These two articles explore how the body shifts between different fuel sources during feeding, fasting, and exercise—and what those shifts actually mean for metabolic health. While fasting and fasted exercise increase fat oxidation, particularly during low- to moderate-intensity exercise, high-intensity exercise relies heavily on muscle glycogen regardless of whether we have eaten beforehand.

The bigger question is whether these differences translate into meaningful long-term adaptations. Research suggests that fasting and fasted training may improve insulin sensitivity, fat oxidation, and glycogen-sparing adaptations in certain contexts, but these benefits aren't universal—and some of the benefits attributed to fasting may simply come from reduced energy intake. For high-intensity training or individuals at risk of low energy availability, exercising fasted may offer little advantage and potentially create additional problems.

Kristi argues against viewing fasting or fasted exercise as inherently superior. Instead, metabolic health depends on the interaction between nutrition, physical activity, and skeletal muscle. Fasting and fasted training can be useful tools, but their value depends on the individual, the type of exercise, and the specific goal.

Read Part 1, Part 2

https://brokenscience.org/fasting-feeding-and-exercise-part-1/