Okay, the Bugs Bunny part is a stretch, but not the relationship between carrots and skin care. And it has nothing to do with the ancient “root” word for carrot, ker, which translates into the word “horn”, which a carrot resembles. Nor is it because ker is also the basis of the word keratin, which is the protein that makes up animal horns, hoofs, claws, and fur – as well as our own hair, fingernails, and is the major protein in the keratinocytes of the stratum corneum layer of our epidermis. There is a different reason.
Actually, carrots are high in β-carotene (itself named after carrots) and something found in other fruits and vegetables and responsible for the orange color of carrots. Here comes the hook: β-carotene is composed of two retinyl groups which are converted in the small intestine into two molecules of retinal, a form of Vitamin A.
Turns out that is a true statement. It was discovered long ago that vitamin A deficiency is one of the causes of night blindness, in this case because of an abnormality in the functioning of the cells of the retina. It was the search for the cause of night blindness that led to the discovery of vitamin A. (Other non-nutritional causes include severe myopia, glaucoma medications that constrict the pupil, cataracts, and retinitis pigmentosa.)
There are, in fact, many “forms” of Vitamin A , collectively known as retinoids, including retinal, retinol, retinaldyhyde, retinyl palmitate, and retinoic acid. All have retina as their root word and enzymes within the body, including in the skin, can transition one compound into another. Retinoic acid is the “active” form and an important hormone-like growth factor for epithelial (e.g. skin) and other cells.
Not surprisingly, vitamin A has many functions besides being absolutely necessary for vision, including gene transcription, immune function, embryogenesis, bone metabolism, and blood cell production. Too little or too much causes problems.
Vitamin A deficiency not only leads to vision issues, it is also the cause of skin disorders in which too much keratin is synthesized. In decades past, oral administration of vitamin A was used to treat such conditions but with only marginal success. Instead, cumulative vitamin A overdosing was found to carry risks of bone, liver, and neurologic injury and congenital malformations (more on the last issue later.)
Because vitamin A is a fat soluble vitamin, and the epidermis is lipophilic (“fat loving”), vitamin A is able to penetrate the stratum corneum and exert effect in deeper layers. As opposed to many other vitamins, vitamin A as found in the diet does not have much direct biological effect but works instead via the active metabolite (biochemical derivative) retinoic acid.
Retinoic acid has direct effect on skin cells through modulation of gene expression within the interior of the cell, in effect, “adjusting” rates of cellular differentiation and proliferation resulting in a more youthful looking state.
Retinoic acid, also known as tretinoin, is the active ingredient in Retin A, and is clinically proved to favorably impact skin in the following ways:
- Reduction of both wrinkles and fine lines
- Increases collagen production
- Smoothes skin texture
- Increases skin thickness
- Improves elasticity
- Diminishes acne
- Improves overall skin tone
- Diminishes mottled pigmentation
- Increases skin hydration
- Stimulates skin repair
- Decreases enlarged pores
Tretinoin can be drying and increase sensitivity to sunlight. More sensitive people may experience redness, scaling, itching, and burning. Physicians recommend gradually increasing use to allow acclimating to the drug over time.
Isotretinoin is the prescription drug Accutane, and oral formulation used to control severe cystic acne vulgaris. It is a powerful drug with potential to cause a variety of birth defects. For that reason, its use is strictly limited and females taking the drug are instructed to use two means of contraception one month before, during, and one month after discontinuing its use.
The related metabolic pathway among these related vitamin A compounds is below:
Retinyl palmitate <=> Retinol <=> Retinaldehyde => Retinoic acid
Retinol and retinyl palmitate are among the most widely used active ingredients in skin care products. Both are forms of vitamin A that can be converted into retinoic acid by enzymes found within the skin. When sufficient amounts of retinol and retinyl palmitate are added to skin cell culture, the amount of retinoic acid in the cells increases. Therefore, at least in theory, topical vitamin A may deliver at least some of the well-established skin benefits of retinoic acid while producing fewer side effects.
Numerous products with retinol and retinyl palmitate are touted as equivalent to retinoic acid in effectiveness yet devoid of its side effects. The reality is a bit more complicated. Many of these products contain too little retinol / retinyl palmitate to have any noticeable effect. The fact that these agents are in the list of ingredients is not enough. Products with high concentrations do exist but may still not deliver the purported benefits for a number of reasons. In particular, a product with highly concentrated retinol may cause skin irritation, especially in people with sensitive skin. Highly concentrated retinyl palmitate is less irritating than retinol (at equivalent levels) but is also less effective. (After all retinyl palmitate is the farthest away from retinoic acid in the metabolic pathway.) Furthermore, conversion rates of various forms of vitamin A to retinoic acid vary among individuals – the same product/concentration may yield visible benefits in some people and little or none in others. Further still, retinol, and to a lesser degree retinyl palmitate, can be degraded by oxidation if formulated, stored and/or used improperly.
Among retinoids precursors, retinaldehyde is the nearest (on the metabolic pathway) to retinoic acid and is likely to match its benefits more closely. However, well-designed products containing retinol and/or retinyl palmitate provide an additional alternative that may be cheaper and, at least for some people, less irritating.
Aside from irritation, drying, and photosensitivity, there remains an open question as to the possible carcinogenic potential of retinyl palmitate. A recent study made public last January (by an interagency group within the NIH) demonstrated that retinyl palmitate speeds photo-carcinogenic effects on hairless mice. The study, entitled Photococarcinogenesis Study Of Retinoic Acid And Retinyl Palmitate and conducted at a federal research center found that animals treated with small doses of retinyl palmitate and ultraviolet light developed skin tumors faster than untreated, light-exposed mice or those treated only with a control cream. To date, however, there is no prohibition against use of retinyl palmitate.
Retinol has been used in topical application for more than 25 years without evidence of increased incidence of carcinoma. In fact, a 1997 study by Moon et al in Cancer Epidemiology showed oral administration of retinol was preventative of squamous cell carcinoma but not basal cell carcinoma.
Overall, the evidence is as yet not clearly conclusive. What many dermatologists recommend is use of retinoid containing skin products along with sun protection if used during the day or, more preferable, use of the products during the nighttime hours.