One of the first associations we associate with colds, flu, and immunity is vitamin C. Although half a century has passed since Pauling’s book on the effects of vitamin C on colds, every now and then a “researcher” appears who will readily declare it a cure for everything from colds. to cancer.

Some research indicates insufficient bioavailability of vitamin C in the skin after oral administration, and it is known that the human body cannot synthesize it due to enzyme deficiencies, so topical application is imposed in response. The effectiveness of vitamin C is, therefore, already etched in our subconscious, the marketing potential is huge, so there is almost no more well-known name in the cosmetics industry that does not have a serum or a similar product with this ingredient in its range. Investing in studies that would disprove the effectiveness of vitamin C is simply not logical from a marketing and profit standpoint.

The potential harmfulness of topical application of vitamin C would be reflected in its pro-oxidative action in reaction with metal ions such as iron or copper (Fenton’s reaction), whose presence on the skin may be conditional on environmental pollution and other cosmetic products (eg make-up). The result is possible to skin irritation and deterioration of collagen and elastin fibers. Numerous studies confirm the opposite, the action of vitamin C as an antioxidant that suppresses free radicals, which are formed under the action of UV rays or as a side effect of normal skin metabolism. Vitamin C is also needed as a cofactor for various enzymes, including prolyl hydroxylase and lysyl hydroxylase involved in collagen biosynthesis.

The number of clinical studies has shown increased collagen synthesis after vitamin C use. Furthermore, vitamin c inhibits the enzyme tyrosinase, which prevents the occurrence and intensity of hyperpigmentation. Articles about tachyphylaxis, ie a kind of tolerance to vitamin C, can also be found on the Internet. The definition of tachyphylaxis is a reduced response of the organism to a particular drug if it is re-administered after a short time. It occurs due to the consumption of the transmitter or mediator through which the drug acts, or due to the desensitization of the receptor on which the drug acts.

You’ve probably met individuals who have never used anything all their life except soap and water and possibly Nivea, and are content and proud of their complexion. Well, in translation, tachyphylaxis means that in case you think of using vitamin C in adulthood in addition to water and soap and possibly Nivea, it will not be effective, because you started using it early. Given the mechanism of action of vitamin C and the available data, this claim is scientifically unfounded. But even in the case of tachyphylaxis, this phenomenon would probably be reversible, or in other words, after a short break, vitamin C could be used with equal efficiency. Some studies suggest the anti-inflammatory effect of topical vitamin C (indirect inhibition of the release of pro-inflammatory cytokines) and its use in skin conditions such as rosacea and acne, as well as promoting wound healing and preventing progressive hyperpigmentation. According to available data, vitamin c is relatively safe to use, ie only in rare cases, in individuals with hypersensitive skin, more serious irritations are possible in terms of redness, burning, and dryness. However, caution should be exercised when applying to the eye area.

The active form of vitamin C-ascorbic acid is unstable, especially under the influence of light, so manufacturers are continuously looking for chemically stable and physiologically active forms and packaging that allow the best efficiency. One of the questions that is often asked is the concentration of vitamin C in the serum.

According to the available data, the effectiveness is proportional to the concentration of up to approximately 20%, but in addition, the form in which vitamin c is present in the product should be taken into account, as it depends on whether it will reach the skin in adequate quantities and/or undergo conversion into a physiologically active form. The optimal concentration, therefore, depends on the form, but in most cases, a concentration higher than 8% is necessary for noticeable efficacy. Concentrations above 20% do not show an increase in effectiveness, but the possibility of skin irritation increases, so most reputable manufacturers use concentrations in the range of 10 – 25%.

L ascorbic acid is probably the most studied and biologically active form. It is a hydrophilic, unstable molecule, which, due to the hydrophobic character of the stratum corneum and the charge of the molecule itself, penetrates poorly. By lowering the pH below 3.5, better stability and penetration is achieved. The addition of ferulic acid lowers the pH and increases the antioxidant capacity of the product.

I note that those with sensitive skin that reacts to acids first test the action of the product on a small piece of skin. Many manufacturers, therefore, use more stable, “non-acidic”, lipophilic and esterified forms of vitamin C that are stable at higher, neutral pH values. Some of them are magnesium ascorbyl phosphate, sodium ascorbyl phosphate, ascorbyl tetraisopalmitate, ascorbyl palmitate, ascorbyl glucoside, ascorbyl methylsilanol pectinate…. In these forms, therefore, the question of the rate of penetration and the degree of conversion to ascorbic acid remains, with each manufacturer emphasizing the comparative advantages of precisely the form it uses in its product. There are no unambiguous studies that would declare some form the best for now, at least as far as I know.

Finally, I can conclude that vitamin C is one of the most studied and relatively safe ingredients for application to the skin, which will justify its role if our expectations are reasonable and if we follow the recommendations of dermatologists and manufacturers.