Fatty liver increases the risk for insulin resistance, cardiovascular disease and non-alcoholic steatohepatitis (NASH) and the number of people with a fatty liver reaches alarming proportions (1-3). One of the pathways that might lead to fattening of the liver is increased dietary fat retention. The low (1.1%) natural abundance (NA) of carbon-13 (¹³C), allows the use of ¹³C-enriched lipids for in vivo MR tracer studies. Here, we show that heteronuclear single quantum coherence (ge-HSQC) spectroscopy offers this opportunity for in vivo detection of hepatic ¹H-[¹³C]-lipid signals in the human liver after a single high-fat meal with ¹³C labeled fatty acids.Two lean, female subjects were included (aged 41 and 30 years; BMI 20 and 18 kg/m2 respectively) in this study. MR experiments were performed on a 3T clinical MR system (Achieva 3T-X, Philips Healthcare, Best, The Netherlands) using a butterfly-loop 1H transmit/receive quadrature coil combined with two 16cm 13C transmit/receive surface coils in quadrature-mode for optimal ¹H receiving sensitivity (Rapid Biomedical GmbH, Rimpar, Germany). The applied ge-HSQC sequence is depicted in figure 1 (4). Water (NSA = 2 x 8) and lipid reference signals (NSA = 2 x 16) were acquired, by turning off the coherence selection gradients and by setting the frequency of the ¹³C RF pulses far off resonance. Minimum TR was set to 2000 ms, 1/2J = 3.95 ms and t1 was set to 6.1 ms. A total of 320 individual acquisitions was averaged for ge-HSQC to come to the ¹³C-edited lipid spectrum. Total duration of the in vivo protocol was approximately 45 minutes. On the test day, subjects arrived at the university after an overnight fast. Subjects were positioned in the MRI scanner in supine position and head first. The surface coil was placed over the right hypochondrium. Scout images were acquired prior to the MR spectroscopy protocol, to accurately position the voxel of 40 x 60 x 60 mm inside the liver. Acquisition was gated using a pressure sensitive sensor. After baseline MRS measurements, subjects consumed a high fat (60E% fat, 36E% carbohydrates and 4E% protein) liquid meal (total energy content 753 kcal, fat content 50.3 g). Additional to this, subject 1 consumed capsules containing 7 grams of U-¹³C algal lipid mixture (Cambridge Isotopes Laboratories Inc., Andover, MA; 0.133 g/kg body weight). The dose was slightly reduced to 5 grams for subject 2 (0.100 g/kg body weight). MRS measurements were repeated 1.5, 3.0, 4.5 and 6.0 hours after the meal to measure the ¹³C enrichment in the IHL pool. Spectra were fitted with the AMARES algorithm in jMRUI (v4). Prior knowledge was used to fix the frequency difference between the two CH₂ peaks at 127 Hz in the ¹³C-edited spectra. The linewidth of these peaks was set equal to the linewidth of the peak in the reference scan. Results are expressed as the ¹³C enrichment (%) of the IHL pool in time.A typical example of in vivo spectra is shown in figure 2. In both subjects the background ¹³C enrichment, i.e. prior to the meal, was close to the expected 1.1 % (1.0 and 0.9 % respectively) and was increased at every time point after the meal. Increased ¹³C enrichment of the IHL pool was already observed after 1.5 hours and peak 13C-enrichment values, 3.2 % and 2.2 % respectively, were found around 3 hours after the breakfast and values stayed above NA (1.1 %) throughout the test day.Here we have demonstrated the feasibility of tracking dietary fatty acids to the liver non-invasively in vivo with ge-HSQC. This technique allows performing in vivo studies to investigate the role of postprandial fatty acid handling in both healthy subjects and subjects at risk for increased liver fat and can give insight in how to influence dietary fat retention to prevent excessive lipid storage in the liver.