Is it possible to hydrate hair?

What role does hydration play in hair?

Once hair emerges from the hair follicles, it is not alive. It is mostly composed of inert keratin filaments, devoid of metabolic activity. However, this does not prevent hair from containing water, nor does it prevent water from playing an important role in its appearance, elasticity and resistance to breakage.

As a reminder, each hair fibre is composed of three overlapping layers: the medulla, the innermost layer, the cortex, accounting for almost 80% of the hair, and the cuticle, the external protective layer. The cortex is made of keratin and also contains melanin pigments, which give hair its colour. It is also the main water storage zone. The cuticle, meanwhile, comprises flat cells that overlap like tiles and whose cohesion is essential for to limit water loss and also prevent its penetration. When these cells are disorganised, often following an alteration of the intercellular lipid cement, the hair become more porous: they retain water less effectively and absorb it more readily.

Structure des fibres capillaires. — Structure of hair fibres.
Source: In vivo human hair hydration measurements using opto-thermal radiometry. International Journal of Thermophysics (2018).

This is why the verb "hydrate" is actually misused when referring to hair. The aim of hair care treatments is not to force water into the fibre. When water penetrates a porous hair strand, it causes a cuticle swelling, further disrupting the scales that form it. This cycle of swelling and contraction, repeating over successive washes and blow-dries, weakens the fibre, increases the risk of breakage and promotes split ends. Indeed, for this reason it is not advisable to let your hair air-dry naturally, as moisture remains for longer. The threat posed by water to hair fibres is illustrated by the images below, taken with an atomic force microscope. They clearly show the lifting of the cuticle when the hair is immersed. Without applying a post-shampoo treatment, or conditioner as the English term has it, to smooth and seal it, the cuticle remains raised and vulnerable to external aggressions.

It is preferable to refer to “conditioning” one’s hair rather than “hydrating” it.

Image par microscopie à force atomique d'un cheveu dans l'air (a) et dans l'eau (b). — Atomic force microscopy image of a hair in air (a) and in water (b).
Source: BALDESCHWIELER J. D. et al. Atomic force microscopy of human hair cuticles: a microscopic study of environmental effects on hair morphology. The Journal of Investigative Dermatology (1995).

How can you "hydrate" your hair?

Moisturising, or more precisely conditioning, hair involves preventing water from entering and leaving it. To do this, the cuticle scales must be thoroughly smoothed. This is achieved notably through the systematic application of a conditioner after each wash. Indeed, the use of a conditioner flattens the cuticle scales along the hair shaft thanks to the cationic surfactants it contains. This treatment also increases the adherence of the cuticle scales to the hair shaft, which enhances the hair’s light-reflecting capacity and reduces friction between the hair fibres.

It is also important to maintain the cuticle’s lipid barrier. This is where emollients and occlusive agents come in. Vegetable oils, such as coconut oil or apricot oil, which are rich in fatty acids, can form a film on the surface of the fibres, thus reducing water evaporation. Some studies have even shown that lauric acid, one of the main components of coconut oil, can penetrate the hair and reduce water-induced cuticle swelling as well as the stress this imposes on the hair fibres.

Other studies have focused on the effect of nanoemulsions on the hydration of textured hair from a woman. Some hair fibres underwent a bleaching process, optionally followed by recolouring. For information, a nanoemulsion is a mixture of two or more immiscible liquids in which one is finely dispersed in the other, forming droplets at the nanometre scale. Specifically, three nanoemulsions were studied: a control nanoemulsion, a version enriched with 7.5% coconut oil, and a third containing 7.5% coconut oil and 0.1% wine lees extract. These were applied to the three hair types, and their hydration was then observed.

	Healthy hair	Bleached hair	Hair bleached and subsequently re-coloured
Control nanoemulsion	7.3	7.3	7.2
Nanoemulsion containing 7.5% coconut oil	7.2	7.3	8.3
Nanoemulsion with 7.5% coconut oil and 0.1% wine lees extract	7.6	7.7	9.6

Comparison of the moisturising effect of different nanoemulsions (arbitrary units).

Source: GOMES A. et al. Nanoemulsion with wine lees: a green approach. Anais da Academia Brasileira de Ciencias (2024).

While cosmetics play an important role, prevention should not be overlooked. Limiting heat sources (hair straighteners, curling tongs), avoiding products containing harsh surfactants and protecting your hair from the sun... are all sensible precautions to adopt to prevent your hair from becoming damaged.