New Orleans — Topical antioxidants are a valuable adjunct to sunscreens for enhancing photoprotection and/or reversing photodamage, said Karen E. Burke, M.D., Ph.D. at the 63rd Annual Meeting of the American Academy of Dermatology (AAD) here.
However, attaining benefits from topical antioxidants depends on careful product selection, since form and formulation influence absorption, activity and stability, adds Dr. Burke, department of dermatology, Mount Sinai Medical Center, New York.
Scientific evidence There is a wealth of scientific evidence from preclinical and clinical studies demonstrating the efficacy of oral and topical antioxidants for providing photoprotection and reversing chronic photodamage. In her own research, Dr. Burke has focused on vitamin E and the trace mineral selenium. In addition, studies from Sheldon Pinnell, M.D., Duke University, Durham, N.C., show the benefits of vitamin C, while others have reported on genistein, the soy isoflavone, and silymarin, which is an extract of the milk thistle plant.
Small, placebo-controlled studies also show benefits of selenium for protecting against skin cancer.
Studies with topically applied and/or oral L-selenomethionine, natural vitamin E (d-alpha-tocopherol), vitamin C, genistein and silymarin have demonstrated their activity for protecting against UV-induced skin damage based on a variety of assessments, including determinations of sunburn cell formation, erythema, edema, blistering, molecular markers of DNA damage and the development of skin cancer itself.
"At least with respect to my research with selenium and vitamin E, topical application is a little more effective than oral, but I still recommend everyone take an oral supplement containing selenium 100 mcg each day, unless, perhaps, they are residents of the Pacific Northwest, where water has a high selenium content," Dr. Burke says.
There are also clinical and histological data to show selenium, vitamin E and vitamin C can reverse wrinkles and other signs of UV-induced photoaging. Because it is a phytoestrogen, genistein would also be expected to stimulate collagen synthesis and reverse photodamage.
"Results of studies with selenium and vitamin E indicate that they may be almost as effective as tretinoin," Dr. Burke says.
While antioxidants are a common ingredient in many cosmeceuticals, their presence in the formulation does not guarantee efficacy. For example, whereas selenium sulfide is very effective for treating tinea versicolor, seborrheic dermatitis and dandruff, it is not absorbed percutaneously and has no photoprotective activity.
"For selenium to penetrate into the skin, it must be in the form of L-selenomethionine, and a concentration of 0.02 to 0.05 percent is necessary to provide maximal protection," Dr. Burke says.
Topical application To achieve activity with topical application, vitamin E must be in the form of d-alpha-tocopherol and not as the mixture of other synthetic conformational isomers of tocopherol or of the esterified tocopherol derivatives. The minimum effective concentration of d-alpha-tocopherol for achieving photoprotective and antiaging activity is 2 percent, whereas 5 percent is optimal.
Similarly, vitamin C must be formulated as L-ascorbic acid since vitamin C ester derivatives are not absorbed. The optimal concentration of L-ascorbic acid to achieve percutaneous absorption is 15 percent to 20 percent, and it should be formulated in an acidic vehicle so that the compound is present in its unionized form.