FFA Analysis in Serum Samples

All of serum samples were stored at −80 °C until use. Sample preparation were performed according to a modified protocol based on Püttmann et al.33 (link). Forty FFAs were analyzed using an ACQUITY ultra-performance liquid chromatography (UPLC) system (Waters Corporation, Milford, USA) equipped with a binary solvent delivery system and an auto-sampler (Waters Corporation, Milford, USA), coupled to a tandem quadrupole-time-of-flight (Q-TOF) mass spectrometry (Waters Corporation, Milford, USA)16 (link). For details, see the Supplementary information. A mixture of all the reference standards at an appropriate concentration was prepared and run after every ten serum samples for quality control. Twenty-four FFAs ratios were calculated by the absolute concentrations of product- to-precursor.