A NEW CORRECTION FACTOR FOR USE IN TRACER ESTIMATIONS OF PLASMA FATTY-ACID OXIDATION

The purpose of this study was to acquire a new correction factor for u se in tracer estimations of plasma fatty acid oxidation that would ful ly account for label fixation during the infusion of fatty acid tracer s. Thus volunteers were infused with C-13-labeled fatty acids and [1-C -14]acetate in the basal state, during hyperinsulinemia-hyperglycemia (clamp), and during I h of cycling exercise. The fractional recovery o f acetate label (i.e., the acetate correction factor) was 0.56 +/- 0.0 2, 0.50 +/- 0.03, and 0.80 +/- 0.03 in the basal state and during the clamp and exercise, respectively. Isotopically determined plasma fatty acid oxidation rates (mu mol . kg(-1). min(-1)) were 1.7 +/- 0.2, 0.8 +/- 0.2, and 6.4 +/- 0.5 (no correction); 2.1 +/- 0.2, 1.0 +/- 0.2, a nd 6.7 +/- 0.5 (bicarbonate correction); and 3.1 +/- 0.2, 1.5 +/- 0.2, and 8.2 +/- 0.4 (acetate correction). We conclude that use of the ace tate correction factor in place of the bicarbonate correction factor s hould improve the accuracy of isotopic measurements of plasma fatty ac id oxidation, because it accounts for label fixation that might occur at any step between the entrance of labeled acetyl-CoA into the tricar boxylic acid cycle until the recovery of label in breath CO2.