Can we switch to fat burning?

Can our body simply switch to fat burning when we are physically active? How does fat burning actually work across different levels of exertion and when is it at its highest? What do I need to consider when training to burn fat? We will summarize the points briefly and succinctly as follows:

Energy supply in the muscle

• ATP is the primary energy source for muscle contractions
• There are four major energy systems: phosphocreatine, glycolysis, carbohydrate oxidation and fat oxidation
• These systems differ in speed and capacity of ATP production
• The breakdown of glucose in anaerobic glycolysis yields 2 ATP, while the aerobic breakdown of glucose via pyruvate to CO2 and H2O generates more than 30 ATP(1)

Fat and carbohydrate metabolism

• Both processes always run in parallel, there is no "switch"
• Fat metabolism requires hydrogen and carbon, which it obtains from carbohydrate metabolism(1)
• Acetyl-CoA is the common fuel for aerobic metabolism, regardless of the source
• The intensity of the load determines the ratio of fat to carbohydrate burning

Influence of training intensity

• At low to moderate intensity, fat oxidation increases
• At high intensity, carbohydrate burning increases, especially from muscle glycogen
• Maximum fat burning is often around 65% of VO2max, but varies from individual to individual

This limit for maximum fat burning (also called FatMax) is also the reason for distinguishing the product BASIC from the product LOAD .

In his article on metabolism ( LINK ), Asker Jeukendrup emphasizes that both metabolic pathways are always active and their relative contributions change depending on the training intensity(2).

Sources:
(1) Peter C. Heinrich et al.: Biochemistry and Pathobiochemistry, Springer
(2) Asker Jeukendrup: The myth of switching to fat metabolism