The idea that hair needs to be hydrated by a constant intake of water is a marketing myth; the concept of "hydrated hair" is a sensory perception rather than an actual biological reality.

noli's read

The community largely agrees that terminology within the hair care industry is misleading, highlighting an important distinction between the physical state of the hair and how it feels to the touch. Scientifically, hair is composed of dead cells (keratin) that cannot absorb or retain water in the same way as living skin. Although a slight water content promotes suppleness (plasticity), excessive saturation causes swelling that can weaken the structure (studies on keratin hygroscopy, often cited in cosmetic research). The use of oils or conditioners does not create internal hydration, but acts by modifying the cuticle's friction coefficient, which influences our perception of softness or "health." The exaggeration lies in the promotion of so-called "hydrating" products that merely seal or lubricate the hair. There is no evidence that hair can be "hydrated" in the sense that a living organism is hydrated.

the sources

1. 1r/HaircareScience