What do you burn on an empty stomach? The surprising truth about fasted cardio

October 16, 2025 •

5 min read

Research has shown that aerobic exercise performed in a fasted state can induce higher fat oxidation during the workout compared to exercising in a fed state. This has led many to question what do you burn on an empty stomach, and whether it’s a superior strategy for weight loss. The reality, however, is more nuanced than simple fat-burning claims suggest.

Quick Summary

When exercising on an empty stomach, your body relies more heavily on stored fat for energy due to low glycogen and insulin levels. While this increases fat oxidation during the workout itself, studies show it doesn’t necessarily translate to greater long-term fat or weight loss compared to fed exercise. Fasted training may offer metabolic benefits like improved insulin sensitivity, but also poses risks such as reduced performance and potential muscle breakdown.

Key Points

Fat vs. Carbs: On an empty stomach, your body's glycogen stores are lower, causing it to rely more heavily on stored fat for energy during exercise.
Acute vs. Chronic Effects: While fasted exercise increases the percentage of fat burned during the session, this doesn't consistently lead to greater overall fat loss compared to fed exercise, as the body compensates later.
Performance Considerations: High-intensity workouts and longer endurance sessions are generally hindered by fasted training due to the lack of readily available carbohydrates for fuel.
Risks of Fasted Training: Exercising on an empty stomach can increase risks like hypoglycemia, muscle breakdown, and higher cortisol levels, especially if done with high intensity.
Metabolic Flexibility: Regular exercise, whether fasted or fed, can improve metabolic flexibility—the body's ability to efficiently switch between fuel sources based on availability.
Personal Preference Matters: The decision to train fasted or fed depends on individual goals, exercise intensity, and how one's body feels. Consistency is the most important factor for long-term results.
The Calorie Deficit is Key: Ultimately, a sustainable calorie deficit over time is the most crucial element for significant fat and weight loss, overshadowing the specific fuel source burned in any single workout.

The Metabolic Switch: Fueling Your Body

During a state of rest or exercise, your body uses two primary fuel sources for energy: carbohydrates and fats. When you eat, your body preferentially uses the glucose from those carbohydrates for immediate energy, converting any excess into glycogen for storage in the liver and muscles. When you haven't eaten for several hours—for example, after an overnight fast—your body's readily available glucose levels are lower. As a result, it shifts its metabolic strategy and relies more on stored fat to meet its energy demands. This physiological switch is a cornerstone of why exercising on an empty stomach is believed to increase fat burning.

The role of glycogen and insulin

After an overnight fast, your liver's glycogen stores are partially depleted, which is the key to understanding fasted training. Low insulin levels further facilitate the breakdown of stored fat for fuel, a process known as lipolysis. With limited glucose available, the body has no choice but to tap into its fat reserves to keep moving. This leads to a higher rate of fat oxidation during the exercise session itself. It is this acute effect that has led to the popularization of 'fasted cardio' for weight loss, though the long-term impacts require more careful consideration.

The reality of fat burning on an empty stomach

It is scientifically established that exercising on an empty stomach increases the percentage of fat burned during the workout. However, this is only part of the story. The total calories burned over a 24-hour period often remain similar regardless of whether you train fed or fasted. Your body is a complex system that seeks balance. If you burn more fat during a morning fasted workout, it may simply compensate by burning less fat and more carbohydrates later in the day when you eat. This compensatory effect is why short-term fat oxidation does not always lead to greater long-term fat loss.

Acute vs. long-term results

The discrepancy between acute fat oxidation and long-term body composition changes is a critical point. While you may see a temporary spike in fat burning during your fasted session, meta-analyses and systematic reviews have shown that there is no statistically significant difference in overall fat loss between fasted and fed cardio groups when total daily calorie intake is the same. This reinforces the fundamental principle that sustainable weight loss is ultimately determined by maintaining a meaningful calorie deficit over time, not just what is burned in a single workout.

Benefits of fasted training

Beyond the potential—and debated—impact on fat loss, fasted training may offer other health benefits. Some studies indicate that regular fasted exercise can enhance whole-body glucose tolerance and improve insulin sensitivity. For individuals with blood sugar regulation issues, this could be a point of interest, though they should consult a healthcare provider before starting. For many people, working out first thing in the morning on an empty stomach can also be a simple matter of convenience, allowing them to fit exercise into a busy schedule without the discomfort of a full stomach.

Risks and considerations of fasted exercise

While fasted training can be beneficial for some, it is not without potential downsides. For high-intensity exercise, such as heavy weight training or long, intense cardio, your body's preferred fuel is glycogen. Without sufficient carbohydrates, performance can suffer, leading to less effective workouts. In some cases, the body may even turn to muscle protein for fuel through a process called gluconeogenesis, which is counterproductive for those aiming to build or maintain muscle mass. Other risks include:

Hypoglycemia: A dangerous drop in blood sugar that can cause dizziness, nausea, or fainting.
Increased Cortisol: Some experts suggest the combination of fasting and high-intensity exercise can chronically elevate the stress hormone cortisol, potentially leading to increased fat storage and muscle breakdown.
Dehydration: Exercising without first consuming fluids can increase the risk of dehydration, especially during more intense sessions.
Post-Workout Hunger: Exercising on an empty stomach can increase appetite, which may lead to overeating later in the day and negating any calorie-burning benefits.

Fasted vs. fed exercise: A comparison


Aspect	Fasted Training (on an empty stomach)	Fed Training (after eating)
Fuel Source	Higher percentage of stored fat and some glycogen, especially during low-to-moderate intensity.	Higher percentage of readily available carbohydrates (glucose/glycogen) from the recent meal.
Acute Fat Oxidation	Higher rate of fat burning during the exercise session.	Lower rate of fat burning during the exercise session, as carbs are the primary fuel.
Long-Term Fat Loss	Studies show no significant difference in overall fat loss compared to fed training, provided total daily calories are equal.	No significant difference in overall fat loss compared to fasted training, based on total daily calorie intake.
Exercise Performance	May be hindered, especially during high-intensity or long-duration workouts due to low glycogen.	Generally supports better performance, especially for longer or more intense sessions.
Muscle Loss Risk	Elevated risk of muscle breakdown (catabolism) to produce glucose if glycogen is depleted.	Lower risk of muscle breakdown as the body has readily available carbs for fuel.
Insulin Sensitivity	Potential for improved insulin sensitivity over time.	Standard exercise benefits, but without the specific insulin sensitivity adaptations of fasted training.
Convenience	Can be more convenient for morning workouts, as no meal is needed beforehand.	Requires planning to eat a digestible snack or meal prior to exercise.

Personalize your approach for success

For most individuals, the optimal approach to exercise fueling is a matter of personal preference, tolerance, and specific fitness goals. For a low-to-moderate intensity morning walk or light jog, exercising on an empty stomach is generally safe and might feel more comfortable. If your goal is high-performance weightlifting or an intense endurance session, fueling up beforehand is a better strategy to ensure you have the energy needed to maximize your workout's effectiveness and protect muscle mass. The ultimate determinant of long-term body composition changes is not when you eat, but what you eat and how consistently you train. Focusing on a balanced diet and regular physical activity, whether fasted or fed, is the most effective path to success. For personalized dietary advice, it is always recommended to consult with a registered dietitian or healthcare provider. A useful resource on exercise and diet can be found on the American Diabetes Association's website, which discusses effective fueling strategies.

Conclusion

While the concept of increased fat burning from exercising on an empty stomach holds some metabolic truth, it is a misleading simplification of the overall weight loss process. The body is a highly adaptive system that compensates for acute fuel utilization, meaning higher fat oxidation during a workout does not guarantee greater long-term fat loss. Fasted training may offer benefits like improved insulin sensitivity, but it also carries risks, especially during high-intensity exercise. Ultimately, the choice to train fasted or fed should align with your personal goals, intensity level, and comfort. The most important factor for success is consistency in a healthy diet and regular exercise, regardless of the timing.

Frequently Asked Questions

Exercising on an empty stomach can increase the percentage of fat burned during the workout because your body has less readily available carbohydrate fuel. However, studies show this doesn’t necessarily lead to greater overall fat loss compared to exercising after a meal.

For most healthy individuals, low-to-moderate intensity exercise on an empty stomach is generally safe. However, high-intensity or long-duration fasted exercise can lead to reduced performance, dizziness, and potential muscle loss.

Low-to-moderate intensity aerobic exercises are typically recommended for fasted training, as they rely more on fat for fuel. Examples include brisk walking, light jogging, cycling, or yoga for durations of 30 to 45 minutes.

Yes, especially during intense or prolonged fasted exercise. If glycogen stores are depleted, your body may start breaking down muscle protein for energy through a process called gluconeogenesis.

For low-intensity exercise, your body's fat stores provide sufficient energy. However, for high-intensity efforts, the lack of readily available carbohydrate fuel (glucose) will likely hinder your performance and endurance.

Potential benefits include convenience, the possibility of improved insulin sensitivity over time, and a focus on fat oxidation during the workout itself. However, these benefits must be weighed against potential risks.

After a fasted workout, it is important to refuel with a balanced meal containing both carbohydrates to replenish glycogen and protein to support muscle repair. Examples include eggs with veggies, oatmeal with fruit, or a protein shake.

The current scientific evidence is mixed and largely does not support the claim that fasted exercise leads to superior weight loss. The most consistent predictor of weight loss is a long-term calorie deficit, regardless of meal timing.