long periods of continuous supplementation with naturally occurring forms, there seems to be evidence supporting further trials to explore the possible photoprotective benefits of carotenoids since they have demonstrated ability to raise the minimal erythema dose in humans.4 In contrast to carotenoids, there is a substantial amount of research that supports the use of retinol and its derivatives for treatment of the clinical manifestations of photoaging. Retinol is the naturally occurring form of vitamin A and it can readily penetrate the epidermis.10 In the body, retinol is naturally converted to numerous derivatives including retinoic acid, the most biologically active form of the vitamin. Other derivatives include retinaldehyde, retinyl esters and the group of oxoretinoids. Synthetic derivatives of retinol have also been designed and marketed including tazarotene, acitretin, etretinate and adapalene.4 The biologic properties of retinoids include antioxidant activity via free radical scavenging, increased fibroblast proliferation, modulation of cellular differentiation and proliferation, increased collagen and hyaluronate and decreased matrix metalloproteinase-mediated extracellular matrix degradation. In sun-exposed skin, retinoic acid stimulates the growth of keratinocytes and fibroblasts resulting in an increase in the production of extracellular matrix. This translates into a number of effects on the skin including an improvement in fine and coarse facial wrinkling, a decrease in tactile roughness and actinic keratoses and a lightening of solar lentigines, or sun spots.10 Retinoids bind to two groups of receptors belonging to the nuclear receptor superfamily which are the retinoic acid receptors (RARs) and the retinoid X receptors. Each receptor group has 3 receptor subtypes: ι, β and γ. Different retinoids exert their effects through different combinations of receptors.4 In human studies, activation of RARs and retinoid X receptors results in molecular changes favoring collagen deposition and increasing dermal thickness. One study proposed that elevated metalloproteinases, resulting from activation of AP-1 and NF-KB by low-dose solar irradiation, degrade collagen and elastin in skin. Researchers found that when human skin is pretreated with
18