The skin has naturally occurring antioxidants that protect against the ravages of free radicals by reducing and neutralizing them (see Chapter 2, Basic Cosmetic Chemistry, for an explanation of free radicals). Antioxidative enzymes that naturally occur in the skin include superoxide dismutase, catalase, and glutathione peroxidase; nonenzymatic endogenous antioxidative molecules are α-tocopherol (vitamin E), ascorbic acid (vitamin C), glutathione, and ubiquinone (better known as coenzyme Q10 or CoQ10).1 However, as part of the natural aging process our defense mechanisms decrease. This leads to an imbalance and increased number of unchecked free radicals, which engender damage to DNA, cytoskeletal elements, cellular proteins, and cellular membranes. Moreover, many of these antioxidant defense mechanisms are inhibited by ultraviolet (UV) and visible light.2,3 Topical antioxidants are currently marketed for the prevention of aging and UV-mediated skin damage. The free radical theory of aging explains why antioxidants are thought to prevent wrinkles, but this theory does not justify the use of antioxidants to treat wrinkles that are already present. Several companies claim that their antioxidant-containing products “treat” wrinkles; however, this is an exaggeration. The only antioxidant that can improve wrinkles that have already been formed is ascorbic acid, through its effects on collagen synthesis.4
In addition to the effects associated with their antioxidative activity, many antioxidants exhibit anti-inflammatory properties or depigmenting activities, which are described in more detail in other sections. There are several important factors to consider when evaluating the efficacy of antioxidants. In order to be considered biologically active, orally administered products must be absorbed and shown to raise antioxidant levels in the skin. Topically administered products must be absorbed into the skin and delivered to the target tissue in the active form and remain there long enough to exert the desired effects. Antioxidants can be activated or inactivated by enzymes in the skin. Some antioxidants are very unstable; therefore, some ingredients such as vitamin C become oxidized and rendered inactive before reaching the target. Stabilizing them in formulation and packaging them to minimize air and light exposure are challenging tasks. Absorption is also important and depends on several factors such as the molecular form of the compound, its pH, whether it is water soluble or fat soluble, and the vehicle that contains the product. This section will discuss the most popular types of antioxidants found in topical cosmetic products.
Fat-soluble antioxidants function in the lipophilic portion of the cell membrane and include vitamin E, idebenone, lycopene, curcumin, and CoQ10.5 Other antioxidants are water soluble and found in hydrophilic areas of the cell. These include ascorbic acid, green tea, silymarin, coffeeberry, and resveratrol.
POLYPHENOLS AND THE CLASSIFICATION OF ANTIOXIDANTS
Polyphenols are widely distributed in nature and in the plant kingdom in particular. They are synthesized by plants in response to environmental hazards that induce enhanced free-radical production....