Quantitative in vivo detection of adipose tissue browning using diffuse reflectance spectroscopy in near-infrared II window

White adipose tissue (WAT) and brown adipose tissue (BAT) biologically function in an opposite way in energy metabolism. BAT induces energy consumption by heat production while WAT mainly stores energy in the form of triglycerides. Recent progress in the conversion of WAT cells to "beige" or "brown-like" adipocytes in animals, having functional similarity to BAT, spurred a great interest in developing the next-generation therapeutics in the field of metabolic disorders. Though magnetic resonance imaging and positron emission tomography could detect classical BAT and WAT in animals and humans, it is of a great challenge in detecting the "browning" process in vivo. Here, to the best of our knowledge, for the first time, we present a simple, cost-effective, label-free fiber optic-based diffuse reflectance spectroscopy measurement in the near infrared II window (similar to 1050-1400 nm) for the quantitative detection of browning in a mouse model in vivo. We could successfully quantify the browning of WAT in a mouse model by estimating the lipid fraction, which serves as an endogenous marker. Lipid fraction exhibited a gradual decrease from WAT to BAT with beige exhibiting an intermediate value. in vivo browning process was also confirmed with standard molecular and biochemical assays.