Regional Free Fatty Acid Uptake Assessed By Positron Emission Tomography In Postabsorptive Humans

BACKGROUND Elevated free fatty acids (FFA) contribute to themetabolic complications of obesity such as insulin resistance. Therefore, defining thetissues and organ FFA uptake should help us understand the impact of FFA onmetabolism.OBJECTIVE To determine whether free fatty acid (FFA) uptake intosubcutaneous and visceral adipose tissue, postural and quadriceps muscle, and liver inhumans in vivo, assessed using positron emission tomography (PET) use of [1-¹¹C]palmitic acid is different in postabsorptive lean and obese adults. Also to validate theadipose tissue uptake results obtained with PET against those obtained usingpreviously established methods (use of [1-¹⁴C]palmitic acid).PATIENTS AND METHODS Volunteers received a continuousintravenous (IV) infusion of [U-¹³C]palmitic acid begun30minutes before the[1-¹¹C]palmitic acid PET/CT study. Blood samples were obtained before and duringthe scan. A dynamic PET scan was acquired over the lower chest/upper abdomen.Tissue palmitic acid uptake rates were calculated using Patlack analysis with a PETleft ventricle-blood hybrid input function. A static thigh PET/CT was doneimmediately after the dynamic scan to measure standardized uptake value, which wasused to calculate thigh adipose palmitic acid uptake. This was followed by an IV[1-¹⁴C] palmitic acid bolus and timed abdominal subcutaneous and thigh adiposetissue biopsies. Palmitic acid flux ([U-¹³C] palmitic acid) adipose14C-palmiticacid content were used to calculate biopsy-derived FFA storage.RESULTSPalmitic acid uptake rates (μmol/kg/min) by PET in upper body (UBSQ) andlower body subcutaneous (LBSQ) fat uptake averaged0.8±0.3and0.31±0.12. In comparison, UBSQ and LBSQ palmitic acid uptake rates (μmol/kg fat/min) by biopsyaveraged0.44±0.2(P=0.05vs. PET) and0.3±0.2(P=NS vs. PET). We foundcorrelations between PET and biopsy derived data for UBSQ and LBSQ (r=0.66,p=0.057and r=0.60, p=0.09, respectively). Palmitic acid flux (μmol/min) by [U-13C]and [1-¹¹C] palmitic acid averaged125±33and119±41respectively, and werecorrelated (r=0.64, p=0.06).CONCLUSIONThis is the first direct comparison of PET vs. biopsy. PET UBSQ palmitic aciduptake is greater than biopsy measured uptake, whereas PET and biopsy LBSQpalmitic acid uptake are similar.