In this second part of her series, Kristi Storuschuk explores how exercising in a fasted state impacts metabolism and fitness adaptations compared to exercising in a fed state.

In the fasted state, fat oxidation is increased due to lower insulin and higher levels of fat-mobilizing hormones, leading to greater use of fat as fuel—especially during low- to moderate-intensity exercise. In contrast, eating before exercise raises insulin, suppresses fat breakdown, and shifts the body toward using carbohydrates, particularly at moderate intensities. However, at high intensities, the body relies heavily on muscle glycogen regardless of whether you've eaten or not, so the feeding state has less of an effect.

Research shows that fasted exercise can enhance metabolic health markers such as insulin sensitivity, especially in overweight or insulin-resistant individuals. These studies also suggest that fasted training may promote better fat adaptation, glycogen sparing, and potentially improved endurance adaptations. That said, the benefits depend on the type of exercise and the person—high-intensity workouts may not benefit from fasted training, and athletes need to be mindful of energy availability. Ultimately, fasted exercise can be a useful tool for health and performance when tailored to individual needs and goals.

https://brokenscience.org/fasting-feeding-and-exercise-part-2/