Research into the effects of fasted cardio has produced mixed results, with some suggesting it may speed up weight loss, while other findings warn against increasing cortisol levels, which can lead to unwanted long-term effects. According to Health, fasted training is performing aerobic exercise after a period of fasting of at least 8-12 hours.

Doctors and scientists have theorized that exercising in a fasted state may force the body to use fats as an energy source since there is no access to carbohydrate reserves. However, scientific evidence highlights the complexity of biochemical processes.

A 2015 study demonstrated that training after an overnight fast was more effective in reducing body fat percentage than training after eating breakfast. However, participants who exercised in a fasted state experienced increased levels of the stress hormone cortisol, which could likely have a negative impact on long-term weight and overall health.

The effectiveness of fasted cardio training likely varies among individuals. Overall, for most adults without blood sugar problems, fasted exercise is likely safe. However, for best results in moderate to high intensity workouts, as well as long endurance workouts (exceeding 1.5 hours), eating before exercise is recommended. Eating complex carbohydrates 1-2 hours before physical activity, such as oats, whole grain toast and fresh fruit, can provide the body with the glucose it needs to support exercise.