Retinol

What is retinol?

Retinol is a form of Vitamin A that is produced in the human body from the hydrolysis of retinyl esters and from the oxidative cleavage of carotenoids to form retinal, followed by reduction to retinol (R).

Is retinol stable?

Retinol degrades rapidly in the presence of light, oxygen, metal ions, and oxidizing agents (R), but formulations have been developed to improve its stability (R, R). However, research suggests that in the epidermis, retinol-binding proteins protect retinol, from photodegradation.

Does retinol get absorbed?

Retinol is lipophilic and has a low molecular weight of 286.45 Daltons, which favours its penetration of the stratum corneum of the skin (R). The minimum concentration for adequate penetration and metabolism of retinol into retinoic acid appears to be 0.025% (R).

In an in vitro study on human skin, only 0.3% and 1.3% of retinol completely penetrated through the skin (R). Another study where 0.3% retinol was applied to forearm skin in vivo, found that although the retinol penetrated deeper into the skin over time, and some was detected in the viable epidermis, most of it remained in the stratum corneum (R). Interestingly, a roll-type massage of pig skin while applying retinol increased its concentration in the epidermis and the dermis 4 hours later (R).

The skin penetration enhancers propylene glycol and oleic acid have been shown to improve the permeation of retinol into human skin (R, R). The use of nanoparticle-coated emulsions and microencapsulation also improved the dermal delivery of retinol (R), R).

What happens after retinol is absorbed?

Most of the retinol that is applied to the skin is not metabolised (R). Of the retinol that does get metabolised, the main metabolites are retinyl esters, with only a small, sometimes undetectable, proportion being converted to retinoic acid (R, R, R).