Submitted via the FutureDerm.com Facebook page:
Would you continue to use your Vitamin C serum if it has turned brown? Skinceuticals says it’s still okay to use it on the package, but if the serum has oxidized, how can it work as an antioxidant? Might it actually be harmful?
At best, using antioxidant creams after they have turned brown is useless. At worst, using such creams is potentially harmful to your skin – but not for the reasons you may think.
Antioxidants work by scavenging loose electrons called free radicals before they can do damage in the system. Vitamin C (as L-ascorbic acid) in particular has been shown to have multiple benefits, including increasing UV protection when applied prior to exposure, and reducing UV damage after exposure (American Academy of Dermatology, 2002). Vitamin C is also essential for the body’s production of collagen. It also lightens skin and increases barrier function when applied topically.
If not stabilized, vitamin C and other antioxidants can become oxidized upon prolonged exposure to light or air, turning brown. Each molecule of vitamin C contains two electrons available for use. When the first is used, the resulting molecule actually becomes more stable than other free radicals and can serve as a free-radical scavenger. After loss of a second electron, the resulting oxidation product, dehydroascorbic acid, can be regenerated or may decay (Journal of the American Academy of Dermatology, 2003).
If twice-oxidized vitamin C (dehydroascorbic acid) is applied topically to the skin, the skin cells will reduce some of the compound back to vitamin C (Journal of Biological Chemistry, 1995). However, some of dehydroascorbic acid will also decay. Chemists know decay has occurred when the molecule’s lactone ring irreversibly opens. I have yet to encounter a study demonstrating application of decayed dehydroascorbic acid has any negative effect.
The rumor pro-oxidant vitamin C becomes damaging to the skin comes from the idea antioxidants absorb free radicals; after a certain point, they must be supersaturated and release free radicals instead. However, when vitamin C absorbs free radicals, it becomes dehydroascorbic acid, and then decays.
Vitamin C has only been shown to act as a pro-oxidant when overdoses are orally ingested in supplement form. Even then, pro-oxidant vitamin C has been shown to have positive effects, helping to create nitric oxide, which relaxes the blood vessels (Cardiovascular Research, 2005).
So, after careful thought and evaluation, I must conclude there is not much evidence to support vitamin C cream causes oxidative damage. I must admit this is a myth I once believed. However, I still do not support the use of “brown” or darkened vitamin C cream, for the following reasons:
- Decreased concentration of vitamin C. Up to 15% L-ascorbic acid is absorbed by the skin (Journal of the American Academy of Dermatology, 2003). If you apply a 15% L-ascorbic acid solution to your skin, and some has visibly decayed, you are not getting the optimal level of benefit.
- pH. Vitamin C is best absorbed at an acidic pH. The more active vitamin C in the solution, the more acidic the pH will be. Conversely, the more decayed vitamin C becomes, the more basic the pH will become, the less effect will occur.
Hope this helps,
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