Ascorbic acid

Vitamin C in skincare is one of the few actives where the evidence is good and the label is nearly useless, and the gap between those two facts is worth twelve hundred words.

What the evidence actually specifies

A review of the clinical literature reports that among the available forms, “L-ascorbic acid is the most biologically active and well studied”.

It then gives three numbers, and each one is a condition on whether a product works at all.

Concentration. “In most cases, for a product to be of biological significance, it needs to have a vitamin C concentration higher than eight percent.”

A ceiling. “Studies have shown that a concentration above 20 percent does not increase its biological significance and, conversely, might cause some irritation.”

pH. L-ascorbic acid is “a hydrophilic and unstable molecule” and also “a charged molecule, which further limits its penetration”. Bringing the pH below 3.5 is described as an effective way to improve both stability and penetration.

So the working range is roughly ten to twenty percent at a pH under 3.5. Outside it, you have a product containing vitamin C that is not doing the thing vitamin C is bought for.

Why the label cannot tell you

Ingredient lists are ordered by weight down to one percent, and unordered below it. They carry no percentages and no pH. That rule is set out in the guide to reading a cosmetics label, and vitamin C is the ingredient where it bites hardest.

A serum at fifteen percent L-ascorbic acid at pH 3.2 and a serum at half a percent at pH 6 can produce the same word in the same position on two ingredient lists. One of them has the evidence behind it and the other has the name.

This is the rare case where a number printed on the front of the pack carries more information than the entire back. A product stating fifteen percent has told you something checkable. A product that only says vitamin C has told you nothing that distinguishes it from a product with a trace.

The derivatives, and the honest problem with them

L-ascorbic acid is unstable, which is inconvenient, so the industry developed esterified forms that are stable at neutral pH: magnesium ascorbyl phosphate, ascorbyl palmitate, ascorbyl glucoside and others.

They solve the formulation problem. Whether they solve the skin problem is less settled, and the review is direct about it: “one study showed that daily application of MAP, ascorbyl-6-palmitate, and other ascorbic acid derivatives did not increase the levels of L-ascorbic acid in the skin.”

That is one study and it is not the end of the matter. It is enough to say that a derivative is not automatically a gentler version of the same thing, and that the evidence base you may be relying on was built on the form that is harder to formulate.

Ferulic acid appears in some formulations for a specific reason rather than as a bonus ingredient: it helps stabilise the molecule and helps achieve the pH below 3.5.

For skin that reacts

The same acidity that makes it work is the reason it stings, and that is not an allergy. A low pH product on a compromised barrier will be felt.

Niacinamide is the usual alternative here, at a neutral pH and with a much wider tolerance. The old advice that the two cannot be used together has not held up well, but using them at different times of day sidesteps the question entirely and costs nothing.

In pregnancy, topical vitamin C is not among the ingredients guidance asks people to set aside, which puts it in a small group alongside Azelaic acid. That is an absence of concern rather than a positive recommendation, and it is a fair thing to raise with a midwife rather than settle here.

Ascorbic acid in the other two aisles

The same molecule is E300 in food, where it is used as an antioxidant to stop browning and to protect fats, and in cured meats it is used alongside Sodium nitrite because it limits the formation of unwanted reaction products.

In supplements it is plain vitamin C. Nothing about the cosmetic argument transfers: the concentration and pH questions are about getting a molecule through the stratum corneum, and swallowing it is a different problem with different answers.