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Keys to Unlocking the Benefits of Vitamin C
By: Lawrence Samuels, MD
Posted: November 29, 2012, from the December 2012 issue of Skin Inc. magazine.
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In addition, many preparations lack a sufficient concentration of free L-ascorbic acid for biologic activity and penetration. L-ascorbic acid concentrations were tested from 5–30%.6 Maximal tissue levels were obtained at L-ascorbic acid concentrations of 20% and maximal concentration for optimal percutaneous absorption was 20%. Tissue levels were saturated after three daily applications, and the half-life of tissue disappearance was about three days.6
pH and stability
Many vitamin C preparations also are not pH-adjusted for optimal biologic activity and penetration. The pKa, or acid dissociation, for L-ascorbic acid is 4.2. The pKa of an ingredient is the pH level where half of the molecule is free acid and biologically active, and the other half is bound and not biologically active. When pKa = pH, there is equal concentration of the acid to its conjugate base. Tissue levels of L-ascorbic acid were enhanced only for formulations with a pH less than 3.5.6 Preparations with a pH higher than 4.0 will oxidize the L-ascorbic acid in the product. Therefore, the molecule must be un-ionized and formulated at pH levels less than 3.5 to penetrate the skin. An appropriate pH of 3.0 will maximize biologic activity, penetration and effectiveness while avoiding skin irritation with a product where the pH is too low (particularly less than 2.5). An ideal pH is between 2.6–2.8.
To evaluate your products for effectiveness, check the ingredient list. Look for products containing L-ascorbic acid and not ascorbyl palmitate, at a concentration of 20%. Many times the concentration will be listed in product materials. If not, an ingredient’s concentration can be obtained by contacting the manufacturer. The company should also be able to tell you the pH of the product to verify it’s within the ideal range.