To evaluate the longitudinal changes in iBAT during high fat diet induced obesityObesity and diabetes are major metabolic disorders associated with dietary intake and sedentary life style. Size and expansion of adipose tissue plays a critical role towards the progression of diet induced obesity. Brown adipose tissue (BAT) is the major tissue for adaptive thermogenesis involving uncoupling protein-1 (UCP1), whose adrenergic receptors can be activated by cold exposure¹. There is a large interest in exploring BAT as therapeutic target for obesity and diabetes^2-3. In our current longitudinal study we performed fat fraction (FF) imaging, histology, mRNA analysis of interscapular BAT (iBAT) in control diet (CD) and high fat diet (HFD) fed rats under both thermoneutral and cold exposed conditions for understanding the metabolic response of iBAT.All animal studies were conducted in compliance with protocols approved by the institutional animal care and use committee. Male Wistar rats of 7, 11 and 15 weeks of age were randomized into CD and HFD cohorts and imaged at thermoneutral and cold exposure conditions. Prior to MR experiments the cold cohort were exposed to 26±0.5 °C for about an hour. The thermoneutral cohort was maintained at 36±0.5 °C during the entire period of imaging. Respiration and body temperature was monitored using a physiological monitoring system. During the course of MRI experiments, isofluorane levels were maintained at 2.0–1.5% in combination of medical air and medical oxygen. MRI experiments were performed using a 7T MRI scanner (ClinScan, Bruker). A 72 mm inner diameter volume coil was used to transmit radio frequency pulses and a 20 mm diameter surface coil was used for reception. Fat-Water Dixon imaging was performed on twenty slices with a slice thickness of 1mm, matrix size 256×256, in-plane resolution of 216x216 μm2, TR=8 ms, averages=1, flip angle=6°, echo bandwidths of 1090 and 1500Hz/pixel, with out-of-phase (1.0ms) and in-phase (2.5ms) echo time. BAT samples were fixed in 10% neutral buffered formalin for 24 hours. Tissue sections were sliced at 5µm and stained for hematoxylin and eosin staining and images were captured at 20× optical magnification with Aperio ScanScope instrument. The mRNA analysis of UCP1 gene was performed on BAT and relative mRNA levels were calculated, normalized to beta actin.Figure 1 shows the representative FF map of iBAT from control and HFD diet fed groups at thermoneutral and cold exposed conditions. Figure 2 shows the box plot of FF computed from interscapular BAT at 7, 11 and 15 weeks of age for the thermoneutral condition. All the pixels from the manually drawn ROI's were utilized to compute the mean and SD as shown in Table1. The FF significantly (P<0.05) increased in HFD fed rats as compared to control group at 11 and 15 weeks of age under thermoneutral condition. Figure 3 shows the FF from iBAT at 7, 11 and 15 weeks of age for rats exposed to cold exposure. Under cold exposed condition, the FF from control group and HFD fed group were significantly (P<0.05) lower than the FF obtained at thermoneutral condition at respective age groups (7, 11 and 15 weeks). Upon cold stimulation, BAT gets activated through sympathetic nervous system and thereby BAT increases its energy demand and burns lipids to produce heat using UCP1⁴. Figure 4 shows the H & E stained sections of iBAT from (A) 7 week (B) 11 week, (C) 15 week control diet and (D) 11 week (E) 15 week HFD fed rats under thermoneutral condition. The histology clearly shows the accumulation of additional lipid droplets due to the HFD contributing to the increase in FF. At 11 weeks, the UCP1 levels from iBAT in HFD fed rats were 2.5 fold (P<0.05) higher than CD fed rats indicating that the excessive fatty acid availability lead to an overproduction of reactive oxygen species associated with increased mitochondrial uncoupling and energy dissipation⁵. The size of the brown adipocytes also increased in HFD fed rats compared to control diet group. The changes in microenvironment during high fat diet can alter the thermogenic potential of iBAT⁶.High fat diet results in increased fat fraction in iBAT due to accumulation of lipid droplets. Histology showed an increase in the size of brown adipocytes which may alter BAT function and theremogenic potential. UCP1 was upregulated in HFD fed groups due to increased mitochondrial uncoupling. With cold exposure the FF was reduced significantly in both control and HFD groups.