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Fuel Selection in Skeletal Muscle Exercising at Low Intensity; Reliance on Carbohydrate in Very Sedentary Individuals
Neusha Barakati1,2, Rocio Zapata Bustos1,2, Dawn K Coletta1,2,3
1Division of Endocrinology, Department of Medicine, The University of Arizona, Tucson, Arizona, USA.
Metabolic Syndrome and Related Disorders
|November 1, 2022
Summary
Sedentary muscle prefers carbohydrates over fat during exercise, regardless of insulin sensitivity. Low mitochondrial content, not insulin resistance, drives this fuel preference in inactive individuals.
Area of Science:
- Exercise Physiology
- Metabolic Regulation
- Skeletal Muscle Biology
Background:
- Insulin resistance is linked to metabolic inflexibility in resting skeletal muscle, favoring carbohydrate over lipid oxidation.
- Measuring fuel oxidation in working skeletal muscle is challenging due to indirect calorimetry limitations.
- The "ΔRER" method offers a way to estimate fuel utilization in exercising muscle.
Purpose of the Study:
- To assess fuel oxidation in working skeletal muscle using the novel "ΔRER" method.
- To investigate the influence of insulin sensitivity and exercise intensity on muscle fuel selection.
- To determine the role of mitochondrial content in dictating fuel choice during exercise in sedentary individuals.
Main Methods:
- Whole-body indirect calorimetry and glucose clamps were used in sedentary volunteers during graded cycle ergometry (15-45 watts).
- The "ΔRER" method was calculated to estimate respiratory exchange ratio (RER) in exercising muscle.
- Muscle biopsies were analyzed for mitochondrial content (citrate synthase activity).
Main Results:
- Active muscle predominantly oxidized carbohydrate at moderate exercise intensities (30-45 watts), irrespective of insulin sensitivity.
- The fraction of fuel derived from fat oxidation was significantly predicted by exercise power output and citrate synthase activity.
- Low mitochondrial content, indicated by lower citrate synthase activity, was a key determinant of fuel choice.
Conclusions:
- In sedentary individuals, working skeletal muscle preferentially utilizes carbohydrates during mild to moderate exercise.
- Mitochondrial content, rather than insulin sensitivity, appears to be the primary driver of fuel selection in this population.
- The "ΔRER" method provides a viable approach for estimating fuel oxidation in exercising skeletal muscle.
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