Retinyl Palmitate

What is retinyl palmitate?

Retinyl palmitate, an ester of retinol and palmitic acid, is the predominant component of retinyl esters (R), the major storage forms of vitamin A in animal tissues (R, R, R). In human skin, retinyl esters account for over 70% of endogenous vitamin A (R).

The epidermis contains approximately 3.5 times more retinyl esters (including retinyl palmitate) than the dermis (R). The amount of retinyl palmitate in the skin may decline with age however. In an experiment on female mice, age-related effects were observed in the stratum corneum, epidermis and dermis, with the level of retinyl palmitate being the highest in the epidermis of mice that were 5 months old, but decreasing by about 4-fold in mice that were between 15 to 17 months old (R).

Retinyl palmitate is also widely used in cosmetics. In 2008, there were 1778 cosmetic products containing retinyl palmitate in the US FDA's Voluntary Cosmetic Registration Program, compared to 160 products containing retinol, 28 products containing retinyl acetate and 3 products containing retinoic acid (R). Its concentration in cosmetic products ranges from 0.1% to 10%, but is generally not more than 1% (R, R).

Is retinyl palmitate stable?

Retinyl palmitate is more thermally stable than retinol (R), but it decomposes in sunlight, potentially forming toxic products associated with photocarcinogenesis (R). This instability can also affect the physical integrity of the cosmetic formulation (R).

Fortunately, the photostability of retinyl palmitate can be improved by adding vitamins C and E (R), butylated hydroxytoluene (BHT) (R, R), pectin (R) and certain sunscreens (R, R, R). Encapsulating retinyl palmitate in a nanoemulgel (R), microcapsules (R) or nanocapsules (R, R) also enhances its stability.

Indeed, a study of commercial cosmetic products containing retinoids found that between 20% to 100% of retinyl palmitate remained after 6 months of storage at 25°C, highlighting the importance of the formulation. In addition, the results for retinyl palmitate were similar to those of retinol, but worse than those of hydroxypinacolone retinoate, a newer retinoid which showed much higher thermal and chemical stability (R).

Interestingly, although retinyl esters in the epidermis degrade rapidly after exposure to sunlight, the same is not true of retinyl esters in the dermis (R).

Does retinyl palmitate get absorbed?

The percutaneous absorption of retinyl palmitate is well-established; it diffuses rapidly into the stratum corneum and epidermis after application (R, R, R). In one study, 18% of topically applied retinyl palmitate was absorbed into human skin from an acetone vehicle (R).

The combined use of pectin and ascorbyl palmitate increases the amount of retinyl palmitate deposited in the epidermis substantially, presumably through their antioxidative effect (R). Encapsulating retinyl palmitate in micelles (R) or liquid crystal emulsions (R) can also help. The addition of glycolic acid on the other hand may not increase the total amount of retinyl palmitate that penetrates into the skin, although it seems to make permeation faster (R).

Interestingly, retinyl palmitate can also be used as the core of polymeric nanocapsules to deliver other active ingredients into deep layers of the skin (R, R).

What happens after retinyl palmitate is absorbed?

To be pharmalogically active, retinyl palmitate must be converted to retinol by cleaving the ester linkage followed by conversion to tretinoin via oxidative processes (R). There is evidence that this does happen - in an experiment on human skin, 44% of the absorbed retinyl palmitate was hydrolysed by esterases in the skin to retinol (R). While no other metabolites were detected, retinol is known to be metabolised to tretinoin in small amounts in the epidermis and dermis (R). Because of the multiple conversion steps required, higher concentrations of retinyl palmitate as compared to retinol or retinal are required in cosmetic formulations to produce similar cellular and molecular changes in the skin (R).

Some of the absorbed retinyl palmitate is likely stored in the skin, as topical application of retinyl palmitate has been shown to alter the physiological levels of endogenous retinoids in mice and human skin (R, R, R). Cultured human fibroblasts have also demonstrated a remarkable uptake and storage of retinyl palmitate in vitro (R).