Measuring VLDL-triglyceride turnover in humans using ex vivo-prepared VLDL tracer

There has been more interest in VLDL-triglyceride (TG) kinetics during the last decade. Unfortunately, robust measurement methods are elaborate and not readily available. Here, we describe a method using unique, ex vivo labeling of the fatty acid moiety of VLDL-TG followed by intravenous bolus infusion in the same person. We found that plasma disappearance of ex vivo-labeled VLDL-TG was comparable to that of in vivo-labeled VLDL-TG and that turnover rates can be safely estimated from the log linear decay of VLDL- TG specific activity. We found minor labeling of the plasma FFA (oleate) pool, which was largely attributable to coinfusion of free [C-14] triolein; VLDL-TG did not contribute substantially to the plasma FFA pool. The plasma decay curve of VLDL-TG was not affected by the presence of tracer in the FFA pool, provided that the data from 2 h after the VLDL tracer bolus infusion was used. The FFA contamination problem was circumvented by minor modification of the VLDL-TG tracer preparation. The approach we describe should expand the opportunity to study processes that cannot be assessed if the FFA precursor pool is labeled. This method for VLDL-TG tracer preparation can allow measurement of VLDL turnover, tissue uptake of VLDL-TG, and oxidation of VLDL- TG.