“Applying 15% Vitamin C for three consecutive days creates a reservoir effect in the skin.”
Firstly, I want to remind you that this study was done on pig skin – not humans. The way that ascorbic acid is stored and metabolized in pig skin may vary from human skin.
Most animals, like pigs, are able to synthesize their own ascorbic acid from glucose, but humans cannot. It’s possible that this data from pigs will be similar to human data, but it’s also very possible that it won’t be. Neither has been proven yet. Presenting an assumption as truth is misleading – but often done in marketing.
I also want to remind you that the way that the ascorbic acid was applied to the skin was not the same way that we apply our skincare. In these experiments, the ascorbic acid solution was applied with a Hill Top Chamber, which occludes the skin, reducing evaporation and theoretically enhancing skin penetration.
For this part of the experiment, Pinnell and his group applied a 15% ascorbic acid solution at pH 3.2 to pig skin for 5 days with a Hill Top Chamber. After the 5th day, application of the ascorbic acid was stopped and ascorbic acid levels in the skin were monitored for an additional 5 days.
After the 3rd day of application of the ascorbic acid serum, the ascorbic acid levels in the skin do appear to reach a peak around 1100 pmol/mg. The deviation around the mean does appear to be reducing with each further day between the 3 subject pigs.
We do need to consider if this theoretical peak amount of ascorbic acid is reached in real-life situations. The living conditions of the pigs in the study were not described, so it’s possible that they were not exposed to natural daylight. It’s understood that UV exposure reduces the amount of ascorbic acid in the skin. UV increases the production of free radicals in the skin, and ascorbic acid is part of the natural antioxidants in the skin which help neutralize these free radicals.
In an experiment using human skin models, it was found that exposure to 16.9 joules/cm² (About 12 minimal erythemal dose equivalent) of UV reduced ascorbic acid levels in the skin model by almost ⅓. This was a higher amount of UV exposure the experimenters expected, they were also unable to detect dehydroascorbic acid in the skin. The study does have some issues which “may be explained by the high levels of ascorbate present in the [tissue] medium…added by the manufacturer to increase collagen synthesis”.
“Vitamin C remains in the skin for 3-4 days and doesn’t wash out”
This marketing claim may be due to some confusion of the term “washout”. In drug experiments a “washout period” refers to the period of time when treatment is stopped, it does not necessarily mean that the skin is washed out.
After applying the 15% ascorbic acid solution to the pig skin, they discontinued application and monitored ascorbic acid levels in the skin. Unfortunately, the methodology in this portion of the experiment isn’t explicitly described. It is unclear, for example, if the pig’s skin was washed each day. The washing procedure is described as “…at the end of the experiment, the formulation was washed vigorously from the skin with water.”
Because most of us use surfactant based cleansers to wash our skin, this data may not be as applicable as the pig’s skin was washed with only water. However, the pig’s skin was removed of stratum corneum before ascorbic acid measurement and the lower layers of skin are likely less affected by the washing and surfactant based-cleanser.
Based on this data, the half-life (the amount of time it takes for the detected ascorbic acid levels to drop by half) was estimated at around 4 days. But as mentioned above, it’s unclear what the living conditions of the pigs were and whether or not they were exposed to sunlight which reduces antioxidant levels in the skin.
Can Vitamin C derivatives increase levels of Vitamin C in skin?
The last portion of the Pinnell experiment looked at whether or not the topical application of Vitamin C derivatives could increase levels of Vitamin C as ascorbic acid in pig skin.
For 24 hours, solutions of dehydroascorbic acid, 10% ascorbyl-6-palmitate, 12% magnesium ascorbyl phosphate, and 15% ascorbic acid were applied to pig skin. Compared to control, only the 15% ascorbic acid solution created a statistically significant increase in ascorbic acid levels in the skin.
For the derivatives, there was no statistically significant difference between the application of the derivative and control (no application of derivatives or Vitamin C) – which implicates that, at least for pig skin, these specific derivatives do not convert to Vitamin C.
For the solutions of dehydroascorbic acid, pig skin levels of ascorbic acid were 7.51 ± 3.34 pmol/mg for 20 mM dehydroascorbic acid and 8.70 ± 2.13 pmol/mg for 1 M dehydroascorbic acid. Where no dehydroascorbic acid was applied levels of ascorbic acid were 9.24 ± 3.55 pmol/mg.
It surprises me how influential this one study on ascorbic acid applied to pig skin has become in terms of marketing language for brands.
Even later studies with Dr. Pinnell as an author leave out that the data are collected from pig skin, “Following topical application, once the skin is saturated with L-ascorbic acid, it remains with a half-life of about 4 d (Pinnell et al, 2001).”
While this experiment is some of the best data we have in terms of ascorbic acid penetration based on formulation, the key point to remember is that human skin cannot be assumed to behave the same as pig skin.
So if you see a claim similar to “15% Vitamin C at pH 3.5 is the most effective concentration”, please imagine me beside you whispering “…for pigs”.
Edit: An error was made in the original version published, pigs can synthesize Vitamin C from glucose, but humans can not. Guinea pigs also cannot synthesize their own Vitamin C.
Edit: An error was made in the original version published, pmmol was corrected to pmol.
Source: Pinnell, S. R., Yang, H. , Omar, M. , Riviere, N. M., DeBuys, H. V., Walker, L. C., Wang, Y. and Levine, M. (2001), Topical L‐Ascorbic Acid: Percutaneous Absorption Studies. Dermatologic Surgery, 27: 137-142. DOI: 10.1046/j.1524-4725.2001.00264.x
Podda, M., Traber, M.G., Weber, C., Yan, L., Packer, L. (1998), UV-Irradiation Depletes Antioxidants and Causes Oxidative Damage in a Model of Human Skin, Free Radical Biology and Medicine, 24: 55-65. DOI: 10.1016/S0891-5849(97)00142-1