Vitamin C: which form, and why ours does not sting.

Vitamin C matters more in this decade than it did in the last, and the reason is structural rather than cosmetic. Your body cannot build collagen without it. The enzymes that hydroxylate and cross-link collagen use vitamin C as a cofactor, which means it is not an optional brightener sitting on top of the process; it is a required part of it. Collagen falls steeply through the transition, with some estimates putting the loss near a third in the first five years after menopause. That is the context in which this ingredient earns its place.

Now the difficulty nobody selling it likes to explain. The most-studied form, L-ascorbic acid, is also the most temperamental molecule in mainstream skincare. It only penetrates reliably at a low pH, around 3.5 or below, which is acidic enough to sting. It oxidises on contact with air, light and warmth, turning yellow and then brown. And a browned serum is not weaker, it is finished; you are applying the oxidation products of a vitamin rather than the vitamin.

“If your vitamin C has gone the colour of weak tea, it is not past its best. It is past.”

For a woman at 47 there is a further problem, and it is the one that made our decision for us. A thinning barrier is exactly the wrong surface for a pH-3 acid. The sting that a thirty-year-old shrugs off becomes days of redness on skin that has started reacting to its own cleanser. I watched client after client buy the well-reviewed L-ascorbic serum, use it for ten days, and abandon it.

So we use tetrahexyldecyl ascorbate, THD ascorbate, at 10%. It is an oil-soluble, stabilised derivative: it does not need an acidic base, so it does not sting, it does not oxidise in the bottle the way L-ascorbic acid does, and being oil-soluble it moves through the skin's lipid layers rather than fighting them. Being honest about the trade, because there is one: the published evidence behind L-ascorbic acid is larger and older. THD has good supporting work and a far better tolerability profile, and the serum you actually apply every morning for six months beats the theoretically stronger one abandoned in a drawer in week two.

How to use it well. Mornings, on clean skin, before sunscreen. Vitamin C is an antioxidant, which means it mops up some of the free radical damage that ultraviolet light causes, and that makes it a partner to sunscreen rather than a substitute for it. Anyone selling it as sun protection is selling you something. Store it out of the bathroom window's light, keep the cap on properly, and expect a good serum to last the bottle rather than a fortnight.

What to expect, and when. Tone evens before anything else does: eight to twelve weeks for most women, longer for stubborn pigmentation, which needs sunscreen discipline more than it needs a serum. Fine texture follows. What it will not do is fill a line or replace the collagen the decade has taken. It supports the machinery that makes collagen, which is a real and worthwhile thing, and a smaller thing than the advertising suggests.

One myth to put down while we are here, because it costs people a good routine: you can absolutely use vitamin C and niacinamide together. That story has a long tail and a thin origin, and I have written out where it came from in the niacinamide piece. Layer them.

Davina Hearne, Naturopath (Diploma, NZ) · Founder, Valentia

Educational only. Always speak to your healthcare provider about your symptoms and before starting a new supplement.