Optical detection methods for carotenoids in human skin.

Resonance Raman spectroscopy and reflection spectroscopy are non-invasive optical quantitative methods for the measurement of carotenoid antioxidant levels in human skin in vivo. Since all tissue carotenoids are derived from the diet, optical monitoring in skin may serve as an objective indicator for fruit and vegetable intake, and more broadly also as an effective biomarker for integrative health. The two optical methods have already found enthusiastic application in the Nutritional Supplement Industry where they are used with large populations to measure skin carotenoid uptake upon consumption of carotenoid-containing dietary supplements. Applications in medical fields such as nutrition science and epidemiology have been awaiting rigorous correlation studies between the optical carotenoid detection methods and the established gold standard detection method of high-performance liquid chromatography, which requires excised tissue samples. In this article we review the principles of the methods along with the current status of validations so the reader can assess the merits of the optical methods in their respective fields of interest.