Vitamin A–Retinoids

The essential role of retinoids in the normal development and keratinization of skin dates back to 1925 when Wolbach and Howe described abnormal keratinization in vitamin-Adeficient enzymes. The topical use of retinoids has been more extensively studied than any other compound in dermatology. Retinoids play an important role in skin development and regulate the growth and differentiation of keratinocytes [44]. The ability of topical retinoids to reverse photoaging as well as chronologic aging makes the use of a retinoid a staple in any cosmetic regimen [45, 46].

Topical vitamin A has the ability to diminish the signs of aging by decreasing fine lines and wrinkling. In addition, there is a normalization and enhancement of elasticity [47]. Improvement of skin tone and texture is a benefit of vitamin A, which enhances skin lightening when used in conjunction with skin lighteners.

Vitamin A is a lipid-soluble molecule whose structure has been amenable to formulation variability resulting in several structurally different forms. The first generation, or nonaromatic form, includes all-trans retinoic acid (tretinoin) and its 9-cis-isomer and 13-cis-isomer (isotretinoin) forms [44, 48]. The other topical forms are polyaromatic isomers of retinoic acid, or arotinoids, which include adapalene and tazarotene [49]. Retinoic acid regulates the growth, differentiation, and normalization of skin by recognizing and binding to specific retinoic acid receptors (RARs) and retinoic X receptors (RXRs), which are ligand-activated transcription factors. These receptors bind to regulatory regions of DNA where they activate gene transcription.Within each receptor family there exist subtypes and multiple isoforms belonging to each subtype.


Vitamin A derivatives used cosmetically in nonprescription products include retinol, retinaldehyde, retinyl palmitate, and retinyl esters. Retinol and retinaldehyde, an intermediate product in the conversion of retinol to alltrans retinoic acid, are natural forms of vitamin A [44]. The most common nonprescription forms used are retinol and retinyl palmitate. In order to be of any benefit, each must convert to the all-trans retinoic acid form in order to bind ligands. Retinol has been shown to penetrate the skin more effectively than retinaldehyde, retinyl palmitate, and even retinoic acid [50]. This was determined by measuring 4-hydroxylase activity,which is directly related to the level of all-trans retinoic acid in the skin [51]. Since each of the vitamin A derivatives must bioconvert to all-trans retinoic acid, the 4-hydroxylase level has become a marker of the bioavailability and effectiveness of these ingredients.

Retinol is the most abundant form of vitamin A in the skin; however, it is extremely sensitive to light and air during formulation in skin care. If retinol is not handled in the correct conditions, it will quickly oxidize, and an irritating, harmful product will result.