In Peter T. Pugliese, MD's, article "Physiology of the Skin: Vitamin C in Skin Care," the power of vitamin C is explored. Following is information about the available forms forms of vitamin C for esthetician use.
Only four or five of the major forms of vitamin C are available to the esthetician, although there are many more. To set the record straight, natural and synthetic vitamin C are chemically identical, and there appears to be no clinically significant difference in either the bioavailability or the bioactivity.
Pure vitamin C or crystalline form. Pure vitamin C is L-ascorbic acid. The L designation refers to how the molecule rotates light. It is interesting that although almost all natural sugars, such as glucose, are D forms, ascorbic acid, which is derived from glucose, is an L form. Crystalline ascorbic acid dissolves easily in water and may have a very faint tinge of yellow. If exposed to air for several hours to several days, it will turn yellow in the presence of iron that may be in the water. Most water-soluble preparations of ascorbic acid will oxidize and turn yellow throughout time unless something is added to control oxidation. Ferulic acid, an antioxidant, is used in one form of aqueous vitamin C to help reduce the oxidation. It takes high levels of crystalline ascorbic acid to penetrate the skin, approximately 5–15%. Preparations of more than 15% tend to be irritating to the skin. The pH should be no higher than 3 in these preparations.
Ascorbyl palmitate. Ascorbyl palmitate is a fat-soluble antioxidant that has a fatty acid, palmitic acid, attached to the molecule. Since it is an amphipathic molecule—that is, one end is water-soluble and the other end is lipid-soluble—it can be incorporated into cell membranes. In the skin, most of it appears to be hydrolyzed (broken apart into palmitate and ascorbic acid) by esterases—which are enzymes that break down esters—after it is absorbed. The ascorbic acid released by the esterase appears to be as bioavailable as ascorbic acid alone. Ascorbyl palmitate is often used in topical preparations because it is more stable than some aqueous forms of vitamin C. Keep in mind when you use this product that if the label states that it is 10% ascorbyl palmitate, you actually are getting only 4.8% ascorbic acid since the bulk of the molecule is palmitic acid.
Magnesium ascorbyl phosphate. Magnesium ascorbyl phosphate is a water-soluble derivative of vitamin C that is nonirritating and more stable than crystalline vitamin C. It appears to have the same ability as vitamin C to promote skin collagen synthesis; however, it is effective in much lower concentrations. Magnesium ascorbyl phosphate should be used on people with sensitive skin, as an example, or if you did not want an acid-related exfoliation. Magnesium ascorbyl phosphate is neutral or slightly alkaline, about the same pH as blood, 7.4. Alhough it is several times more stable than vitamin C, exposure to light and air will degrade it throughout time. To be safe, use a brown bottle or blue glass for longer shelf life.