Evidence from studies examining the effects of fasted exercise is mixed: Some show it may speed weight loss, but others suggest fasted cardio can increase cortisol levels and lead to undesirable long-term effects. This was reported by Health.
Fasted cardio is doing aerobic exercise (which raises your heart rate) after fasting for at least 8 to 12 hours. Many doctors and scientists have theorized that training on an empty stomach may force the body to use fat for fuel because it does not have access to carbohydrate stores. But scientific evidence shows that actual biochemical processes are more complex.
A 2015 study found that exercising after fasting overnight was more effective at reducing body fat than exercising after breakfast. However, participants who fasted before exercise showed increased levels of the stress hormone cortisol, which can negatively impact long-term weight loss and health.
Most likely, the effectiveness of fasted cardio varies from person to person. However, it is probably safe to exercise on an empty stomach for most adults who do not have blood sugar dysregulation. Therefore, this approach can be tried, but moderate to high intensity workouts and long endurance training (more than 1.5 hours) will benefit more from eating before exercise. Eating complex carbohydrates, such as oats, whole-wheat bread, and fresh fruit, one to two hours before exercise can provide the glucose needed for physical activity.
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