Understanding How Does Not Eating Affect Your Cardio Performance?

October 11, 2025 •

5 min read

Some research indicates that performing cardio in a fasted state can lead to higher fat oxidation, or fat burning, during a workout. However, this practice, commonly known as fasted cardio, prompts an important question: how exactly does not eating affect your cardio performance and overall health?

Quick Summary

Exercising on an empty stomach, or fasted cardio, can increase the use of stored fat for fuel but may compromise intensity and endurance during longer or high-intensity workouts. It presents risks like low blood sugar and potential muscle loss. Personal goals and tolerance determine if this approach is suitable.

Key Points

Fasted vs. Fed Fuel: In a fasted state, the body primarily uses stored fat for energy due to depleted glycogen, while a fed state relies on recent carbohydrate intake.
Performance Limitations: High-intensity or prolonged cardio sessions are negatively affected by fasting, leading to reduced endurance, earlier fatigue, and lower overall intensity.
Muscle Loss Risk: Exercising intensely on an empty stomach can trigger gluconeogenesis, where the body breaks down muscle protein for fuel, potentially hindering muscle growth and repair.
Hypoglycemia Warning: A primary risk of fasted cardio is low blood sugar, which can cause dizziness, nausea, and fainting, especially in individuals with metabolic conditions.
Listen to Your Body: The best approach depends on individual tolerance, fitness level, and goals. Experts recommend starting slow and prioritizing hydration and recovery.
Post-Workout Nutrition is Key: Consuming a balanced meal with carbohydrates and protein after any workout, especially a fasted one, is crucial for effective muscle recovery and energy replenishment.

The Science Behind Fasted Cardio

When you engage in a cardiovascular workout, your body needs energy to fuel your muscles. This energy comes from two primary sources: carbohydrates, which are stored as glycogen in your muscles and liver, and fat, stored in adipose tissue. In a fed state, your body preferentially uses carbohydrates from your recent meal and stored glycogen for immediate, high-intensity energy. In a fasted state, typically after an overnight fast of 8-12 hours, your glycogen stores are depleted, forcing your body to rely more on stored fat for fuel.

This shift in energy source is the central premise of fasted cardio. The idea is that with less glycogen available, your body becomes more efficient at tapping into fat reserves, potentially increasing fat burning. While some small studies have shown a higher rate of fat oxidation during fasted, low-to-moderate intensity exercise, this does not always translate to greater overall fat loss. The body is a complex system that seeks to maintain balance, and any increased fat burning during exercise may be compensated for by burning more carbohydrates later in the day.

Impact on Performance and Energy Levels

Fuel is essential for optimal exercise performance. Without a ready supply of carbohydrates, your body's ability to perform at high intensities or for extended durations is compromised. This effect becomes more pronounced the higher the intensity or the longer the workout. Signs of low fuel can include fatigue, lightheadedness, and reduced stamina. A runner might notice they can't maintain their usual pace, or someone in a HIIT class might be unable to complete all the reps at maximum effort. Conversely, for lighter activities like a brisk walk or moderate cycling, the impact on performance is less noticeable, and the potential for fat oxidation might be marginally higher.

Risks of Exercising on an Empty Stomach

While fasted cardio can be effective for some individuals, it is not without risks. One of the most common is the risk of hypoglycemia, or low blood sugar. This can cause symptoms like dizziness, shaking, nausea, or even fainting, which can be dangerous, especially when exercising away from home.

Another significant risk is muscle breakdown. In the absence of sufficient carbohydrates for fuel, the body can resort to a process called gluconeogenesis, converting protein from muscle tissue into glucose for energy. This can be counterproductive for those aiming to build or maintain muscle mass. Additionally, fasted workouts, particularly high-intensity ones, can increase levels of the stress hormone cortisol, which can also contribute to muscle breakdown if elevated chronically.

Fasted vs. Fed Cardio: A Comparison


Feature	Fasted Cardio (Empty Stomach)	Fed Cardio (Pre-Workout Snack)
Primary Fuel Source	Stored fat due to low glycogen.	Carbohydrates from recent meal/glycogen.
Fat Burning	Higher fat oxidation during exercise, but overall 24-hour fat loss may be similar.	Relies less on fat during exercise, but total daily calorie burn is often higher due to improved intensity.
Performance	Impaired for high-intensity or long-duration efforts; risk of fatigue and reduced stamina.	Enhanced performance, allowing for higher intensity and greater endurance.
Muscle Impact	Risk of muscle protein breakdown via gluconeogenesis.	Better supports muscle maintenance and growth by providing adequate protein and carbs.
Risks	Higher risk of hypoglycemia, dizziness, and fainting.	Lower risk of low blood sugar, but can cause digestive discomfort if meal is too large or too close to workout.
Cortisol Levels	Potentially elevated, particularly with intense exercise.	Typically lower, reducing the risk of chronic stress response.

Who Should Be Cautious About Fasted Cardio?

While some healthy individuals may tolerate fasted exercise well, it is not for everyone. Certain populations should exercise caution or avoid the practice altogether:

Individuals with diabetes: For those managing diabetes, especially those on insulin, fasted exercise can lead to dangerously low blood sugar levels (hypoglycemia).
People with low blood pressure or blood pressure-related issues: Dizziness and lightheadedness from low blood sugar can be exacerbated in individuals with already low blood pressure.
Pregnant women: Fasted cardio is generally not recommended for pregnant women due to the potential risks to both mother and baby.
Athletes or individuals training for high-intensity events: Performance during high-intensity or long-duration training can be significantly hampered without proper fueling.
Anyone new to exercise: For beginners, understanding your body's response to exercise and building foundational fitness is more important than experimenting with fasted states.

Tips for a Healthy Fasted Workout

If you are a healthy individual who prefers or feels better exercising on an empty stomach, consider these tips to minimize risks:

Keep it low to moderate intensity: Stick to activities like walking, light jogging, or cycling, keeping sessions under an hour.
Prioritize hydration: Drink plenty of water before, during, and after your workout. Dehydration can worsen the effects of fasting.
Listen to your body: Pay close attention to any signs of dizziness, weakness, or nausea. Stop immediately and consume a small, easily digestible snack if you feel unwell.
Fuel up post-workout: Plan a balanced meal containing carbohydrates and protein to replenish energy stores and support muscle recovery within a couple of hours of finishing.

The Importance of Overall Nutrition

The timing of your food intake is a relatively minor detail compared to your overall nutritional habits. A balanced, nutrient-rich diet that supports your energy needs is the most crucial factor for cardiovascular health and exercise performance. Focusing on whole grains, fruits, vegetables, lean proteins, and healthy fats provides the consistent fuel your body needs for both exercise and daily function.

Conclusion

So, does not eating affect your cardio? The answer is a clear yes, but whether the effect is positive or negative depends heavily on the context. For light, moderate-intensity exercise, a fasted state may promote fat oxidation for some, though the benefits for long-term fat loss are debatable and often negligible compared to total daily energy balance. For more intense or prolonged activities, exercising on an empty stomach can severely hinder performance, accelerate fatigue, and risk muscle loss. Ultimately, the decision comes down to your personal goals, the type of exercise, and how your body responds. The most important takeaway remains consistency in both exercise and overall healthy nutrition, ensuring proper fueling and recovery regardless of when you choose to move your body.

One authoritative source on this topic is the Mayo Clinic, which provides valuable information on how to properly fuel your workouts: https://www.mayoclinic.org/healthy-lifestyle/fitness/in-depth/exercise/art-20045506.

Frequently Asked Questions

While fasted cardio may increase fat oxidation during the workout itself due to lower glycogen stores, long-term studies show no significant difference in total fat loss between exercising fasted versus fed, as the body adapts over the course of the day.

For most healthy people, low-to-moderate intensity fasted cardio is safe, but it can cause side effects like dizziness or nausea due to low blood sugar. High-intensity or long-duration fasted exercise is riskier and may not be safe for everyone, especially those with certain health conditions.

There is a risk of muscle protein breakdown, especially during intense or prolonged fasted workouts. When carbohydrate fuel is depleted, the body may convert protein from muscle tissue into glucose for energy, which is why a post-workout meal with protein is crucial.

Common side effects include lightheadedness, dizziness, weakness, shaking, and nausea, all of which can result from low blood sugar. Excessive fatigue and reduced performance are also possible.

It is recommended to consume a balanced meal or snack containing protein and carbohydrates within 30 minutes to two hours after your workout to help with muscle recovery and replenish energy stores.

No, individuals with diabetes, especially those taking insulin or other glucose-regulating medications, should avoid fasted cardio without a doctor's supervision. The risk of dangerously low blood sugar levels (hypoglycemia) is a major concern.

Yes, overall nutrition is more important than meal timing around a single workout. The body compensates for increased fat burning during fasted exercise by burning more carbohydrates later, so a healthy, balanced diet throughout the day is key.