Is It Bad to Run After Not Eating All Day? Understanding the Risks and Rewards

October 30, 2025 •

4 min read

According to sports nutrition research, exercising in a fasted state can lead to higher fat oxidation, but skipping meals all day to run is risky. The practice of fasted running, especially after not eating for an extended period, can severely deplete your energy stores, impacting performance and health.

Quick Summary

Running after not eating all day significantly increases health risks like hypoglycemia and muscle breakdown due to depleted glycogen stores. While it may increase fat oxidation in low-intensity scenarios, performance is impaired, especially for high-intensity or long-duration runs, and it is not safe for everyone.

Key Points

Hypoglycemia Risk: Running on empty after an entire day increases the risk of low blood sugar, causing dizziness and weakness.
Performance Impairment: Glycogen depletion from fasting severely limits your ability to perform high-intensity or endurance runs effectively.
Muscle Loss: In the absence of glucose, the body may break down muscle protein for fuel through a process called gluconeogenesis.
Elevated Stress: Fasted, strenuous exercise can increase the stress hormone cortisol, potentially leading to muscle loss and compromised immunity.
Not for Beginners or High Intensity: Fasted running is only considered for short, easy efforts by experienced athletes; it is not suitable for beginners or intense workouts.
Fueling is Key for Performance: For best results and recovery, fueling with carbohydrates before exercise is generally recommended over a fasted state.

The Science of Fasted Exercise and Energy Depletion

Your body primarily fuels exercise using carbohydrates stored as glycogen in your muscles and liver. After an overnight fast, these stores are low. If you have not eaten all day, your glycogen reserves will be critically low, and your blood sugar (glucose) will be low. In this state, your body shifts to using fat for energy. While this might sound beneficial for fat loss, fat is a less efficient fuel source for high-intensity efforts, and the process of converting fat and protein into usable energy is slower than using glycogen.

The Body's Emergency Fuel System

When glycogen is depleted, the body may begin a process called gluconeogenesis, where it creates glucose from non-carbohydrate sources, including amino acids from muscle tissue. This can lead to muscle protein breakdown, which is counterproductive for strength and overall fitness. For those who have gone all day without fuel, the risk of tapping into muscle mass is much higher.

Significant Health Risks of Running on Empty

Running after a prolonged fast carries several significant health risks that can compromise both performance and safety.

List of Potential Dangers:

Hypoglycemia: Critically low blood sugar can cause dizziness, lightheadedness, weakness, nausea, and in severe cases, fainting. This poses a serious injury risk, particularly if running outdoors. This is especially risky for individuals with diabetes.
Increased Cortisol Levels: Running in a highly fasted state, particularly if it's high-intensity, can raise levels of the stress hormone cortisol. Chronically high cortisol can lead to muscle loss, increased fat storage around the midsection, and reduced training adaptation.
Muscle Protein Breakdown: As mentioned, your body may catabolize muscle tissue for fuel. This means you lose valuable muscle mass instead of gaining fitness.
Impaired Performance: With limited glycogen, you will likely feel sluggish and unable to sustain high intensity or endurance. This can make workouts feel much harder and lead to reduced overall calorie burn compared to a properly fueled session.
Weakened Immune System: Strenuous exercise in a fasted state, especially prolonged sessions, can temporarily suppress the immune system, making you more susceptible to illness.
Low Energy Availability (LEA): Long-term under-fueling, which could be replicated by running regularly on empty, can lead to Relative Energy Deficiency in Sport (RED-S). This syndrome can cause hormonal disruption, bone density loss, and fatigue.

Fasted vs. Fueled Running: A Comparison


Feature	Fasted Running (Not Eating All Day)	Fueled Running (Proper Nutrition)
Energy Source	Primarily stored fat, then muscle protein.	Primarily glucose from recent carbohydrate intake and glycogen stores.
Performance	Significantly impaired, especially high-intensity or long-duration.	Optimized for intensity and endurance, preventing fatigue.
Fat Oxidation	May be higher during the workout, but doesn't guarantee greater overall fat loss.	Fat oxidation occurs consistently as part of a healthy, regular training routine.
Risks	High risk of hypoglycemia, muscle breakdown, increased cortisol, and injury.	Minimal risk, focused on performance and recovery.
Digestive Comfort	Can avoid stomach issues associated with pre-run meals.	Can cause GI distress if timing or food choices are poor.
Recovery	Impaired due to depletion of glycogen and potential muscle damage.	Enhanced by post-run protein and carbohydrate replenishment.
Best Use Case	Occasional, easy, short runs for experienced, healthy individuals.	All types of runs, especially high-intensity intervals or long-distance.

Best Practices for Safer Fasted Running (For Short Periods Only)

For healthy, experienced runners interested in occasional, low-intensity fasted training, the "not eating all day" scenario should be avoided. A safer approach involves an overnight fast, typically done in the morning. Here's how to do it correctly:

Stick to Low Intensity: Limit fasted runs to easy-paced, low-intensity cardio, such as a jog or brisk walk. High-intensity exercise demands glucose, which your body won't have in a fasted state.
Keep it Short: Don't exceed 30-60 minutes. The longer you go, the more you deplete your energy stores and increase risk.
Prioritize Hydration: Drink plenty of water before and during your run. Consider an electrolyte supplement to help maintain balance, as fasting can lead to greater sodium loss.
Monitor Your Body: Listen closely to your body. If you feel dizzy, nauseous, or excessively weak, stop immediately and refuel.
Refuel Post-Run: The recovery period is crucial. Replenish your glycogen and repair muscle tissue with a balanced meal of carbohydrates and protein within 30-60 minutes of finishing.

A Note on Extreme Fasting

The practice of regularly running after not eating for an entire day is not a recommended training strategy and carries significant health risks. It is a form of severe energy deficit that can lead to compromised performance, health issues, and a potential for disordered eating. Most athletes and health experts advise against it, emphasizing that fueling for performance is a far more effective and safer long-term strategy. For more detailed information on athletic performance and nutrition, consulting a resource like Examine.com can be beneficial.

Conclusion

While some potential benefits of short fasted workouts exist for experienced and healthy individuals, running after not eating all day is a dangerous practice. The risks of hypoglycemia, muscle breakdown, and impaired performance far outweigh any perceived benefits. For optimal health and training, it is recommended to fuel your body correctly before and after exercise. Listen to your body and prioritize consistent, well-fueled training over extreme or unsafe practices.

Optional Outbound Link

For further reading on the pros and cons of fasted cardio, check out this comprehensive guide: The Pros and Cons of Running on an Empty Stomach.

Frequently Asked Questions

Immediate symptoms can include dizziness, lightheadedness, a shaky feeling, fatigue, and a sudden drop in energy due to low blood sugar (hypoglycemia) and depleted glycogen stores.

While running in a fasted state may cause your body to burn a higher percentage of fat during the workout, this doesn't necessarily lead to greater overall fat loss. Consistent, fueled training can be more effective for long-term weight management by improving exercise intensity and duration.

Occasional, short, and easy-paced fasted runs are generally considered safe for healthy and experienced individuals. However, running after a prolonged fast of an entire day significantly increases risks and is not recommended.

Signs of low blood sugar include feeling shaky, weak, dizzy, or confused. It is important to stop exercising immediately and consume a quick source of carbohydrates if you experience these symptoms.

When your primary fuel source, glycogen, is severely depleted from prolonged fasting, your body may use protein from muscle tissue to create glucose for energy, a process called gluconeogenesis. This is less efficient and leads to muscle breakdown.

Individuals with certain medical conditions, such as diabetes or hypoglycemia, should avoid running on an empty stomach. Beginners and those planning high-intensity or long-duration runs should also always fuel beforehand.

Stop running immediately. Sit or lie down to prevent fainting and injury. Consume a fast-acting carbohydrate source, like a sports drink or a piece of fruit, to quickly raise your blood sugar levels.