Noninvasive analyses of polyunsaturated fatty acids in human oral mucosa in vivo by Fourier-transform infrared spectroscopy

The aim of this study was to develop a noninvasive method to observe polyunsaturated fatty acids (PUFAs) behavior in the human body using Fourier transform infrared spectroscopy. For the noninvasive measurement of human oral mucosa, we have used infrared spectroscopy with a suitable attachment for an in vivo attenuated total reflectance system. The fatty acid contents in the tissues were determined by gas-chromatography mass-spectrometry after methylation. The alkene C-H stretching vibrations of unsaturated fatty acids in dietary oils showed infrared absorption bands with various peak positions and intensities at around 3010 cm^-1 depending on the extent of unsaturation and their species. The diurnal fluctuation of the alkene peak position of oral mucosa suggested that the contents of PUFAs were increased gradually in the early afternoon, and these data were supported by the direct determination of fatty acid species in oral mucosa where the relative increase of arachidonic and docosahexaenoic acids was observed in the early afternoon. This diurnal change of alkene peak position resembled the pattern of a "lipid factor" change calculated with the factor analysis applied to the overall infrared spectrum. We could monitor the diurnal fluctuations of PUFA contents of human oral mucosa noninvasively using a reagent-free infrared analysis system. The measurement of alkene and methylene infrared bands may provide a useful tool for detecting changes in PUFA balance in the human body.