Whether you should work out on an empty stomach depends on your goals, the type of exercise, and your individual health status. Exercising in a fasted state, often called fasted cardio or fasted training, means performing physical activity before eating, typically in the morning after an overnight fast. This practice has been popularized with claims that it enhances fat burning and improves body composition, but the scientific evidence presents a nuanced picture.
The main argument for working out on an empty stomach is that it increases fat oxidation during exercise. When you have not eaten, your insulin levels are low, and your body is more likely to use stored fat as fuel rather than carbohydrates. Studies confirm that aerobic exercise performed in a fasted state does indeed increase fat burning during the workout itself compared to exercising after eating[2][4]. This phenomenon is called fat oxidation and is measurable in the short term.
However, increased fat burning during a single session does not necessarily translate into greater fat loss or better body composition over time. A systematic review from 2017 found that fasted exercise training programs do not lead to significantly more fat loss than non-fasted training when looking at longer-term outcomes[2][4]. The body appears to compensate for the increased fat burning during exercise by adjusting metabolism later in the day, possibly by reducing overall energy expenditure or increasing fatigue, which can reduce total daily calorie burn[2][4]. This means that while you may burn more fat during the workout, your total fat loss over weeks or months may be similar whether you eat before or after exercise.
The type of exercise also matters. Fasted cardio, which is typically moderate-intensity aerobic exercise, may benefit from fasted conditions in terms of fat oxidation. However, for high-intensity interval training (HIIT) or resistance training aimed at building muscle, working out on an empty stomach may not be optimal. Muscle growth and strength gains rely on adequate energy and nutrient availability, especially protein and carbohydrates. Some research suggests that intermittent fasting, which includes periods of fasting and feeding, can promote skeletal muscle growth and differentiation through mechanisms involving autophagy and the mTOR pathway, but this is under specific controlled conditions and may not apply to all fasted workouts[1].
From a practical standpoint, exercising on an empty stomach can be uncomfortable for some people, causing dizziness, weakness, or reduced performance, especially during high-intensity or long-duration workouts. Others may find it convenient and feel better exercising before eating. Psychological factors and personal preference play a role in adherence and consistency, which are critical for long-term fitness and health outcomes.
In terms of weight loss strategies, intermittent fasting approaches that include fasting periods but allow eating during other times have shown promise for sustainable weight loss and improved eating behaviors compared to traditional calorie restriction[3]. This suggests that timing of food intake relative to exercise can be part of a broader nutritional strategy but is not the sole determinant of success.
In summary, working out on an empty stomach can increase fat burning during the session but does not guarantee greater fat loss or better body composition in the long run. The choice should be guided by your fitness goals, the type of exercise, how your body responds, and your overall nutrition plan. For muscle building and high-intensity training, eating before exercise may be more beneficial. For moderate aerobic exercise and fat loss, fasted workouts can be an option but are not superior to fed workouts in the long term.
Sources:
[1] PMC article on intermittent fasting and muscle growth, Nutr Metab (Lond). 2025 Sep 30;22:109
[2] UNSW article on fasted cardio and fat loss science, 2025
[3] CU Anschutz Health and Wellness Center study on intermittent fasting, 2025
[4] Futura Sciences article on fasted cardio and fat oxidation, 2025
[5] Nature article on HIIT and fitness, 2025
