The practice of running on an empty stomach, often called fasted cardio, has gained popularity for its supposed benefits, primarily increased fat burning. However, the physiological response is complex, with both advantages and disadvantages for the runner. Understanding the science behind training in a glycogen-depleted state is crucial for making an informed decision about your running routine.
The body's energy sources on an empty stomach
When you haven't eaten, your body's primary source of quick energy—glycogen, stored carbohydrates in your muscles and liver—is low. After an overnight fast, your body is forced to rely more heavily on an alternative fuel source: stored body fat. While this sounds like an effective weight-loss strategy, the full picture is more nuanced. During low-intensity, steady-state cardio, your body can efficiently tap into fat reserves. However, for higher-intensity or longer-duration efforts, the slower conversion of fat to energy can cause performance to suffer. This fuel switch also has other physiological consequences, including hormonal changes and potential muscle breakdown.
Potential benefits of fasted running
- Increased fat utilization: Running on an empty stomach is shown to increase fat oxidation, where the body burns more fat for energy. Some studies suggest this effect can last throughout the day, although research is conflicting on whether this translates to greater total fat loss over time.
- Improved insulin sensitivity: Exercising in a fasted state can enhance the body's response to insulin, helping regulate blood sugar levels. This can be particularly beneficial for individuals with insulin resistance.
- Enhanced endurance for experienced athletes: Seasoned endurance runners may find that occasional fasted runs help their bodies become more efficient at using fat as fuel, building metabolic flexibility and resilience for long distances.
- Mental toughness: Pushing through a workout with limited fuel can challenge your mental resilience and discipline, skills that are valuable during races or tough training sessions.
- Reduced digestive issues: For runners who experience gastrointestinal discomfort from eating too close to a run, a fasted session can eliminate that problem.
Significant risks and drawbacks
Despite the potential benefits, fasted running comes with substantial downsides, especially for beginners or those engaging in high-intensity exercise.
- Decreased performance: Without readily available carbohydrates, your speed, power, and overall endurance will likely decrease. High-intensity interval training (HIIT) or long runs without fuel are often less effective.
- Higher risk of muscle breakdown: When glycogen stores are depleted, your body may turn to muscle protein for energy, a process called muscle catabolism. This can lead to a loss of valuable muscle mass over time.
- Hormonal imbalances: The combination of fasting and exercise, both of which are stressors, can elevate the stress hormone cortisol. Chronically high cortisol can hinder fat loss and potentially disrupt reproductive hormones, especially in women.
- Increased risk of hypoglycemia: Low blood sugar can cause dizziness, lightheadedness, nausea, or even fainting, which is especially dangerous during a run. Individuals with diabetes or other blood sugar conditions should exercise extreme caution.
- Hitting the wall ('bonking'): If you're a marathoner or endurance runner, running on empty can lead to a severe energy crash, where you suddenly feel like you can't continue.
Fasted vs. Fueled Running: A Comparison
| Feature | Running on an Empty Stomach (Fasted) | Running After Eating (Fueled) |
|---|---|---|
| Primary Fuel Source | Stored body fat, with a higher reliance on fat oxidation due to low glycogen. | Recently consumed carbohydrates and stored glycogen, with some fat utilization. |
| Performance (Intensity & Duration) | Limited, especially for high-intensity or long-duration workouts, due to slower fat-to-energy conversion. | Higher intensity and longer duration are possible thanks to readily available carbohydrates. |
| Fat Burning | Higher fat burning during the workout but conflicting evidence on whether this results in greater total fat loss over 24 hours. | Lower fat burning during the workout, but no significant difference in overall fat loss compared to fasted exercise in most studies. |
| Muscle Preservation | Increased risk of muscle breakdown for fuel, particularly with intense or prolonged exercise. | Muscle protein is spared as long as carbohydrates are available for fuel. |
| Gastrointestinal Issues | Generally lower risk, as there is no food to upset the stomach during exercise. | Higher risk of bloating, cramping, or nausea for those with sensitive stomachs or if eating too close to the run. |
| Recovery | Slower recovery, as glycogen stores are depleted and must be replenished post-run. | Faster and more effective recovery by providing the body with necessary nutrients immediately. |
How to safely run on an empty stomach (and who should avoid it)
For those who prefer or are curious about fasted running, it is generally considered safe for short (under 60 minutes), low-intensity sessions. Listen to your body and be aware of warning signs like dizziness, nausea, or excessive fatigue. Always prioritize hydration by drinking water before you head out, and fuel up properly within 30 minutes to an hour after your run with a balanced meal containing carbohydrates and protein to aid recovery.
However, fasted running is not for everyone. Beginners, long-distance runners (over 60 minutes), and those with medical conditions such as diabetes should avoid fasted exercise. If you have a history of disordered eating or suspect you have hormonal imbalances, consult a healthcare professional.
Conclusion
Whether you should run on an empty stomach depends on your goals, experience, and overall health. While it can enhance fat burning during the workout and improve metabolic efficiency for some, especially during short, easy runs, it can also lead to decreased performance, muscle loss, and hormonal issues, particularly for more intense or longer efforts. Research suggests that overall, fasted running does not provide a significant long-term advantage for weight loss compared to running while properly fueled. By understanding the physiological trade-offs and listening to your body, you can make the best decision for your fitness and well-being. For a comprehensive overview of how to properly fuel for runs of all types, consider this expert guide from REI.
