The various forms of retinoids.

Retinoids encompass vitamin A, its metabolites and its derivatives (both natural and synthetic). The retinol, theretinoic acid and the retinal are the three available forms of the vitamin A. These molecules belong to the family of retinoids of the first generation. Their history can be traced back to ancient Egypt, where beef liver compresses (containing retinol) were used to treat blindness, hence their names which derive from the "retina" of the eye.

It was in 1931 that the retinol molecule was first isolated by Swiss chemist Paul KARRER, from mackerel liver oil. The first study using tretinoin (or all-trans retinoic acid) to treat acne was published in 1943. The effectiveness of this organic compound on signs of ageing was demonstrated in the 1980 by American dermatologist Albert KLIGMAN. Retinoids are keratolytic agents that stimulate cell renewal.

Retinoids can come in various forms, each having a specific action.

• Retinol (approved for use in cosmetics)

The most popular retinoid in skincare, it is considered one of the most effective compounds to counteract the degradation of the skin's support fibres. It stimulates the production of collagen and elastin for smoother and more elastic skin. Moreover, retinol is recognised for its antioxidant properties which prevent premature ageing of skin cells. It also has an effect on the skin's melanin production, helping to reduce the appearance of brown spots that can appear with age.

Generally recommended for mature skin, it is also a prime choice for oily and/or acne-prone skin. Its keratolytic action helps to eliminate dead cells present on the surface of the epidermis. The pores are unclogged and the skin texture is refined, thus preventing the appearance of blackheads and/or comedones.

• Retinal (approved for use in cosmetics)

From a chemical perspective, retinal is an intermediate metabolite between retinol and retinoic acid. In other words, when applied to the skin, retinol is oxidised into retinal and then metabolised into retinoic acid, its active form that can effectively combat photo-ageing of the skin. However, it is prohibited in cosmetics as it is too irritating.

Thus, retinal is more directly converted into retinoic acid in the skin than retinol, which must first undergo oxidation. Retinal is relevant in skincare because to be active, its concentration can be lower than that of retinol. Indeed, retinal can have a smoothing and regenerating effect at concentrations ranging from 0.015% to 0.1%.

• Retinoid esters (permitted in cosmetics)

These compounds are typically used due to their superior chemical stability compared to retinol or retinal, but also because they are the gentlest retinoids for the skin. The most common are retinyl acetate (INCI: Retinyl acetate), retinyl propionate (INCI: Retinyl propionate), retinyl palmitate (INCI: Retinyl palmitate) and retinyl linoleate (INCI: Retinyl linoleate). They are easily identifiable by the suffixes "ate". However, retinol esters are less effective on wrinkles and acne than retinol or retinal, as they must undergo more transformation in the skin before taking the form of retinoic acid. The esters are nevertheless relevant if one has sensitive skin but still wishes to start incorporating retinoids into their beauty routine.

• Retinoic Acid (prohibited in cosmetics)

This acidic form of Vitamin A, also known as tretinoin, is among the substances banned in cosmetic products according to European Regulations due to its potential to cause irritation. This active ingredient is only available on prescription and is used in the treatment of severe acne.

Note: Regardless of the form of retinoids chosen, all are contraindicated for pregnant women and photosensitising (they increase the skin's sensitivity to UV rays).