Identification of human brown/beige adipose tissue using near-infrared time-resolved spectroscopy

Brown/beige adipose tissue (BAT) is expected to contribute to protecting lifestyle-related diseases. The purpose of this study was to examine if near-infrared time-resolved spectroscopy (NIR_TRS) is capable of distinguishing BAT from muscle and white adipose tissue (WAT). We analyzed the optical characteristics of tissues in the supraclavicular region, where BAT deposits can be located, and deltoid and abdominal regions in 36 participants (16 men and 20 women) who were apparently healthy individuals, with a median age of 44.5 years, in winter and summer. They also had a median body fat percentage of 28.3% and a deltoid and abdominal adipose tissue thickness of 0.85 cm and 1.71 cm, respectively. The total hemoglobin concentration [total-Hb] and the reduced scattering coefficient (μs′) were determined using nearinfrared time-resolved spectroscopy (NIRTRS) with a 3 cm optode separation for supraclavicular and deltoid regions and a 2 cm optode separation for abdominal region. The results regarding data collected in winter were the following: deltoid (μ_s' = 9.6 [9.1, 10.4] cm^-1, [total-Hb] = 114.9 [107.0, 127.7] μM); abdominal (μ_s' = 9.0 [7.9, 10.1] cm^-1, [total-Hb] = 11.2 [8.0, 16.0] μM); and supraclavicular (μ_s' = 7.9 [7.2, 8.7] cm^-1, [total-Hb] = 60.7 [48.9, 74.7] μM)) in winter. Some data are overlapped between groups of muscle and BAT. These results indicated that [total-Hb] and μs′ show region-specific characteristics. We conclude that using [total-Hb] - μs′ relationship determined by NIR_TRS is a useful strategy to distinguish BAT from other tissues in a simple, rapid, and non-invasive manner.