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Vitamin C is one of the few skincare ingredients with a ton of independent research to back up its properties, like its brightening and anti-wrinkle benefits. Those of you who are vitamin C enthusiasts may have noticed that with some vitamin C serums, you end up with slightly stained orange-brown skin after a few days of use – sort of like fake tan. I’ve often wondered why but didn’t really dig into it past a quick Google search (which found nothing), so I just put it off as a weird side effect of vitamin C oxidising.
But recently, I came across a diagram in a peer-reviewed paper on a completely different topic that accidentally told me exactly why vitamin C does this: ascorbic acid eventually oxidises to erythrulose!
As most of you may know, vitamin C in the form of ascorbic acid oxidises easily to dehydroascorbic acid, which has an orange-brown colour. It does this when it’s stored in water, as well as on your skin. Oxygen and light exposure will speed up the oxidation reaction. This reaction is reversible, so you can get back the ascorbic acid if you have the right antioxidants in the formula.
However, dehydroascorbic acid also decomposes, and it does it irreversibly. It turns into a number of things, but for our purposes here, we only care about the fact it turns into 2,3-diketogulonic acid, which then turns into erythrulose, according to the reactions I’ve drawn below:
Erythrulose is an ingredient in fake tans, and is often included alongside the more commonly found dihydroxyacetone or DHA. These ingredients react with proteins in the dead stratum corneum layer of your skin to produce brown compounds called melanoidins that stay on your skin until the dead cells come off (after around a week). The reaction is a Maillard reaction, like the reactions that happen when meat and baked goods turn brown with heat.
Related post: The Science of How Fake Tan Works
Erythrulose is notable for giving a slightly redder tan than DHA that stains more slowly, less streakily and lasts longer. It’s included in tans from St Tropez, Bondi Sands, Jergens, Dove and Models Prefer that have DHA as the main ingredient. It’s also the sole ingredient in Deciem’s Hand Chemistry Glow Oil and Hylamide Glow Radiance Booster.
So that’s why vitamin C serums can stain – the ascorbic acid essentially turn into fake tan on your skin! Mystery solved.
How to Avoid Vitamin C Staining Your Skin
So now that we know why vitamin C serums can stain, how can we prevent this from happening? There are a few approaches:
Preventing the vitamin C from oxidising is the obvious way of keeping the erythrulose down in the first place. You can use an antioxidant-rich formulation like vitamin C + vitamin E + ferulic acid (e.g. Timeless or Paula’s Choice). What I’ve found helpful with my low-tech, budget-friendly DIY vitamin C serum is to immediately use oils and creams on top to protect the vitamin C from the air. Using it at night, away from high energy sunlight, will also help.
Related post: Water-Based Vitamin C Serums
Use oxidation-resistant forms of vitamin C
Ascorbic acid is by far the most unstable form of vitamin C and oxidises very easily. While the other forms of vitamin C aren’t as researched and require your skin to take a few extra steps before working, they also won’t oxidise anywhere near as badly on your skin. Some of these oxidation-resistant vitamin C ingredients include:
- magnesium ascorbyl phosphate (e.g. from The Ordinary)
- sodium ascorbyl phosphate
- ascorbyl tetraisopalmitate (also known as tetraisopalmitoyl ascorbic acid or ATIP, e.g. again from The Ordinary, who seem to have every variation of vitamin C, and from Moogoo)
- tetrahexyldecyl ascorbate (THDA, e.g. from Sunday Riley, Medik8)
Related post: Oil-Based Vitamin C Serums
Wash your hands after applying vitamin C serums
Fake tan ingredients stain dead skin cells, and your hands tend to have a thicker stratum corneum. This means that sometimes your hands will turn brown even if your face doesn’t noticeably stain! To avoid stained hands, just wash them with soap immediately after applying the vitamin C serum.
Apply vitamin C serum evenly over face
Some of you might not mind the colour – it can actually give you a nice rosy glow! If that’s you, then apply the serum as evenly as possible and regularly exfoliate your face so the tan fades evenly. If you notice streaking, increase the waiting time between applying vitamin C serum and your next product (try half an hour as a starting point).
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