Vitamin C in skincare — what it does and why it is so difficult to formulate.

Vitamin C — specifically L-ascorbic acid — has more clinical evidence behind it than almost any other antioxidant in skincare. The evidence for its role in collagen synthesis, its effect on pigmentation, and its antioxidant protection against UV-induced oxidative damage has been building for decades. It is also one of the most difficult ingredients to formulate and use correctly.

What vitamin C is

L-ascorbic acid is the bioavailable form of vitamin C that the skin can use directly. The body cannot synthesise vitamin C — it is obtained from diet and, in the context of skincare, applied topically. Other forms appear in skincare — sodium ascorbyl phosphate (SAP), ascorbyl glucoside, magnesium ascorbyl phosphate (MAP), tetrahexyldecyl ascorbate (THD) — and these derivatives convert to ascorbic acid in the skin to varying degrees. L-ascorbic acid remains the most studied form and, at the right concentration and pH, the most effective.

What it does

Antioxidant protection. L-ascorbic acid neutralises free radicals generated by UV radiation and environmental pollution. It is water-soluble, which allows it to protect the skin's aqueous environment — complement to vitamin E, which is fat-soluble and protects the lipid layers. Applied in the morning, vitamin C intercepts oxidative damage before it triggers the cascade that degrades collagen and causes pigmentation changes. This preventative role is well-supported.

Collagen synthesis. Vitamin C is required for the hydroxylation of proline and lysine — two amino acids essential to stable collagen structure. Without adequate vitamin C, collagen cannot be properly formed. Applied topically, L-ascorbic acid has been shown to stimulate collagen gene expression in dermal fibroblasts, with effects measurable over months of consistent use.

Addresses hyperpigmentation. Vitamin C inhibits tyrosinase, the enzyme central to melanin production, reducing the formation of new pigment. Paired with the collagen effect, consistent vitamin C use produces measurable improvement in skin tone evenness over time. The effect is on prevention and gradual lightening — not rapid bleaching.

The stability problem

L-ascorbic acid is highly unstable in water. It oxidises readily on exposure to air, light, and heat — turning the product from colourless or pale yellow to orange or brown. An oxidised vitamin C product has lost its activity and, critically, the oxidised byproducts may themselves cause oxidative stress in the skin. A product that is orange or brown when purchased has been compromised.

This instability is the primary reason for the wide range in vitamin C product quality. Formulation and packaging determine whether a 20% L-ascorbic acid serum is effective or ineffective. Key variables:

• pH. L-ascorbic acid requires a low pH — between 2.5 and 3.5 — to penetrate the skin effectively. At higher pH, absorption is significantly reduced.
• Packaging. Opaque, air-restricted packaging — pump dispensers, opaque tubes — limits oxygen and light exposure. Clear glass dropper bottles with repeated air contact are the worst-case scenario for stability.
• Storage. Keep away from heat and direct light. Vitamin C is one of the few skincare ingredients that benefits meaningfully from refrigeration.

Derivatives — what they offer and what they sacrifice

The instability of L-ascorbic acid has driven the development of derivatives designed to be more stable in water at a higher pH — and therefore more cosmetically elegant.

Sodium ascorbyl phosphate (SAP) is water-soluble, stable, and converts to ascorbic acid in the skin. Evidence suggests it is effective at 5–10%.

Ascorbyl glucoside is among the most stable water-soluble derivatives. Evidence for efficacy is building but less extensive than for L-ascorbic acid.

Tetrahexyldecyl ascorbate (THD) is oil-soluble, highly stable, and penetrates well. Evidence is positive, though the conversion to active ascorbic acid in the skin is less fully characterised.

All derivatives are a tradeoff: better stability and cosmetic experience in exchange for less certainty about the final active concentration. For people who find L-ascorbic acid too irritating or impractical, a derivative at a clinically relevant concentration is a reasonable alternative.

Concentration matters

The clinical evidence for L-ascorbic acid concentrations in consumer products sits between 10% and 20%. Below 10%, penetration is unlikely to be sufficient for the collagen synthesis and antioxidant effects. Above 20%, irritation increases without proportionate additional benefit. 15% at pH 3.5 or below, in stable packaging, is a well-established benchmark.

Where it fits

Vitamin C is a morning ingredient. Its antioxidant function is most relevant when the skin is about to be exposed to UV and environmental oxidative stress. It does not make sense to apply it at night, when the barrier is in its repair phase and there is no environmental challenge to defend against. Apply after cleansing, before a moisturiser with SPF.

The Lux & Glo position

The Lux & Glo ritual does not include vitamin C. The three steps are designed around the barrier — gentle cleansing, niacinamide strengthening ceramide synthesis, squalane and shea reinforcing the lipid matrix. Vitamin C is a specific active for antioxidant protection and pigmentation goals that the barrier-first routine does not address.

It is a well-supported ingredient. Whether it belongs in a routine depends on whether its goals — antioxidant protection, collagen maintenance, pigmentation — are current priorities for the skin using it. If they are, a stable, correctly concentrated, properly pH-adjusted L-ascorbic acid or a well-researched derivative is worth adding — in the morning, in opaque packaging, stored away from heat.