Skin care products containing amino acids often promote the benefits conferred on the skin by these molecules. Natural amino acids found in the skin have been found to improve the health of skin through antioxidation, membrane stabilization and increasing skin’s natural moisturizing factor (NMF),1 a compound known to increase hydration and firmness of skin. In addition, amino acids in the stratum corneum have been found to increase the penetration of water to lipids, which plumps skin and decreases the appearance of wrinkles.1 However, limited information is available on percutaneous absorption of amino acids, which this column will quantify by assembling previously published data.
Researchers performed a literature search using PubMed and EMBASE at the library at University of California, San Francisco, to find studies focused on amino acid absorption. Emphasis was placed on studies that included quantitative data on amino acid concentration, either within skin layers or in the acceptor solution of a diffusion chamber (DC). Data found on this topic was largely from in vitro experiments with animal skin, as measuring concentration in an acceptor solution is simpler and more definitive than measuring the quantity in human blood or a biopsy sample. Moreover, the methods of quantification varied, making comparison difficult. Permeability coefficients calculated from in vitro studies at a neutral pH ranged from 0.162 to 10 x 10-5 cm/hr and percent absorption calculated from in vivo studies ranged from 27–96%. This suggests that the amino acids contained in creams and lotions may be readily absorbed; however, the precise amount (and benefit) has yet to be characterized. The following studies are organized by test method and substrate.