Hyaluronic acid is a glycosaminoglycan – a large sugar molecule – that occurs naturally in the skin, connective tissue, and eyes. It’s one of the most efficient humectants known, capable of holding up to 1000 times its weight in water. That number gets cited constantly, but what matters practically is what happens at different molecular weights.
High molecular weight HA (over 1 million Da) sits on the skin’s surface, forming a film that reduces transepidermal water loss. It’s effective for surface hydration but doesn’t penetrate. Low molecular weight HA (under 50,000 Da) penetrates into the upper dermis, where it can interact with the extracellular matrix and trigger collagen synthesis. Some formulations use hydrolysed HA or sodium hyaluronate (the salt form, smaller molecule, slightly better penetration) to optimise this.
The skin naturally produces HA via fibroblasts, but production declines with age – by around age 40, skin retains roughly half the HA it did at 20. This decline correlates directly with reduced skin plumpness and increased fine line visibility.
Application on damp skin makes a meaningful difference. HA draws moisture from its environment – if you apply it to dry skin in a dry environment, it can pull moisture from the lower layers of skin upward, which is the opposite of the intended effect. Apply to damp skin and seal with a moisturiser.
Polyglutamic acid (PGA) is a newer humectant that outperforms HA in some hydration studies and also inhibits hyaluronidase, the enzyme that breaks down existing HA. These two work well together for a compound hydration effect.