Hyaluronic acid: what benefits for hair?

What beneficial effects does hyaluronic acid have on hair?

The hyaluronic acid is a molecule naturally present in the body, renowned for its remarkable capacity to attract and retain water. Depending on its molecular weight, it can even bind up to 1,000 times its weight in water. However, while this quality is useful for the skin, it is less relevant for hair. Indeed, each hair fibre comprises three overlapping layers: the medulla, the innermost layer; the cortex, which accounts for nearly 80% of the hair; and the cuticle, the outer protective layer. This latter layer is composed of flat cells that overlap like roof tiles, and their cohesion is essential not only to limit water loss but also to prevent water from penetrating the hair.

In fact, water is not really beneficial for hair. When it penetrates into a porous hair fibre, where the cuticles are misaligned, it causes a swelling of the cuticle and exacerbates its disorganisation, thereby promoting the formation of split ends. This is why it is inadvisable to allow the hair to air-dry, as moisture remains trapped for longer. Moreover, despite occasional claims, hyaluronic acid does not form a protective film on the hair surface: being hydrophilic, it cannot adhere to a lipid-rich surface such as that of the hair fibre.

Thus, the use of hyaluronic acid to "moisturise" hair that is dry, whether straight, curly or coily, is not as relevant as one might think.

To date, there have been few studies on the hair-related properties of hyaluronic acid. One of these evaluated the effects of hyaluronic acid of different molecular weights on the mechanical properties of hair fibres. Strands were treated with a spray containing 0.25% hyaluronic acid and then subjected to tensile tests to measure their strength. The results showed that only low-molecular-weight hyaluronic acid significantly increased the tensile strength of hair fibres (+16%), without altering their elongation at break.

Effets de l'acide hyaluronique sur la résistance à la traction de fibres capillaires. — Effects of hyaluronic acid on the tensile strength of hair fibres.
Source: WANG J. et al. Improving the mechanical properties of damaged hair using low-molecular-weight hyaluronate. Molecules (2022).

Another study reached the same conclusions. Researchers assessed the effect of four different conditioners enriched with biopolymers (collagen, hyaluronic acid, a 50/50 collagen–hyaluronic acid blend and a 50/50 collagen–hyaluronic acid blend with 30% chitosan), compared to a conditioner without biopolymers, on the mechanical properties of human hair. Brown hair strands from a 20-year-old woman were washed, dried, treated with one of the conditioners for 30 minutes, then rinsed and left to air dry. The hair was subsequently subjected to mechanical testing (Young’s modulus and elongation at break) using a tensile testing machine. The results, shown in the graph below, demonstrate that the presence of hyaluronic acid in the conditioners significantly increases Young’s modulus, corresponding to an improvement in the strength of the hair fibres.

Although further studies are needed, hyaluronic acid could enhance hair strength.

Note : It is sometimes claimed that hyaluronic acid has the ability to boost hair growth or slow hair loss. However, although hyaluronic acid injections can indeed have this effect, no study suggests that topically applied hyaluronic acid can do the same.

Scalp: how does it benefit from hyaluronic acid?

Hyaluronic acid is particularly beneficial for the scalp, a region often overlooked.

Just like the skin of the face, the scalp can become dehydrated, for example following overly frequent washes with harsh shampoos. Thanks to its hygroscopic properties, the hyaluronic acid acts like a sponge to retain water within the tissues and restore the scalp’s water balance. Furthermore, when of high molecular weight, hyaluronic acid can form a film on the surface of the scalp, helping to protect it from external aggressions and offset the drying effect of certain surfactants in shampoos.

Beyond its hydrating properties, the hyaluronic acid also demonstrates notable anti-inflammatory effects for scalps prone to irritation, redness or itching. Studies in vitro have shown that this molecule can modulate the immune response by acting on certain inflammatory mediators such as interleukin-8, produced by skin cells under stress. By reducing the release of these cytokines, hyaluronic acid limits local inflammation and soothes sensations of discomfort. Moreover, its high affinity for the CD44 receptor allows it to inhibit the production of prostaglandin E2 (PGE2) and matrix metalloproteinases, which are also implicated in itching and irritation.