Ascorbic acid
The active form of vitamin C; well-evidenced for photoageing at 10–20 % and low pH, but unstable.
L-ascorbic acid is the form of vitamin C the skin actually uses: an electron donor that regenerates other antioxidants, a required cofactor for collagen cross-linking, and a tyrosinase inhibitor.
Getting it in
The key absorption work showed uptake rises steeply below pH 3.5 and plateaus around 20 %, above which more does nothing[1]. That is why serious formulas are acidic and sit at 10–20 %.
The evidence
Two double-blind, vehicle-controlled split-face studies found significant improvements in wrinkling, texture and photodamage scores after three to six months[2][3]. Applied before sun exposure it also reduces UV-induced erythema[1], which is why it belongs in the morning under zinc oxide or another sunscreen.
Stability
It oxidises in water, air and light. Amber packaging, low water activity and ferulic acid all help; a brown serum has already lost the plot. sodium ascorbyl phosphate trades potency for stability.
Evidence by claim
Safety & limits
Sources · 3
- [1]Pinnell SR, Yang H, Omar M, et al.. Topical L-ascorbic acid: percutaneous absorption studies. Dermatologic Surgery (2001)doi:10.1046/j.1524-4725.2001.00264.x
- [2]Fitzpatrick RE, Rostan EF. Double-blind, half-face study comparing topical vitamin C and vehicle for rejuvenation of photodamage. Dermatologic Surgery (2002)doi:10.1046/j.1524-4725.2002.01196.x
- [3]Humbert PG, Haftek M, Creidi P, et al.. Topical ascorbic acid on photoaged skin. Clinical, topographical and ultrastructural evaluation: double-blind study vs. placebo. Experimental Dermatology (2003)doi:10.1034/j.1600-0625.2003.00008.x