Vitamin C

Vitamin C is the most studied antioxidant in skincare and the most difficult to formulate. Every derivative on this list exists to solve problems created by the pure form, and each solves them at a cost. Understanding that trade-off is most of what you need to compare products.

What vitamin C does

Three well-documented roles.

Antioxidant. It operates in the water phase, donating electrons to neutralise free radicals generated by UV and pollution. It also regenerates vitamin E after oxidation, which is why the two are so often formulated together.

Collagen synthesis. Ascorbic acid is an essential cofactor for prolyl and lysyl hydroxylase, the enzymes that stabilise collagen during formation. Without adequate vitamin C, collagen cannot form properly. This is basic biochemistry rather than a cosmetic claim, and it is the origin of scurvy.

Pigmentation. It interacts with tyrosinase, the enzyme that initiates melanin production.

The pure form

Ascorbic acid is vitamin C as the body uses it. Nothing needs converting, so it is active on application.

Its problems are equally direct. It oxidises readily in water, in air, in light and with warmth, degrading first to dehydroascorbic acid and then further, turning yellow and then brown. And it requires a low pH, typically below 3.5, both to remain stable and to penetrate. Some skin finds that acidity irritating.

Research commonly cites diminishing returns above about 20 percent, with penetration plateauing while irritation continues to rise.

Practical tip: colour is the most reliable indicator of degradation. Clear or pale straw is fine. Deep yellow, orange or brown means significant oxidation and lost potency.

The derivatives, and what each trades away

Sodium ascorbate is the sodium salt. Stable at near-neutral pH, so it needs no acidic formulation and is considerably gentler. It must dissociate and convert in the skin, so delivery is less direct.

Magnesium ascorbyl phosphate, often shortened to MAP, adds a phosphate group and a magnesium salt. Stable, water-soluble, tolerant of near-neutral pH. Skin phosphatase enzymes remove the phosphate to release ascorbic acid. Gentle, well suited to sensitive skin and to emulsions.

Aminopropyl ascorbyl phosphate is a related phosphate derivative with an aminopropyl group, developed for improved stability and penetration over MAP.

Ascorbyl glucoside attaches a glucose molecule. Very stable, water-soluble, converted by glucosidase enzymes in the skin. Among the gentlest options.

Ascorbyl palmitate attaches palmitic acid, making it oil-soluble. Suits anhydrous and oil-phase formulations where water-soluble forms will not disperse. Frequently present at low levels as a formula antioxidant rather than a skin active, so its position in an ingredient list matters.

Tetrahexyldecyl ascorbate, usually shortened to THD ascorbate, is the most lipophilic. Long fatty chains make it compatible with the lipid-rich barrier, so it penetrates readily without needing low pH and is markedly less irritating. Very stable.

The rule that applies to all of them

Every derivative must be converted to ascorbic acid by skin enzymes before it can act. That conversion is neither immediate nor complete.

Which means the delivered dose of active vitamin C is always lower than the applied concentration, and comparing a derivative percentage against an ascorbic acid percentage is not like for like. A 15% THD ascorbate serum is not equivalent to a 15% ascorbic acid serum.

Marketing rarely acknowledges this. It is the single most useful thing to know when comparing vitamin C products.

Why formulations combine them

Water-soluble and oil-soluble forms reach different compartments. A fast-acting surface form alongside a slower, deeper-penetrating one covers more ground than either alone.

Formulas carrying both ascorbic acid and THD ascorbate, for instance, are doing exactly this: the free acid works at the surface immediately while the ester converts more gradually at depth.

Formulation strategies for stability

Three approaches, and the one a product uses tells you a lot about its texture and feel.

Low pH. Keeps ascorbic acid stable and penetrating. Most effective, least comfortable.

Anhydrous. Remove water entirely and suspend ascorbic acid in silicone or oil. It cannot oxidise without an aqueous phase, and activates on contact with the skin's own moisture. Very stable, but produces an occlusive texture that must be applied last.

Derivatives. Stable at comfortable pH in a normal water-based formula. Gentlest, least direct.

None is simply better. They suit different skin and different routines.

Practical notes

Packaging matters more for vitamin C than almost any other ingredient. Opaque, air-restricting containers are worth looking for, and a jar you open daily is close to the worst option.

Vitamin C is complementary to sunscreen rather than a substitute. SPF blocks UV before it enters; antioxidants deal with free radicals generated by what gets through.

Vitamin C is not photosensitising and is generally regarded as photoprotective.

The claim that vitamin C and niacinamide cancel each other out comes from older research where the two were combined under heat. In modern formulations at skin-relevant temperatures this is not considered a practical concern, and the two are frequently formulated together deliberately.