The phenomenon of skin aging may be categorized broadly into photoaging caused by exogenous factors and physiological aging induced by endogenous factors. A near-infrared diffuse reflectance (NIR-DR) spectroscopy method was recently proposed for non-invasive evaluation of changes that occur within the skin of hairless mice that were divided into an ultraviolet-B (UVB)-irradiated group (photoaging) and a non-irradiated group (physiological aging). In the present study, a non-invasive method to

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... nvasive method to assess changes in human skin was developed and tested. For investigation of photoaging and physiological aging, NIR-DR spectra in the 8000 to 4000 cm -1 region were measured from the outer forearm (sun-exposed) and the inner upper arm (sun-protected) of 86 females ranging in age from 23 to 69 years. The obtained spectra were subjected to principal component analysis (PCA). PCA results suggested that contributions of both types of aging occurring within the skin can be monitored and quantitatively assessed using the 5990 to 5490 and 5000 to 4480 cm -1 regions of second derivative NIR-DR spectra, respectively. This study demonstrates that for human skin, NIR-DR spectroscopy and PCA may allow non-invasive assessment of the degree of photoaging and physiological aging due to degenerative changes in protein elasticity and reduction in protein quantity, respectively.