To optimize percutaneous absorption and full activity of vitamin C, the precise formulation is of utmost importance (15). Fortunately, the skin level of vitamin C can be increased significantly by topical application. Topical absorption was proven by radioactive-labeling studies in pigs. After treatment with 10% vitamin C cream, 8.2% was found in the dermis, and 0.7% was in the blood (4). Formulations containing 5%, 10%, 15%, 20%, or 25% vitamin C were tested: after 24 hours, 20% resulted in the highest skin levels, with maximized concentration in the skin after three days (16). Indeed, the level of vitamin C in the skin attained by topical application was over 27 times the level attained by high oral intake (16).
Since L-ascorbic acid is an inherently unstable molecule—making it an excellent antioxidant—creation of an effective topical delivery system is crucial. Many products contain stable derivatives which are not metabolized by the skin (such as ascorbyl-6- palmitate or magnesium ascorbyl phosphate) and therefore have no activity (16). Other formulations do not result in measurable absorption of vitamin C because they are not at the correct pH. Delivery of L-ascorbic acid depends upon removing the ionic charge achieved optimally at a pH of 3.5 (16). Having the pH below the pKa of ascorbic acid (pHa=4.2) gives optimal activity as an antioxidant.