This question is about Running
You can run on an empty stomach, as research shows that people who fasted and trained at lower intensity levels (50%-70% of maximum heart rate) had better endurance levels compared to those who ate before going on a run . This means that if your run is under 60 minutes, you may be able to run on an empty stomach and immediately refuel afterwards with little detriment to performance.
However, if your run is over 60 minutes (or if you exceed 80% of your maximum heart rate), it’s recommended to eat some carbohydrates and electrolytes (in the form of a sports drink or gel) mid-run to maintain energy.
Keep in mind that if you feel light-headed, dizzy, or disoriented, stop to refuel and rehydrate, as these are signs that your body needs food.
Zouhal, H., Saeidi, A., Salhi, A., Li, H., Essop, M. F., Laher, I., Rhibi, F., Amani-Shalamzari, S., & Ben Abderrahman, A. (2020). Exercise Training and Fasting: Current Insights. Open access journal of sports medicine, 11, 1–28. https://doi.org/10.2147/OAJSM.S224919