Vitamin C is an antioxidant that we ingest through our diet and one of the most widely used skin care treatments due to its properties to combat premature skin ageing.
Fruits and vegetables are the main sources of this vitamin: kiwis are one of the fruits with the highest Vitamin C content, followed by strawberries and oranges, among others. Eating a diet rich in this vitamin does not interfere with the levels of Vitamin C in the skin cells, but for it to exert its antioxidant function for the skin, it must be administered topically through the use of cosmetics.
Ascorbic acid (pure vitamin C) is one of the most potent antioxidants, provided it is formulated under the right conditions. The biggest problem with this ingredient is its instability in formulation, as it oxidises and is photosensitive: it degrades in the presence of oxygen and solar radiation. To avoid this, it is not enough to use hermetic, opaque packaging; there are alternative ingredients derived from ascorbic acid that are more stable. Also, the combination of Vitamin C with other antioxidants such as Vitamin E and ferulic acid has been shown to be effective in slowing down the possible oxidation it may undergo.
What is the role of Vitamin C as an antioxidant for the skin?
Vitamin C and its role in premature skin ageing
Vitamin C acts as a cofactor in the synthesis reaction of collagen fibres, the main components of the dermis giving the skin turgidity and support. Collagen is formed and destroyed naturally and constantly, in a more or less balanced ratio. As we age, the rate at which collagen is formed slows down compared to the rate at which it is destroyed, which is why Vitamin C is the gold standard in anti-ageing cosmetics.
Vitamin C also has a high antioxidant power, which means that it interferes with the formation of free radicals, neutralising them and thus preventing them from causing cell damage. This functionality gives it the name “scavenger”. Ageing, genetic factors, diet and lifestyle play all a role in the formation of radicals, which consequently affect the various organs of the body, including the skin. Using cosmetics with antioxidants for the skin, such as vitamin C, reduces the harmful effects of these naturally formed free radicals.
Vitamin C and its role in dark spot formation
Vitamin C, as a consequence of being a powerful antioxidant for the skin, interferes with the melanogenesis reaction, reducing the synthesis of melanin, the pigment responsible for colouring the skin. Vitamin C therefore helps to reduce the appearance of hyperpigmentation common in ageing skin and in hormonal changes during pregnancy.
Not only does it reduce the formation of melanin, but applying Vitamin C to the skin under the effect of sun exposure also regulates melanin production. On the other hand, applying it after sun exposure reduces the erythema or redness that this exposure causes, so it has a photoprotective effect on the skin.
For all these reasons, Vitamin C is commonly found in products that claim to have an illuminating effect on the skin, to improve radiance, and to combat the signs of skin ageing.
Different forms of stabilised Vitamin C
Because vitamin C is not very stable in ordinary cosmetic formulations, it is usually recommended to use stabilised forms of the molecule. These are esters of ascorbic acid that are stable against oxidation by both oxygen and light, and are better tolerated by the skin because they must be formulated at pHs closer to the physiological pH of the skin. Some of these stabilised forms of Vitamin C are: 3-O-Ethyl ascorbic acid, Ascorbyl glucoside, Aminopropyl ascorbyl phosphate (MAP) or Sodium ascorbyl phosphate (SAP).
- 3-O-Ethyl Ascorbic Acid (INCI: Ethyl Ascorbic Acid): It is a pH 4.0 to 5.5 stable ascorbic acid derivative. It contains a high concentration of ascorbic acid compared to other Vitamin C derivatives (86%), and is known to have good skin penetration.
- Ascorbyl glucoside (INCI: Ascorbyl Glucoside): It is a pH 5.0 to 8.0 stable ascorbic acid derivative. When applied topically, it is hydrolysed by the endogenous α-glucosidase enzyme, which converts it into the active form of ascorbic acid.
Recommendations when formulating with Vitamin C as an antioxidant for the skin
The importance of the pH of the formula
The highest efficacy is obtained depending on the pH of the formulation:
- For ascorbic acid: the formula should have a pH of around 3.5. At a pH below 4, ascorbic acid loses its ionic charge, which allows it to pass through the stratum corneum of the epidermis more easily.
- For 3-o-Ethyl ascorbic acid: the formula should have a pH around 4.5.
- For ascorbyl glucoside: the formula should have a pH around 6.
Not all concentrations are valid
In addition to pH it is important to formulate the cosmetic with an adequate concentration of Vitamin C in order to be effective. It is estimated that to demonstrate maximum antioxidant efficacy on the skin, vitamin C should be at a concentration between 5 and 15% in a cosmetic. It is not advisable to exceed the 15% concentration, as pure Vitamin C could cause skin irritation since it is formulated at an acidic pH.
The format of the cosmetic affects its efficacy as an antioxidant for the skin.
The type of packaging also interferes with the efficacy of the cosmetic, as opaque packaging minimises the possible entry of light and therefore prevents oxidation. Therefore, single-dose ampoule-type cosmetic formats are very common for high concentrations of these active ingredients. In this case, the contact of Vitamin C in the cosmetic with the air is minimised, as once opened, it is applied to the skin. Lately, powder formats have been very well welcomed by the consumer, as there is no waste of water in their manufacture. In addition to the environmental benefits of using powder formulas, in this case they are favourable to prevent Vitamin C from coming into contact with the water in the formulation itself and oxidising.
Myths about Vitamin C
Vitamin C stains skin
Pure vitamin C is photosensitive and unstable in the sun, so if a cosmetic with a high concentration of vitamin C is applied, it can accumulate in the pores and, when oxidised, acquire a dark colour. To prevent the pores from becoming dark, it is important to maintain proper daily skin care and not to use high concentrations of Vitamin C in excess. However, in no case does vitamin C cause stains on the skin. On the contrary, this vitamin reduces the appearance of spots by intervening in the synthesis of skin pigment.
A cosmetic product with yellow or orange Vitamin C means that it contains a high concentration of this vitamin.
When the Vitamin C formula changes colour and becomes dark (in creams, from white to yellow, or in solutions, from transparent to orange or brown), it is an indication that the Vitamin C has been degraded and therefore loses its antioxidant effectiveness for the skin.
Ascorbic acid is metabolised to dehydroascorbic acid and then to erythrulose. Erythrulose is a dark-coloured compound and is used in some self-tanning product formulations to darken skin tone. Therefore, if the product changes colour, it means that the ascorbic acid has already been converted to erythrulose in the formula before it comes into contact with the skin, thus losing its efficacy.
Vitamin C should not be used in daytime cosmetics or in summer.
Because Vitamin C is not photosensitising, it can be used both day and night, without interfering with its effectiveness.
Moreover, Vitamin C cosmetics can be used in any season of the year. In summer it is especially important for neutralising free radicals that are formed in the skin due to sun exposure.