J. Cosmet. Sci., 61, 303–309 (July/August 2010) 303 Regional variation in the free amino acids in the stratum corneum MARTY VISSCHER, MARISA ROBINSON, and R. RANDALL WICKETT, Skin Sciences Institute, Division of Neonatology and Pulmonary Biology, Cincinnati Children’s Hospital Medical Center, Department of Pediatrics, Cincinnati, OH 45229 (M.V.), and The James L. Winkle College of Pharmacy, University of Cincinnati, Cincinnati, OH 45267 (M.R., R.R.W.). Accepted for publication May 7, 2010. Synopsis Regional differences in water-binding free amino acids (FAAs) in the stratum corneum (SC) may be expected, since differences in skin biophysical properties are well known. The objective was to determine whether dif- ferences in skin hydration as a function of body site may arise from differences in the chemical makeup of the skin, specifi cally the FAAs. Levels were quantifi ed from serial SC samples collected from the forearm, calf, back, torso, and jaw in two studies using HPLC methods. FAA levels were higher from the calf versus the forearm and lower from the jaw compared to torso and back skin. Body site variations in skin hydration could not be attributed to differences in FAA levels. INTRODUCTION Many factors are known to infl uence in vivo natural moisturizing factor (NMF) levels. These include ambient humidity (1), age (2–5), skin condition (6), exposure to solvents (7), and water (i.e., soaking) (7,8). Differences in NMF as a function of body site may be ex- pected, as regional variations in skin biophysical properties and appearance are well docu- mented. Variations in skin friction and hydration (9), and in roughness and scaliness across body sites (10), have been reported. SC thickness varies widely by location (11), with the genital area and facial skin the thinnest and the palms and soles the thickest. Anatomical site also affects water diffusion and the rate of barrier damage (12) and repair (13). Water-soluble amino acids (FAA) constitute about 40% of the natural moisturizing factor (NMF) (1,14). Additional water-soluble materials, including lactates, urea, sugars, and ions, are also components of NMF. Recent work in our group has focused on the effects of various perturbations on free amino acids in the upper stratum corneum (SC) using an HPLC-based method from serial tapes (D-Squames®) (7,8). We have examined the effects of lipid extrac- tion and warm water soaking on SC NMF (FAA) levels in vivo. These studies revealed a loss of NMF due to warm water soaking alone, which was intensifi ed by acetone/ether extraction of the skin and which led to a regeneration of NMF over a four-hour time period, and proved