Fat oxidation tracks with fatty acid availability at low but not high plasma fatty acid concentrations

Abstract

The purpose of this study was to examine the effect of changes in fatty acid (FA) availability on resting FA oxidation and reesterification. We measured whole‐body fat oxidation (indirect calorimetry) and plasma concentrations of FA and glycerol in seven healthy, non‐obese women (BMI: 22.9±1.0 kg/m2) during and after an overnight (16h) lipid and heparin infusion (LIPID; 0.11 g fat/kg/h) compared with an overnight infusion of saline (SALINE). Although LIPID increased plasma FA concentration more than 2‐fold compared with SALINE (1.10±0.16 vs. 0.41±0.09 mM at 0700h the next morning, P<0.05), this augmented FA availability did not increase fat oxidation. The infusions were stopped at 0700h, and by 0900h plasma FA concentration during LIPID dropped to half that found during SALINE (0.20±0.06 vs. 0.43±0.07 mM, P<0.05). Importantly, this fall in plasma FA concentration was paralleled by a lower fat oxidation in LIPID vs. SALINE (2.0±0.6 vs. 4.0±0.5 umol/kg/min, P<0.05). Up to 3h after the infusions the ratio of plasma FA‐to‐glycerol concentration was lower during LIPID vs SALINE (3.76±0.49 vs. 8.02±2.71; P<0.05), suggesting LIPID had a persistent effect on endogenous fat metabolism by increasing FA reesterification. These data indicate that: 1) plasma FA availability influences lipid oxidation at relatively low but not high plasma FA concentrations, and, 2) elevated FA availability may augment intracellular FA reesterification, thereby limiting endogenous release of FA for oxidation and other metabolic fates.