The concept of "locking in hydration" in hair is a marketing misnomer; so-called hydrating hair products do not add water to the hair fiber, but instead act as conditioning agents to smooth and protect the hair surface.

noli's read

The community largely agrees that hair, being composed of dead cells without an active metabolism, cannot be "hydrated" in the biological sense like skin. Research in cosmetic science, particularly as explained by experts like Lab Muffin, confirms that hair products deposit protective films (often cationic surfactants or oils) onto the cuticle. These ingredients reduce friction, improve shine, and protect against external stressors, which is sensorially perceived as softness or hydration. The idea that one can seal water inside the hair is technically exaggerated, as the keratinized structure does not retain moisture the way living tissues do. The confusion stems from an inadequate transposition of skincare jargon to hair care. This consensus on the forum is robust, with users overwhelmingly rejecting the myth of internal hydration via topical products.

the sources

1. 1r/HaircareScience