volunteers who applied the 0.1% retinol formulation for 36 weeks showed improvement in all major aging signs assessed including wrinkles under the eyes, fine lines, and tone evenness compared to placebo control. Moreover, tryptophan fluorescence, a non-invasive indicator of epidermal thickness, increased in the active-treated group and not in the placebotreated group indicating cell proliferation was accelerated in vivo. Retinol 0.1% applied to a monolayer of epidermal keratinocytes increased gene expression of all three forms of hyaluronic acid synthases (HAS) and increased hyaluronic acid production in human skin equivalents.27 Subjects treated with the same 0.1% retinol demonstrated an increase in hyaluronic acid accumulation in the epidermis compared to those treated with vehicle.
The dermal effects of retinol have also been confirmed. Retinol 0.04% lotion applied three times a week for 24 weeks to the forearm significantly increased glycosaminoglycan expression and procollagen 1 immunostaining in naturally aged skin.28 Clinical assessment resulted in significant improvement in fine wrinkling score between retinol-treated and vehicle-treated groups. In a comparative study, tretinoin 0.1%, retinol 0.1%, and vehicle were applied to forearm skin once weekly under occlusion for four weeks.29 Genes for collagen type 1 and collagen type 3 were upregulated with both retinoids and increases in procollagen I and procollagen III protein expression observed mostly in the papillary dermis. Application of 1% retinol to naturally aged skin was shown to increase fibroblast growth and collagen synthesis while decreasing matrix metalloproteinases.30 In addition having collagen boosting effects, retinol has also been shown to increase elastin fiber formation.31 Retinol 0.04% added to cultured dermal fibroblasts resulted in an increase in elastin protein synthesis and elastin fiber formation. Application of the same retinol 0.04% to human skin explants confirmed an elastin fiber network on biopsy using Luna staining. Thus, it appears that retinol confers similar benefits to retinoic acid on skin with less irritation.
Clinical studies confirm the benefits of retinol on aging skin. An eight-week, double-blind, split-face, vehicle-controlled randomized study was conducted on female subjects with moderate photodamage.18 Subjects applied 0.1% stabilized retinol or vehicle for eight weeks to each side of the face. Retinol treatment resulted in a progressive improvement over time with significant improvement over vehicle in all wrinkle parameters, pigmentation, elasticity, firmness, and overall photodamage at eight weeks. The test retinol was well tolerated with no erythema, scaliness, or edema noted. Improvement was seen in the vehicle-treated group but far less than the active group and is believed to be due to the moisturizing effects of the vehicle containing glycerin and the passive lightening that occurs during the winter months. Studies have also confirmed that topical retinol remains beneficial with long-term use. In a one-year, double-blind,
