Vitamin C is one of the few skincare ingredients with a decent amount of independent research to back up its properties, like its brightening and anti-wrinkle benefits.
But those of you who are vitamin C enthusiasts may have noticed that with some vitamin C serums that contain ascorbic acid, you end up with slightly darkened skin after a few days of use. The stained skin looks red or orange-brown, sort of like fake tan. It can happen on your face, as well as your hands and fingernails. It can even stain your hair orange!
I’ve often wondered why this happens, but I 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!
Ascorbic acid oxidation
As you may already know, vitamin C in the form of ascorbic acid (specifically L-ascorbic acid) oxidises easily. This is when it decomposes and turns into a number of other substances that likely have much less benefits for skin, and it gradually darkens in colour during this process.
Ascorbic acid oxidises in products, especially when it’s in the presence of water, as well as on your skin. Oxygen and light exposure will also speed up the oxidation reaction.
The first step in the oxidation process is when the ascorbic acid is converted to dehydroascorbic acid. 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 this process is irreversible. It turns into a number of different substances, 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 work by reacting with proteins in the dead stratum corneum layer of your skin to produce brown compounds called melanoidins. These melanoidins stay on your skin and give it a brown colour, until the dead cells slough off (after around a week).
The reaction is a Maillard reaction, similar to 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. It stains more slowly, less streakily and lasts longer.
It’s included in tans from Isle of Paradise, St Tropez, Tan-Luxe, Jergens and Dove that have DHA as the main ingredient. It’s also the sole ingredient in Deciem’s 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! And you’ll notice that it has that signature soy sauce-like fake tan smell too.
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 levels down in the first place.
You can use an antioxidant-rich formulation like the classic trio of vitamin C, vitamin E and ferulic acid (e.g. Skinceuticals, Timeless or Paula’s Choice). These extra antioxidants can act to undo the initial, reversible step of the oxidation process (ascorbic acid → dehydroascorbic acid) both in the bottle and on your skin.
After applying any ascorbic acid product (like my low-tech, budget-friendly DIY vitamin C serum), it can be helpful to immediately use oils and creams on top, to try to protect the vitamin C from the air until it absorbs into the skin.
Using vitamin C at night, away from high energy sunlight, will also help prevent accelerated oxidation.
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. However, it’s the best researched form vitamin C ingredient in terms of its benefits on the skin.
The other forms of vitamin C (vitamin C derivatives) aren’t as well researched, and require your skin to take a few extra steps to convert it to an active form before working, but they have one big advantage – they won’t oxidise anywhere near as badly on your skin, and should cause much less staining!
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)
- tetrahexyldecyl ascorbate (THDA, which seems to be the same as ATIP but with a different name for some reason e.g. from Sunday Riley, Medik8)
Related post: Vitamin C Esters: Are ATIP and THDA the Same Thing?
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 than your face, particularly around your nails. This means that sometimes your hands will turn brown even if your face doesn’t noticeably stain!
Your nails also contain keratin, which can also participate in these browning reactions.
To avoid stained hands and nails, just wash them with soap immediately after applying the vitamin C serum. And make sure you scrub them thoroughly!
Apply vitamin C serum evenly over face
Some of you might not mind the colour – it can actually give you a nice rosy glow, which is why a lot of people use fake tan!
If that’s you, then just apply the serum as evenly as possible. Also make sure that you regularly exfoliate your face so the dead skin layer is even and the tan stains uniformally, and fades evenly.
If you notice streaking, it might be due to a later product moving the erythrulose around on your skin. You should be able to reduce the chances of this happening by increasing the waiting time between applying vitamin C serum and your next product – try half an hour as a starting point.
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