Vitamin A is an essential compound for a normal and healthy human function. Like other vitamins, it takes part in a variety of bodily functions including maintenance of: the immune system, good vision, teeth, bones, soft tissue membranes and skin and it even takes part in blood synthesis and brain function. However, while this is an essential ingredient for our bodies, humans do not have the capability of synthesising it and therefore our intake relies solely on our diet. Vitamin A can be found at higher levels in meat (especially liver), fish, poultry (including eggs) and dairy. However, it can also be found in various vegetables and fruits as well such as peppers, carrots, sweet potatoes, cantaloupe, apricots etc following a general rule “the brighter yellow and orange colour the higher the amount”. Nevertheless, it can also be found in non-yellow or orange vegetables such as broccoli and spinach.
Interestingly, the term vitamin A does not necessarily point to a specific molecule. Vitamin A is actually a family of compounds with different molecular structures. This family is generally divided into two sub-classes: the retinols and the carotenes. Retinols are the compounds found in animal sources whereas carotenes are found in plant based sources. Unfortunately, even though the two different sub-classes belong to the same family, they also exhibit different functions and therefore, both are needed to be consumed. Basically, don’t just eat your meat, have the veg too!
Vitamin A can be found in the human skin but especially in the epidermis (the outer layer of skin) in significant amounts which sadly decrease due to UV-exposure but possibly also naturally through ageing. Generally, carotene concentrations have been found to be always higher than the retinol, in all skin layers, with a ratio of 25:5 in the epidermis.  Interestingly, a study published by the Department of Dermatology at the University Hospital of Sweden, concluded that there is no significant difference in concentration of carotenes or retinols related to age or sex.  Although this study was based on the analysis of 22 healthy subjects’ skin (and 4 cadavers!) it is a little dated as it was published in the 80’s.
Similarly to other vitamins, retinol in its free alcohol form is unstable and therefore it is found in tissue and skin care products as its ester derivative. As a result, when you are looking out for this ingredient look for retinol, retinyl acetate or palmitate. Other generations of the retinol molecule also exist but are usually barred behind prescription only products and include: retinaldehyde, tretinoin, isotretinoin, etretinate, acitretin, arotinoid, adapalene and tazarotene. 
Similarly, carotenes, also a family of compounds, including alpha, beta and gamma-carotene are classed as pro-vitamin A molecules meaning that they get converted into vitamin A in your body by an enzyme.  As a result, their absorption and therefore, efficiency is directly dependent on how much of that enzyme your body has and by extension how much of the carotene it converts to vitamin A.
However, as explained in other posts, other than the fact that skin care products might include ingredients that are good even if they are not in an active form, the method of application is also crucial. Even though vitamin A is extremely beneficial for our bodies when taken orally, either in food or tablets, its activity might not be the same as when applied topically as a cream, gel, serum etc. This is because sometimes molecules don’t have the right structure or size to pass through our skin barrier making them less potent or even useless when used in skin care.
So is there any scientific evidence on vitamin A, whether in its retinol or carotene forms, being beneficial in skin care and if so how much does a product need to contain?
In a study published in 2000 by the Department of Dermatology at the University of Michigan, it was found that the topical application of vitamin A, specifically retinol, can indeed reverse the effects of ageing skin but the method of action is not fully understood.  It is speculated that this could be due to protection of the formed collagen from degradation (breaking down) but it could be also due to stimulation of collagen production, or both!  Whatever the method of success, the study which was based on 72 individuals of all ages (20-80+), also showed that retinol’s positive effects were the same for young or aged skin, and even for already photo-damaged skin, meaning that whatever you skin situation might be, retinol could be beneficial. 
These results were supported by other studies, including a study that only looked at individuals of 80 years old + (which let’s face it is the harshest, worst case scenario skin) and found that the application of retinol did indeed revitalise ageing skin but also visibly reduced the appearance of wrinkles.  Let me tell you, the before and after pictures in this scientific paper look very convincing! As if this wasn’t enough retinol has also been linked to increased cell renewal resulting in a more toned and brighter skin.  Other than a possible connection to collagen production, it is believed that retinoids (class of retinols) possess an anti-oxidant activity by picking up the skin damaging radicals but also activate growth factors and keratin production. [4, 6]
However, bear in mind that scientific studies are done over a period of time and so, like with any skin care product, you have to be consistent and persistent to see the full effects. The studies ranged from 4 weeks to 6 months and used a retinol concentration of 0.3-1% as opposed to the very low 0.08% or so usually found in cosmetics. [7, 8] And even though some individuals have reported skin irritation upon retinol usage, it is considered safe to use. 
Vitamin A derivatives are considered the most effective comedolytic agents, meaning they inhibit the formation of blemishes that arise from trapped oil or skin cells in pores.  For this reason, topical application of retinoids have been used to treat acne for nearly 40 years now.  A study by the University of Rio de Janeiro describes vitamin A as the gold standard for topical treatment resulting in thicker skin, which is highly beneficial as our skin becomes thinner with age.  Perhaps then it is not surprising that vitamin A has been classed, by some, as an anti-ageing ingredient. However, although there are a lot of studies showing the positive effects of vitamin A in anti-ageing functions, the vast majority of them are in vitro, meaning outside an organism (so by taking samples of skin) instead of in vivo (on an organism).  This could potentially make a difference as even retinol has to be converted to its active form, retinoic acid, and its effectiveness would obviously be directly proportional to how much of it was actually converted. And this could vary within an organism, especially with age, skin type and other factors.
Although the effectiveness of vitamin A in preventing or reversing UV-damage is not as contested as its other functions, many studies fail to address how and if photo-inactivation (light changing the structure of the molecule) affects its function and efficiency.  However, it is well established that UV-radiation decreases the amount of vitamin A in our skin  and so topical replenishment is beneficial. And although vitamin E is a lot more strongly connected to UV-protection (especially when combined to vitamin C, see post “Vitamins C & E – Do they work in skin care?”) a study based on 14 individuals alternatively taking vitamin E or vitamin A (retinol) for a few months, concluded that vitamin A was more effective in reducing shallow wrinkling caused by UV-light. 
Carotenoids (class of carotenes), on the other hand, are part of what gives colour to our skin  and can therefore also be found in some tan enhancing sun creams. Although they do not appear to have as much UV-protection activity as other molecules, they can be beneficial. Additionally, there is some evidence that skin cancer is linked to lower levels of carotenoids in the skin  implying that decrease in their concentration might cause skin cancer or perhaps it is part of the cancer mechanism to use up these molecules.
Interestingly, some studies suggest that oral supplementation of beta-carotene as a single antioxidant could lead to an imbalance of the skin’s anti-oxidant defence and result in detrimental effects.  There is a study that advises to avoid using beta-carotene alone for UV-protection  but it is based on oral supplementation not topical application and of course, you wouldn’t take vitamin A alone neither from your diet or supplements or in your skin care.
Overall, there are many studies on vitamin A but it is nowhere near as researched as vitamins C and E. Sadly, vitamin A is still used a lot more often in pharmaceuticals than cosmetics. Perhaps this is because it is not as thoroughly studied or perhaps its conversion to the biologically active form is more effective when taken orally than topically. Additionally, it is very evident that the vast majority of these studies are focused on retinol and not so much on the carotenes which are actually the ones found in higher concentrations in our skin. Perhaps this is due to the varied effectiveness of carotenes in every individual, depending on how much our skin can convert to actual vitamin A, making retinol a more direct approach. More studies are definitely needed but there is a fair amount of positive results to at the very least give this ingredient a try, especially when combined with others like vitamins C and E.
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