Abstract
Brown adipose tissue (BAT) mediates adaptive thermogenesis upon food intake and cold exposure, thus potentially contributing to the prevention of lifestyle-related diseases. 18F-fluorodeoxyglucose (FDG)–positron emission tomography (PET) with computed tomography (CT) (18FDG–PET/CT) is a standard method for assessing BAT activity and volume in humans. 18FDG–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 O2-dependent light absorption in the tissue in a non-invasive manner, without using radiation. Among NIRS, time-resolved NIRS (NIRTRS) 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 NIRTRS 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 (NIRCWS) is employed for measuring relative changes in oxygenation in tissues. In this review, we provide evidence for the validity of NIRTRS and NIRCWS in estimating human BAT characteristics. The indicators (IndNIRS) examined were [oxy-Hb]sup, [deoxy-Hb]sup, total hemoglobin [total-Hb]sup, Hb O2 saturation (StO2sup), and reduced scattering coefficient () in the supraclavicular region, as determined by NIRTRS, and relative changes in corresponding parameters, as determined by NIRCWS. The evidence comprises the relationships between the IndNIRS investigated and those determined by 18FDG–PET/CT; the correlation between the IndNIRS and cold-induced thermogenesis; the relationship of the IndNIRS to parameters measured by 18FDG–PET/CT, which responded to seasonal temperature fluctuations; the relationship of the IndNIRS and plasma lipid metabolites; the analogy of the IndNIRS to chronological and anthropometric data; and changes in the IndNIRS following thermogenic food supplementation. The [total-Hb]sup and [oxy-Hb]sup determined by NIRTRS, but not parameters determined by NIRCWS, exhibited significant correlations with cold-induced thermogenesis parameters and plasma androgens in men in winter or analogies to 18FDG–PET. We conclude that NIRTRS can provide useful information for assessing BAT-d in a simple, rapid, non-invasive way, although further validation study is still needed.
Highlights
Human adipose tissues are of a variety of types, such as white (WAT) and brown adipose tissue (BAT) [1]
The correlation coefficients of parameters determined by NIR continuous wave spectroscopy (NIRCWS) and the uptake of 18F-fluorodeoxy glucose (FDG) are presented under cold-exposed condition. μs′, reduced scattering coefficient determined by NIR time-resolved spectroscopy (NIRTRS); oxy-Hb, oxygenated hemoglobin (Hb); deoxy-Hb, deoxygenated Hb; total-Hb, total Hb; Hb O2 saturation (StO2), tissue hemoglobin oxygen saturation; adjusted supraclavicular StO2 (adjStO2), adjusted StO2 in the supraclavicular region relative to the deltoid muscle; VO2BAT, oxygen consumption in BAT; SUVmean, the mean standardized uptake value of the radioactivity (SUV) assessed by 18FDG–positron emission tomography (PET)/computed tomography (CT); maximal standardized uptake value (SUVmax), the maximal SUV assessed by 18FDG–PET/CT; BAT volume, evaluated by summating all voxel volume with SUV >2.0 assessed by 18FDG–PET/CT; Ref. no., reference numbers are obtained from the list of references in this paper; NM, not mentioned; ND, could not be determined. #Data obtained under thermoneutral conditions. *P < 0.05
The correlation coefficients of tissue-oxygenated hemoglobin, deoxygenated Hb, total Hb, tissue Hb oxygen saturation (StO2,), and optical scattering parameters as determined by NIRTRS under thermoneutral conditions and the uptake of 18F-fluorodeoxy glucose (FDG) or cold-induced thermogenesis are presented. μs′, reduced scattering coefficient determined by NIRTRS; adjusted StO2 in the supraclavicular region relative to the deltoid muscle; SUVmean, the mean standardized uptake value of the radioactivity (SUV) assessed by 18FDG–PET/CT; SUVmax, the maximal SUV assessed by 18FDG–PET/CT; BAT volume, evaluated by summating all voxel volume with SUV >2.0 assessed by 18FDG–PET/CT; Ref. no., reference number is obtained from the list of references in this paper. *P < 0.05
Summary
Human adipose tissues are of a variety of types, such as white (WAT) and brown adipose tissue (BAT) [1]. Human BAT is reported to be related to lower adiposity [body mass index (BMI), the percentage of whole body fat (%BF), and visceral fat area (VFA)] [6,7,8,9] and increased glucose sensitivity [10]. In experimental studies, repeated exposure to cold environment enhanced the BAT activity and improved glucose tolerance in obese counterparts [11] and patients with type 2 diabetes mellitus [12] as well as in healthy individuals [9, 13, 14]. Increasing BAT activity or volume may aid in combatting obesity and chronic diseases, such as type 2 diabetes mellitus
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