Metabolism of parenterally administered fat emulsions in the rat: studies of fatty acid oxidation with 1-13C- and 8-13C-labelled triolein

Inhibition of fatty acid β-oxidation attenuates the reinforcing effects and palatability to fat

OBJECTIVE

We investigated the effects of the energy signal on the reinforcing effects and palatability to fat in mice.

METHODS

To examine the effects of postingestive energy signal, mice were injected with 400 micromol/kg body mass of mercaptoacetate, a beta-oxidation blocker or with saline (control group). Reinforcing effects and palatability response were examined with the conditioned place preference and one-bottle tests, respectively.

RESULTS

In conditioned place preference tests, the mercaptoacetate group exhibited reinforcing effects when offered a 40% sucrose solution, which is not metabolized by the beta-oxidation pathway, but not when offered 100% corn oil. The control group exhibited reinforcing effects when offered the sucrose solution and when offered corn oil. In one-bottle tests, the control group exhibited palatability to corn oil, but the mercaptoacetate group did not. Consumption of sucrose solution was identical for both groups.

CONCLUSIONS

These results suggest that postingestive energy signal influences the reinforcing effects and palatability of fat.

Breath [13CO2] recovery from an oral glucose load during exercise: comparison between [U-13C] and [1,2-13C]glucose

The purpose of the present experiment was to compare 13CO2 recovery at the mouth, and the corresponding exogenous glucose oxidation computed, during a 100-min exercise at 63 +/- 3% maximal O2 uptake with ingestion of glucose (1.75 g/kg) in six active male subjects, by use of [U-13C] and [1,2-13C]glucose. We hypothesized that 13C recovery and exogenous glucose oxidation could be lower with [1,2-13C] than [U-13C]glucose because both tracers provide [13C]acetate, with possible loss of 13C in the tricarboxylic acid (TCA) cycle, but decarboxylation of pyruvate from [U-13C]glucose also provides 13CO2, which is entirely recovered at the mouth during exercise. The recovery of 13C (25.8 +/- 2.3 and 27.4 +/- 1.2% over the exercise period) and the amounts of exogenous glucose oxidized computed were not significantly different with [1,2-13C] and [U-13C]glucose (28.9 +/- 2.6 and 30.7 +/- 1.3 g, between minutes 40 and 100), suggesting that no significant loss of 13C occurred in the TCA cycle. This stems from the fact that, during exercise, the rate of exogenous glucose oxidation is probably much larger than the flux of the metabolic pathways fueled from TCA cycle intermediates. It is thus unlikely that a significant portion of the 13C entering the TCA cycle could be diverted to these pathways. From a methodological standpoint, this result indicates that when a large amount of [13C]glucose is ingested and oxidized during exercise, 13CO2 production at the mouth accurately reflects the rate of glucose entry in the TCA cycle and that no correction factor is needed to compute the oxidative flux of exogenous glucose.

Whole body and leg acetate kinetics at rest, during exercise and recovery in humans

We have used a constant [1,2-(13)C]acetate infusion (0.12 micromol x min(-1) x kg( 1)) for 2 h at rest, followed by 2 h of one-legged knee-extensor exercise at 65% of leg maximal workload, and 3 h of recovery in six post-absorptive volunteers to quantify whole-body and leg acetate kinetics and determine whether the whole-body acetate correction factor can be used to correct leg substrate oxidation. The acetate whole-body rate of appearance (R(a)) was not significantly different at rest, during exercise or during recovery (365-415 micromol x min(-1)). The leg net acetate uptake was similar at rest and during recovery (approximately 10 micromol x min(-1)), but increased approximately 5-fold with exercise. At rest the leg acetate uptake (approximately 15 micromol x min(-1)) and release (approximately 5 micromol x min(-1)) accounted for 4 and 1.5 % of whole-body acetate disposal (R(d)) and R(a), respectively. When the leg acetate kinetics were extrapolated to the total body skeletal muscle mass, then skeletal muscle accounted for approximately 16 and approximately 6% of acetate R(d) and R(a). With exercise, leg acetate uptake increased approximately 6-fold, whereas leg acetate release increased 9-fold compared with rest. Whole-body acetate carbon recovery increased with time of infusion at rest and during recovery from 21% after 1.5 h of infusion to 45% in recovery after 7 h of infusion. Leg and whole-body acetate carbon recovery were similar under resting conditions, both before and after exercise. During exercise whole-body acetate carbon recovery was approximately 75%, however, acetate carbon recovery of the active leg was substantially higher (approximately 100%). It is concluded that inactive skeletal muscle plays a minor role in acetate turnover. However, active skeletal muscle enhances several-fold acetate uptake and subsequent oxidation, as well as release and its contribution to whole-body acetate turnover. Furthermore, under resting conditions the whole-body acetate correction factor can be used to correct for leg, skeletal muscle, substrate oxidation, but not during exercise.

Differential oxidation of individual dietary fatty acids in humans

BACKGROUND

Dietary fatty acids that are more prone to oxidation than to storage may be less likely to lead to obesity.

OBJECTIVE

The aim of this study was to determine the effect of chain length, degree of unsaturation, and stereoisomeric effects of unsaturation on the oxidation of individual fatty acids in normal-weight men.

DESIGN

Fatty acid oxidation was examined in men consuming a weight-maintenance diet containing 40% of energy as fat. After consuming the diet for 1 wk, subjects were fed fatty acids labeled with (13)C in the methyl or carboxyl position (10 mg/kg body wt). The fatty acids fed in random order were laurate, palmitate, stearate, oleate, elaidate (the trans isomer of oleate), linoleate, and linolenate blended in a hot liquid meal. Breath samples were collected for the next 9 h and the oxidation of each fatty acid was assessed by examining liberated (13)CO(2) in breath.

RESULTS

Cumulative oxidation over the 9-h test ranged from a high of 41% of the dose for laurate to a low of 13% of the dose for stearate. Of the 18-carbon fatty acids, linolenate was the most highly oxidized and linoleate appeared to be somewhat conserved. (13)C recovery in breath from the methyl-labeled fatty acids was approximately 30% less than that from the carboxyl-labeled fatty acids.

CONCLUSIONS

In summary, lauric acid is highly oxidized, whereas the polyunsaturated and monounsaturated fatty acids are fairly well oxidized. Oxidation of the long-chain, saturated fatty acids decreases with increasing carbon number.