MEASUREMENT OF FREE FATTY-ACID KINETICS DURING NONEQUILIBRIUM TRACER CONDITIONS IN MAN - IMPLICATIONS FOR THE ESTIMATION OF THE RATE OF APPEARANCE OF FREE FATTY-ACIDS

Background This study aimed to document the applicability and variabil ity of free fatty acid (FFA) kinetic parameters during non-equilibrium and equilibrium tracer conditions in man. Methods FFA kinetic paramet ers were assessed after an overnight fast in six healthy nonobese and three obese subjects as well as in three patients with non-insulin-dep endent diabetes mellitus (NIDDM) by infusion of [C-14]-palmitate of 60 min (study A) and 10 min duration (study B). Results The kinetic para meters estimated from the upstroke and downstroke of the plasma FFA sp ecific activity curve (non-equilibrium) were not statistically differe nt within studies A and B. Furthermore, there were no significant diff erences in any of. the FFA kinetic parameters between studies A and B. The averaged plasma levels of FFA obtained during the up- and downstr oke from studies A and B were higher in obese subjects and NIDDM patie nts than in non-obese subjects (P < 0.01). The averaged total rate of appearance (TRa) of FFA was higher in obese subjects than in non-obese subjects (P < 0.02). The TRa and metabolic clearance rate (MCR), esti mated from non-equilibrium conditions, were about 25% higher than the apparent values obtained from steady-state measurement in all subjects combined (P < 0.01), suggesting considerable recirculation of. label from hydrolysis of labelled esterified fatty acids. Indeed, in three n on-obese subjects, the radiolabel in esterified fatty acids was approx imately 50% of labeled FFA at 60 min of label infusion. The coefficien ts of variation of the kinetic parameters were consistently larger in study A than in study B. Conclusion FFB kinetic parameters can be esti mated with sufficient precision using nonequilibrium data from short-t erm labelled palmitate infusion. Short-term label infusion has the adv antage that label recirculation is prevented and exposure to radiation is limited.