Good evidence
L-Ascorbic Acid
INCI: Ascorbic Acid · also vitamin C, LAA, pure vitamin C
Vitamin C in its native, unesterified form. It is the version the body actually uses and the version nearly all of the topical research was done on. It is also the version that oxidizes if you look at it wrong.
Why it is here
The version of vitamin C the studies were done on, and the version that dies if you leave the cap off. The whole serum is built around keeping it alive long enough to matter.
Ascorbic acid donates electrons, which lets it quench the free radicals ultraviolet light generates in skin. Separately it is a required cofactor for the two enzymes that hydroxylate proline and lysine during collagen synthesis, which is the mechanistic link between vitamin C and the dermis. It needs a low pH, roughly 3.5 or below, to be in the uncharged form that crosses the stratum corneum.
The receipts
A 5 percent vitamin C cream produced significant improvement in global score versus its own excipient, a significant increase in skin microrelief density and a decrease in deep furrows, with ultrastructural evidence of elastic tissue repair.
Topical ascorbic acid on photoaged skin. Clinical, topographical and ultrastructural evaluation: double-blind study vs. placebo. Experimental Dermatology, 2003. RCT, n = female volunteers with photoaged skin, treated over 6 months. PMID 12823436
A stable aqueous solution of 15 percent L-ascorbic acid with 1 percent alpha-tocopherol gave significant protection against erythema and sunburn cell formation, reaching an antioxidant protection factor of 4-fold; either vitamin alone was protective but the combination was superior.
UV photoprotection by combination topical antioxidants vitamin C and vitamin E. Journal of the American Academy of Dermatology, 2003. in vivo animal, n = pig skin, 4 days of application. PMID 12789176
In practice
- Typical use
- 10 to 20 percent in serums. 15 percent is the level used across the Duke photoprotection work and the level Bright Side uses. Above 20 percent there is no added benefit in the published work and stability gets harder.
- Pairs with
- Vitamin E, Ferulic Acid, Vitamin C + E + Ferulic
- Keep apart from
- copper-tripeptide: copper ions catalyze the oxidation of ascorbic acid in solution, which is well established chemistry (PMID 5347591). Whether it matters on skin at cosmetic levels has not been tested. Separate them by routine step or time of day and you have removed the question.. retinal: not dangerous, but the low pH vitamin C needs and the irritation retinal can cause stack up on the same skin. Vitamin C in the morning, retinal at night, is the simplest answer.
- Cautions
- Low pH means a tingle on freshly exfoliated or compromised skin. Oxidized vitamin C turns yellow then brown; color change means it is past its useful life. Use sunscreen with it, not instead of it: antioxidants reduce oxidative damage but do not absorb ultraviolet light.
Questions people ask
Why does my vitamin C serum turn orange?
That is oxidation. Ascorbic acid gives up electrons readily, which is exactly why it works and exactly why it degrades. Exposure to air, light, heat and metal ions pushes it to dehydroascorbic acid and then further. A yellow tint is early. Deep orange or brown means the active is largely spent and the serum should be replaced.
Is 20 percent vitamin C better than 15 percent?
Not in the published work. The Duke group established percutaneous delivery plateaus, and 15 percent is the concentration carried through the photoprotection studies that everyone cites. Higher numbers mostly buy you more stinging, a harder formulation problem and a marketing figure. Fifteen is the studied level, so fifteen is what we use.
Can I use vitamin C with niacinamide?
Yes. The old warning came from 1960s work heating the two together in solution, which formed nicotinic acid and caused flushing. Room-temperature cosmetic formulations do not reproduce that. The practical issue is pH, since vitamin C wants an acidic base and niacinamide prefers neutral. In our routine they sit in different products for that reason.