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Resonance Raman Spectroscopy measures skin carotenoids
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3818359/
"RRS is a form of laser spectroscopy that detects the characteristic vibrational/rotational energy levels of a molecule. Carotenoids are particularly well suited to RRS..."
"...there are real and significant disadvantages to the use of plasma or serum carotenoid concentrations, including ... the relatively short half-life of carotenoids in blood. Adipose tissue is thought to be a more stable depot of carotenoids..."
"Cholesterol transporters, such as scavenger receptor class B1 type 1 protein (SR-B1) and Cluster of Differentiation 36 membrane protein (CD 36), appear to facilitate absorption of carotenoids in the intestine, and these transporters may also facilitate carotenoid absorption in the epidermal layers of the skin."
"...smokers had about a 20–30% reduction in skin carotenoid levels when compared to non-smokers."
"UV irradiation of the skin of the forearm resulted in significant decreases in beta-carotene and lycopene. In addition, infrared irradiation also led to reductions in skin carotenoids. The authors speculate that solar irradiation in the UV, IR and visible regions produced free radicals in skin that deplete carotenoids."
"The enzyme that controls the metabolism of provitamin A carotenoids into retinal (precursor to retinol), beta-carotene 15,15’-monoxygenase 1 (BCMO1), is known to be polymorphic. In 2009, Leung et al.66 reported that two common polymorphisms in this gene were associated with fasting plasma beta-carotene concentrations."
"These observations suggest that skin carotenoid RRS scores might be useful as a surrogate marker for general antioxidant status."
"...our group subsequently developed and evaluated portable 488 nm solid state laser based compact instruments in field settings. One such evaluation included our study of 3 and 4 year-old children who were assessed in a preschool setting..."
"Recognizing that the solid state lasers used in portable research-grade RRS devices are still relatively expensive, we have been exploring alternative optical methodologies to assess skin carotenoid status noninvasively but at reduced cost. For example, skin reflectance-based methodologies have been evaluated by other groups, including the group of Sies and Stahl, for many years. We have developed a variation on this method using topical pressure"
https://sci-hub.tw/10.1093/ajcn/71.3.795
"The skin carotenoid level was measured at the inside of the arm because this area is easily accessible to reflection photometry and is only moderately exposed to natural sunlight. In addition, the carotenoid level on the inside surface of the arm correlates well with serum beta-carotene concentrations . Reflection spectra were collected noninvasively between 350 and 850 nm with a Multiscan OS 20 spectrophotometer..."
"A comparison of skin carotenoid levels was difficult because concentrations were measured in different areas of the body, and different skin layers were included. Reflection spectroscopy provides spectroscopic data from surfaces of skin, which includes epidermis and dermis. The carotenoid levels measured in the present study are similar to those found in studies in which skin free of subcutaneous fat was analyzed... The levels appear to be much higher when subcutaneous fat is included in the sample... there is still some doubt about the levels of carotenoids at the target sites and further research is necessary to measure the time dependency of carotenoid uptake and turnover in the skin."