Retinal

What is retinal?

Retinal, also known as retinaldehyde, is a precursor of retinoic acid that is produced in the body from ingested retinoids such as carotenoids and retinyl esters. Yellow and green vegetables are rich sources of carotenoids, while fish liver oil and animal fats contain retinyl esters (R).

Does retinal get absorbed, and what happens after it is absorbed?

Retinal appears to exert its biological activities only after being transformed to retinoic acid (R), so its metabolism within the skin after its absorption is as important to its efficacy as its ability to penetrate the skin.

The minimum concentration for adequate penetration and metabolism of retinal to tretinoin seems to be 0.025%, as lower doses failed to cause significant induction in the activity of a tretinoin-specific enzyme. Interestingly, retinal was found to be a better inducer of enzyme activity than retinol at a concentration of 0.025%, but at higher concentrations of 0.05% to 1%, retinal and retinol were equally effective (R).

Unsurprisingly, a 10-fold higher concentration of retinal than tretinoin is required to induce a significant increase in the activity of the enzyme (R). This is because in the skin, retinal is converted by 2 pathways. One leads to retinol and retinyl esters, which are the major metabolites and function as retinoid stores; the other to retinoic acid. The metabolism occurs in human keratinocytes, with keratinocytes undergoing differentiation oxidising retinal more efficiently (R).

An experiment in which retinal was applied topically to human skin in vitro discovered that low but significant amounts of retinoic acid were detected in the epidermis, and to a lesser extent, in the dermis (R). 0.05% retinal has also been observed to penetrate well into the epidermis of excised human skin, raising the levels of endogenous retinoids in addition to being metabolised to retinoic acid (R). These findings support the hypothesis that retinal is predominantly converted to storage forms of vitamin A, while delivering relatively small amounts of retinoic acid.

The addition of an alpha-hydroxy acid, such as glycolic acid, seems to result in better bioavailability of retinal, as the epidermal and dermal concentrations of retinal and retinoic acid were higher after treatment with retinal plus glycolic acid, as compared to treatment with retinal alone or glycolic acid alone. This leads to higher delivery of retinoic acid, which potentiates the biological activities while avoiding the side effects usually observed with high concentrations of topical tretinoin (R).