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Endogenous substrate oxidation during exercise and variations in breath 13CO2/12CO2
J F Gautier1, F Pirnay, B Jandrain
1Department of Medicine, University of Liège, Belgium.
Journal of Applied Physiology (Bethesda, Md. : 1985)
|January 1, 1993
Summary
Acipimox, an inhibitor of fat breakdown, significantly increased carbohydrate use during exercise in men. This shift altered expired CO2 isotope ratios, highlighting the need for controls in substrate oxidation studies.
Area of Science:
- Exercise Physiology
- Metabolic Research
- Isotope Tracing
Background:
- Understanding substrate utilization during exercise is crucial for optimizing performance and health.
- Adipose tissue lipolysis significantly contributes to energy production during prolonged physical activity.
- Accurate measurement of substrate oxidation is essential for metabolic research.
Purpose of the Study:
- To investigate the impact of inhibiting lipolysis on substrate utilization during prolonged exercise.
- To determine how altering endogenous substrate use affects the 13C/12C ratio in expired CO2.
- To assess the reliability of using 13C-labeled substrates to measure exogenous substrate oxidation under manipulated metabolic conditions.
Main Methods:
- Six healthy male volunteers performed 3-hour treadmill exercise at moderate intensity (approx. 45% max O2 uptake).
- Participants ingested either a placebo or Acipimox (a lipolysis inhibitor) 75 minutes prior to exercise.
- Indirect calorimetry and isotope-ratio mass spectrometry (IRMS) of expired CO2 were used to measure substrate oxidation and isotopic enrichment.
Main Results:
- Acipimox effectively inhibited the rise in plasma free fatty acids and glycerol, preventing increased lipid utilization.
- Total carbohydrate oxidation increased significantly under Acipimox (182 g/3h) compared to placebo (128 g/3h).
- Lipid oxidation decreased significantly with Acipimox (57 g/3h) versus placebo (84 g/3h).
- Expired CO2 delta 13C showed a significant, sustained rise under Acipimox, mimicking exogenous carbohydrate oxidation.
Conclusions:
- Inhibiting lipolysis with Acipimox markedly shifts substrate utilization towards carbohydrates during prolonged exercise.
- Changes in expired CO2 13C/12C ratios under altered substrate utilization can lead to erroneous conclusions about exogenous fuel use.
- Control experiments are mandatory when using 13C-labeled substrates and IRMS to study substrate oxidation, especially when endogenous metabolism is manipulated.