Burning fat and carbohydrate during exercise

Body exercise and burning fat

In a paper published in The Journal of Physiology, Helge, Stallknecht, Richter, Galbo, and Keins from Copenhagen shed light on fat oxidation during exercise and physical activity. Their observations suggest that fat oxidation during exercise reflects a fine interplay between the cardiovascular, neurological, endocrine and muscle metabolic systems.

During exercise and physical activity, the primary fuels used by muscles are carbohydrate and fat. When mild exercise is performed there is a tendency to burn relatively more fat and less glucose, but as exercise becomes more intense, a higher fraction of the energy demands of the muscle are supplied by glucose, until at the highest intensities almost only carbohydrates are used. Is this shift in fuel source a property of the muscle itself, or does it represent the interplay between what is happening in the muscle and the exercise-related responses in the rest of the body"

The study, performed at the Copenhagen Muscle Research Centre at the University of Copenhagen, examined muscle fuel utilisation in response to graded exercise performed with only one leg. Nine healthy males performed one-leg exercise at 25, 45, and 85% of maximal workload. Their results showed that, when only a small mass of muscle is contracting, and blood flow and oxygen supply are not limited by central circulatory capacity, the shift in fuel source from fat to glucose as exercise intensity increases does not occur.

Helge et al.