Plasma and serum samples were thawed on ice and 50 µl aliquots were mixed with 150 µl MeOH for protein precipitation. Samples were then centrifuged for 15 min at 15800×g. The supernatant was transferred to a new tube and then evaporated to complete dryness in a vacuum concentrator. Prior to LC-MS analysis, samples were reconstituted in 50 µl MeOH:H2O 1:1 and the two isotopically labelled internal standards phenylalanine (D5) and palmitic acid (D4) (Cambridge Isotope Laboratories) were added at a final concentration of 5 µM.