Hyaluronic acidHA · Sodium hyaluronate
A humectant that holds water at the skin's surface: hydration, not rejuvenation.
Hyaluronic acid is a large sugar molecule found throughout the body, most abundantly in skin, joints and connective tissue. It holds water, and it holds a great deal of it, which is what gives the tissues it occupies their plumpness and cushioning.
In skincare it is a humectant, meaning it draws water into the outer layer of skin and holds it there. That produces a real but temporary effect: skin looks plumper and fine lines look softer while it is hydrated, and the effect fades as the hydration does.
It is not a filler and does not rebuild anything. The injectable dermal fillers that share the name work by physically occupying space under the skin, which is an entirely different intervention from a serum applied to the surface.
There is a practical trick to using it. In dry air, a humectant can draw water from deeper in the skin and lose it to the atmosphere, which is why it should be applied to damp skin and sealed with a moisturiser rather than used alone in a dry room.
In joints, hyaluronic acid is a major component of synovial fluid, contributing to lubrication and shock absorption. Injectable versions are used in osteoarthritis with mixed and much debated evidence, and oral supplements have weaker support still.
It is a well tolerated, inexpensive ingredient that does one thing reliably, which makes it useful and unremarkable rather than an active worth building a routine around.
In practice, use it as a hydration step under a moisturiser, and reserve the expectation of structural change for retinoids and for sun protection.