Muscle fat provides energy during moderate exercise in trained men who have not eaten

At rest, fat oxidation provided 66 ± 5% of total energy expenditure in trained male cyclists.

• During 120 minutes of moderate intensity exercise, fat oxidation rates increased, correlating well with changes in plasma free fatty acid concentrations.
• The contribution of free fatty acids (FFA), other fat sources, plasma glucose, and muscle glycogen to total energy expenditure during exercise was 28 ± 3%, 15 ± 2%, 12 ± 1%, and 45 ± 4%, respectively.
• Both the use of other fat sources and muscle glycogen declined with the duration of exercise.
• Fluorescence microscopy showed a 62 ± 7% net decline in muscle lipid content in type I fibres immediately after exercise, with no change during recovery.
• Intramuscular triglyceride stores are an important substrate source during moderate intensity exercise in endurance-trained male athletes following an overnight fast.

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