What does hyaluronic acid actually do in a rinse-off shampoo?

Hyaluronic acid (listed as sodium hyaluronate) is a humectant: it binds water molecules and holds them at the hair fibre surface. Even though shampoo is rinsed off, a thin hydrating film remains on the cuticle. This film reduces friction between strands, so hair feels smoother, detangles more easily, and reflects light better — hence the glossier look.

The effect is most noticeable on porous hair — lengths that have been bleached, coloured, or regularly heat-styled. Porous hair loses water quickly and feels rough; the film from hyaluronic acid temporarily patches those gaps, slowing moisture loss and taming flyaways. It does not "repair" the internal structure, but it improves how the hair behaves day to day.

For lasting hydration, follow with a conditioner or mask that contains occlusive ingredients (oils, butters, ceramides) to seal in the moisture the hyaluronic acid helped attract.