The Error in the Estimation of Net Leg Total FFA Balance from Palmitate Balance is Small at Rest and During Exercise

Abstract

The study of plasma free fatty acid (FFA) balance across the limb is complicated by the fact that FFA are simultaneously taken up and released. The addition of fatty acid tracer (i.e. palmitate) infusion to arteriovenous balance techniques allows for the partitioning of net limb FFA balance into uptake and release. This method relies on the estimation of total FFA balance from palmitate balance and assumes that the net balances of the remaining individual FFA (∼72% of total FFA pool) will be in the same direction as palmitate with a magnitude that is in direct proportion to their respective concentrations. PURPOSE: To compare directly measured net leg total FFA balance data with those that are estimated from net leg palmitate balance at rest and during exercise. METHODS: Eight men (26 ± 1 yr, 77.4 ± 3.7 kg) were infused with [1-13C]palmitate during 90 min of rest and 60 min of exercise at 45 and 65% VO2peak. Femoral arterial and venous blood samples drawn simultaneously at rest and during exercise were analyzed for individual FFA concentrations and palmitate enrichment and limb blood flow was determined by thermodilution. RESULTS: Directly measured net leg total FFA balance data were highly correlated to estimated values at rest (r = 0.96; P <0.01) and during exercise (r = 0.89; P <0.01). The absolute residuals (estimated minus directly measured net leg total FFA balance) were smaller at rest (±6.3 ±7.3 umol'min1) than during exercise (range of −14.2 ± 47.8 to −29.4 ± 50.0 umol'min1). The residuals were standardized by dividing rest and exercise values by their respective average SD of the residuals to account for the much smaller net leg total FFA balance values at rest compared to exercise (∼ −46 vs. 188 umol'min1). These standardized residuals were similar at rest and during exercise. Examination of Cohen's d values indicated a small effect size at rest and during exercise (Cohen's d = +0.31 and −0.11 to −0.25, respectively). Similar results were found when a fatty acid with a greater relative concentration than palmitate (oleate, ∼ 41 % of total FFA pool) was used to estimate net leg total FFA balance. CONCLUSION: The error of estimation of net leg total FFA balance from palmitate balance is small at rest and during moderate intensity exercise allowing for the accurate determination of tracer-derived leg FFA uptake and release. Supported by NIH Grant R01 AR42906