. Follow with a moisturiser to seal the hydrated layer.

In very dry climates (humidity below 30%), apply a moisturiser or facial oil after the HA to prevent the HA from drawing water from your skin rather than the air. Without that seal, HA can paradoxically be drying in arid conditions. This is the only case where HA can backfire.