Vitamin C - why it can stain the skin and how to apply it according to a doctor and analyses by BeautyDecoders.

Why can a serum skin staining ?

Pure vitamin C (ascorbic acid) is easily oxidized by light and oxygen. To stabilize it, manufacturers add antioxidants, opaque, hermetically sealed bottles, acidic pH formulas and sometimes denatured alcohol to facilitate absorption, at the cost of additional irritation.

But once on the skin, ascorbic acid is once again exposed to light and air: it can be rapidly denatured. Once oxidized, it undergoes a cascade of chemical reactions. According to a 2011 article on vitamin C degradation products, oxidized ascorbic acid becomes dehydroascorbic acid, then 2,3-diketogulonic acid, which turns into erythrulose. Erythrulose is precisely one of the ingredients in self-tanners: on contact with the skin, it reacts with amino acids in the stratum corneum to form pigments (melanoidins), which color the dead cells of the epidermis until they are naturally shed. Hence those orange or brown areas.

Always finish with a sun cream in the morning (or a moisturizer in the evening) after your vitamin C. This prevents contact with the air and protects the skin and vitamin C from UV rays. Wait until the vitamin C is well absorbed before applying the next skin care product: the aim is to avoid oxidation. Once absorbed, vitamin C becomes a formidable antioxidant.

Apply just after cleansing to enhance absorption, then follow with lighter textures to richer ones (lotion, serum, cream). pH is crucial: a study by Dr Pinnell has shown that it should be below 3.5 for optimum penetration and better stability. The presence of alcohol can improve penetration, and other antioxidants help stabilize the formula, which leads to the next tip.

Prefer serums combining other antioxidants. The best combination is vitamin C + vitamin E + ferulic acid: the antioxidant activity of ascorbic acid is multiplied in synergy with vitamin E. Together, they limit the damage caused by exposure to the sun, provided sun protection is always used. For examples of well-formulated serums, see the selection of best vitamin C serums.

Buy your pure vitamin C in an opaque, airtight container, ideally stored in a cool place. It's easy to recognize an oxidized serum: if it takes on a yellow-orange color, it should be discarded. Tip: order your serum online rather than on the shelf, to avoid it having been stored for too long in a warehouse or boutique.

Some active ingredients do not mix well with pure vitamin C. For the retinol, As retinol is also an irritant, never start with both at the same time: retinol first, then vitamin C once it's well tolerated. You should also avoid using copper peptides at the same time as vitamin C.

And if you have the sensitive skin ?

Pure vitamin C isn't for everyone: in the case of sensitive skin or redness, it's best to opt for a derivative. Less potent than pure vitamin C, derivatives have the advantage of being highly stable (no risk of oxidation), water- or fat-soluble, depending on your skin type, and easy to combine with other active ingredients, including retinol. For details and examples, see the page on vitamin C derivatives.

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