Near-Infrared Time-Resolved Spectroscopy for Assessing Brown Adipose Tissue Density in Humans: A Review

Brown adipose tissue (BAT) mediates adaptive thermogenesis upon food intake and cold exposure, thus potentially contributing to the prevention of lifestyle-related diseases. (18)F-fluorodeoxyglucose (FDG)–positron emission tomography (PET) with computed tomography (CT) ((18)FDG–PET/CT) is a standard method for assessing BAT activity and volume in humans. (18)FDG–PET/CT has several limitations, including high device cost and ionizing radiation and acute cold exposure necessary to maximally stimulate BAT activity. In contrast, near-infrared spectroscopy (NIRS) has been used for measuring changes in O(2)-dependent light absorption in the tissue in a non-invasive manner, without using radiation. Among NIRS, time-resolved NIRS (NIR(TRS)) can quantify the concentrations of oxygenated and deoxygenated hemoglobin ([oxy-Hb] and [deoxy-Hb], respectively) by emitting ultrashort (100 ps) light pulses and counts photons, which are scattered and absorbed in the tissue. The basis for assessing BAT density (BAT-d) using NIR(TRS) is that the vascular density in the supraclavicular region, as estimated using Hb concentration, is higher in BAT than in white adipose tissue. In contrast, relatively low-cost continuous wavelength NIRS (NIR(CWS)) is employed for measuring relative changes in oxygenation in tissues. In this review, we provide evidence for the validity of NIR(TRS) and NIR(CWS) in estimating human BAT characteristics. The indicators (Ind(NIRS)) examined were [oxy-Hb](sup), [deoxy-Hb](sup), total hemoglobin [total-Hb](sup), Hb O(2) saturation (StO(2sup)), and reduced scattering coefficient ([Formula: see text]) in the supraclavicular region, as determined by NIR(TRS), and relative changes in corresponding parameters, as determined by NIR(CWS). The evidence comprises the relationships between the Ind(NIRS) investigated and those determined by (18)FDG–PET/CT; the correlation between the Ind(NIRS) and cold-induced thermogenesis; the relationship of the Ind(NIRS) to parameters measured by (18)FDG–PET/CT, which responded to seasonal temperature fluctuations; the relationship of the Ind(NIRS) and plasma lipid metabolites; the analogy of the Ind(NIRS) to chronological and anthropometric data; and changes in the Ind(NIRS) following thermogenic food supplementation. The [total-Hb](sup) and [oxy-Hb](sup) determined by NIR(TRS), but not parameters determined by NIR(CWS), exhibited significant correlations with cold-induced thermogenesis parameters and plasma androgens in men in winter or analogies to (18)FDG–PET. We conclude that NIR(TRS) can provide useful information for assessing BAT-d in a simple, rapid, non-invasive way, although further validation study is still needed.