Sprague-Dawley rats were fed with a HFD (60% calories from fat) (n=4) and a normal diet (10% calories from fat) (n=3) for 6 weeks. Seven rats were scanned using a 3T GE MR750 scanner (GE Healthcare, Milwaukee, WI, USA) with a custom-built surface coil (inner diameter: 3.7 cm; outer diameter: 5.5 cm). The time-course HP ¹³C MRS was examined every 2 weeks in the course of 6 weeks. A HyperSense DNP (Oxford Instruments, Abingdon, UK) was used to hyperpolarize [1-¹³C] pyruvic acid. Two mL of HP [1-¹³C] pyruvate solution was injected into the tail vein within 20 seconds. Dynamic MRS data were acquired using FIDCSI (GE Healthcare) with 5 kHz acquisition bandwidth, 4096 points, and a slice-selective radiofrequency pulse with 10° flip angle. The MR spectra from a slab spanning 15 mm of the rat liver were acquired every 2 seconds for 120 seconds from the beginning of the injection. During MRS examinations, the respiration and body temperature of the rats were monitored using an MR-compatible small animal monitoring and gating system (Model 1025, SA Instruments Inc., NY, USA). All the spectra were processed by the SAGE software (GE Healthcare).The body weight of HFD-induced obese rats was significantly increased compared to normal rats at 6 weeks after feeding (p=0.034) (Table 1). In addition, the fat percentage of the liver in an obese rat was 17.5% which is higher than that of a normal rat, 6.6% (Fig. 1). Simultaneously, the HFD-induced obese rats showed significantly increased levels of serum alanine aminotransferase (ALT), aspartate aminotransferase (AST), lactate dehydrogenase (LDH), and low-density lipoprotein (LDL)-cholesterol compared to normal rats (p≦0.05) (Table 1). In the dynamic ¹³C MR spectra acquired at 6 weeks after feeding, the obese rats showed significantly increased ratios of [1-¹³C] lactate to [1-¹³C] pyruvate and [1-¹³C] alanine to [1-¹³C] pyruvate (p≦0.05) (Table 2, Figs. 2-4). The ¹³C spectral outcomes are positively correlated with the enzyme levels of ALT and LDH in the HFD-induced obesity.The levels of [1-¹³C] lactate and [1-¹³C] alanine are potentially considered as important biomarkers for the high-fat diet-induced obesity.