The Problem With Vitamin C Claims
Vitamin C is one of the most cited ingredients in skincare, but the category is far less coherent than the marketing suggests. A serum labelled 20% vitamin C and a serum labelled 5% vitamin C are not simply different strengths of the same thing. They may contain different forms of vitamin C, formulated at different pH levels, with different stabilities, and with fundamentally different penetration profiles. The percentage alone communicates almost nothing meaningful about what the skin receives.
Understanding why requires some context about how vitamin C derivatives work, why stability determines everything, and why the source of the vitamin C itself has a material effect on how the ingredient behaves in a formulation.
The Forms of Vitamin C Used in Skincare
L-ascorbic acid is the bioavailable form. It is the only form of vitamin C that the skin can use directly without needing to convert it first. It is also highly unstable, oxidising rapidly on contact with air, light, and heat. A formulation of 20% L-ascorbic acid that has begun to oxidise is delivering progressively less active vitamin C with every application, regardless of what the label says.
Stabilised derivatives, including ascorbyl glucoside, magnesium ascorbyl phosphate, and ascorbyl tetraisopalmitate, solve the stability problem but introduce a conversion step. The skin must convert these precursors to L-ascorbic acid, which requires specific enzymatic activity. Conversion efficiency varies, which is why a 5% L-ascorbic acid formulation can outperform a 20% ascorbyl glucoside formulation despite the apparent concentration difference.
This is also where the source of vitamin C enters the equation in ways that go beyond standard ascorbic acid chemistry.
Natural Vitamin C Sources and Their Profiles
Kakadu plum, native to northern Australia, contains the highest known natural concentration of vitamin C of any food source: up to 100 times the vitamin C content of oranges by weight. Beyond the raw concentration, its vitamin C is accompanied by a suite of naturally occurring phytonutrients, gallic acid, ellagic acid, and quercetin, which have antioxidant properties of their own and may enhance the stability and skin benefit of the vitamin C present.
This is distinct from pure ascorbic acid, which is typically synthesised and delivers only the ascorbic acid molecule without these co-factors. Whether the co-factors matter clinically is an ongoing research question, but the case for a vitamin C superfruit extract lies partly in this complexity of action rather than the single-ingredient simplicity of synthesised ascorbic acid.
A 5% vitamin C superfruit extract derived from Kakadu plum is therefore not a lower-dose formulation compared to a conventional 20% ascorbic acid serum. It is a different kind of formulation with different stability characteristics, a more complex phytonutrient profile, and a gentler pH requirement than free L-ascorbic acid.
Stability: Why the Percentage on the Label Becomes Meaningless
Free L-ascorbic acid requires a formulation pH below 3.5 to remain stable. At this pH level, the product is highly acidic, which is why conventional vitamin C serums tingle or sting on application, particularly on sensitised or reactive skin. The pH also disrupts the skin’s acid mantle and can cause the kind of surface disruption that, over time, impairs the barrier the product is meant to protect.
As the formulation pH rises, L-ascorbic acid converts to dehydroascorbic acid and then to diketogulonic acid, both of which are inactive. The yellowing and browning that conventional vitamin C serums undergo after opening is visible evidence of this conversion in progress. By the time a product is visibly brown, most of the active vitamin C has been lost.
Stabilised forms and encapsulated vitamin C both address this in different ways. The vitamin C brightening serum formulated around Kakadu plum extract can be developed at a pH closer to the skin’s natural range, which means less surface disruption, better compatibility with other actives, and a more stable product over its shelf life.
Combining Vitamin C with Other Actives
Vitamin C’s interactions with other skincare actives are frequently misunderstood. The historically cited conflict with vitamin B3, based on the possibility of forming nicotinic acid, has been substantially revised by more recent research. The temperatures required for this reaction to occur meaningfully do not apply under normal cosmetic use conditions, and the combination may be synergistic for brightening rather than antagonistic.
The more practical consideration is pH compatibility. A low-pH L-ascorbic acid formulation applied before a higher-pH vitamin B3 product can raise the surface pH of the vitamin C before it has fully absorbed, reducing its penetration. Allowing adequate absorption time between steps, or using a formulation that stabilises both actives together, resolves this. Formulations that already contain both vitamin C and vitamin B3 at a balanced pH address the layering problem by eliminating it.
What to Look for in a Vitamin C Formulation
Stability markers matter more than the number on the label. Colour consistency across the product’s lifespan, airless packaging, and a realistic pH range that does not require a highly acidic formulation to maintain bioavailability are the practical indicators of a well-formulated vitamin C product.
Source quality matters alongside stabilisation method. Vitamin C derived from documented natural sources with high baseline concentration and accompanying phytonutrients offers a different formulation basis than synthetic ascorbic acid alone. Neither is inherently superior, but they are different, and the differences are meaningful for consumers building a brightening routine around vitamin C as a primary active.
Mark Martin is a freelance writer and editor based in New York City. He has written extensively on topics ranging from technology, business and lifestyle. His work has been featured in major publications such as The New York Times, The Wall Street Journal and Wired Magazine.