Should I do cardio if I haven't eaten?

October 10, 2025 •

5 min read

Research on the effectiveness of exercising on an empty stomach, or 'fasted cardio,' is mixed, with some studies suggesting a potential boost in fat oxidation during exercise. However, this strategy is not suitable for everyone and depends heavily on individual goals, health status, and workout intensity.

Quick Summary

This article explores the debate surrounding fasted cardio, detailing its purported benefits, such as enhanced fat oxidation, and potential risks, including reduced performance and muscle breakdown. It examines the science behind how the body fuels itself during fasted versus fed states and offers guidance for determining if exercising before eating is appropriate for your fitness level and objectives.

Key Points

Fasted Cardio Theory: The belief is that with low glycogen stores from fasting, your body will burn more fat for energy during cardio.
Conflicting Evidence: While some studies show higher fat oxidation during a fasted workout, most research shows no significant difference in overall fat loss compared to fed cardio, as the body compensates later.
Performance vs. Intensity: Fasted cardio may hinder high-intensity workouts due to lower energy, but can work well for low-to-moderate steady-state exercise.
Risk of Muscle Loss: Intense or prolonged fasted workouts increase the risk of muscle breakdown for fuel, which can be detrimental to body composition.
Safety First: Listen to your body and watch for signs of low blood sugar, like dizziness. Proper hydration is essential.
Individual Choice: The best approach depends on personal preference, health status, and workout goals. Consistency is more important than timing for fat loss.
Post-Workout Nutrition: After a fasted workout, a balanced meal of protein and carbs is crucial for recovery and muscle repair.

The Science Behind Fasted Cardio vs. Fed Cardio

When you engage in cardiovascular exercise, your body requires fuel to power your muscles. The primary sources of this fuel are carbohydrates (stored as glycogen) and fat. The core theory behind performing cardio on an empty stomach, often after an overnight fast, is that with depleted glycogen stores, your body is forced to tap into its fat reserves for energy.

How Your Body Uses Fuel

Fasted State: After not eating for several hours, your insulin and glycogen levels are low. In this state, your body's hormonal environment is conducive to breaking down fat for energy, a process known as lipolysis and fat oxidation. This can lead to a higher percentage of fat being burned during the workout itself.
Fed State: When you eat before a workout, especially a meal rich in carbohydrates, your blood sugar and insulin levels rise. Your body prefers to use this readily available glucose for energy, as it is the most efficient fuel source for intense exercise.

The Compensatory Effect

While some short-term studies show that fasted cardio burns more fat during the exercise session, this benefit often doesn't translate to significantly greater fat loss over a 24-hour period. The body is a highly adaptive machine and may compensate later in the day by reducing its fat-burning rate or increasing carbohydrate oxidation. A 2014 study on women showed no significant difference in body composition changes between groups who exercised either fasted or fed, provided their overall caloric intake was controlled. The overall energy balance (calories in versus calories out) remains the most critical factor for fat loss, not simply the timing of your cardio.

Fasted Cardio: Is It Right for You?

The decision to perform cardio on an empty stomach is highly individual and depends on several factors, including your fitness level, goals, and how your body responds.

Benefits of Fasted Cardio

Improved Metabolic Flexibility: Regularly training in a fasted state can train your body to become more efficient at switching between using fat and carbohydrates for fuel.
Increased Fat Oxidation: Some research suggests a higher rate of fat oxidation during exercise, which can be beneficial for specific performance goals, such as for endurance athletes.
Convenience: For many, exercising first thing in the morning before eating is simply more convenient and easier to fit into a busy schedule.
Reduced Digestive Discomfort: Some individuals experience digestive issues or nausea when exercising soon after eating, which can be avoided by training fasted.

Risks and Considerations

Decreased Performance and Intensity: If you lack fuel, your energy levels may be lower, making it difficult to maintain high intensity or endure long training sessions. High-intensity interval training (HIIT) is particularly demanding and may be compromised when fasted.
Muscle Breakdown: Without readily available glucose, the body may resort to breaking down muscle protein for energy, a process called gluconeogenesis. This can be counterproductive for those aiming to build or preserve muscle mass.
Hypoglycemia and Discomfort: Low blood sugar can cause dizziness, lightheadedness, nausea, or shakiness, which can increase the risk of injury.
Elevated Cortisol Levels: Fasted, high-intensity exercise can elevate cortisol, a stress hormone, which can potentially hinder recovery and, in extreme cases, affect hormonal balance, especially in women.
Not Ideal for Endurance: If you plan a long, strenuous workout lasting over an hour, eating beforehand is generally recommended for optimal performance.

Comparison Table: Fasted vs. Fed Cardio


Feature	Fasted Cardio	Fed Cardio
Primary Fuel Source	Stored fat (due to lower glycogen)	Recently consumed carbohydrates
Potential Fat Burn (During Workout)	Higher fat oxidation rate	Lower fat oxidation rate
Potential Fat Burn (Over 24 Hours)	No significant difference in overall fat loss shown by most studies	No significant difference in overall fat loss shown by most studies
Performance for High-Intensity	Potentially compromised due to lower energy	Supports high performance with available fuel
Performance for Low-Intensity	Generally sustained without issue	Well-supported and sustained
Risk of Muscle Breakdown	Increased risk, especially with intense or long workouts	Lower risk, with readily available energy sources
Energy Levels and Stamina	Can feel lower or more sluggish	Often higher and more stable
Best For	Lower-intensity, shorter duration workouts or those prioritizing metabolic flexibility	High-intensity workouts, longer endurance sessions, or muscle building goals

How to Approach Fasted Cardio Safely

If you decide to incorporate fasted cardio into your routine, do so with caution:

Start Gradually: Ease into it with short, low-intensity sessions, like a 20-30 minute brisk walk. Observe how your body responds and adjust accordingly.
Stay Hydrated: Always drink plenty of water before, during, and after your workout. Some may benefit from electrolytes, especially during longer sessions.
Listen to Your Body: Pay close attention to warning signs like dizziness, nausea, or severe fatigue. If you feel unwell, stop the exercise and refuel.
Prioritize Post-Workout Nutrition: The meal after your fasted session is crucial for recovery. Consume a balanced meal with protein and carbohydrates within 30-60 minutes to replenish glycogen and support muscle repair.
Target the Right Intensity: Reserve high-intensity training for when you have adequate fuel. Save fasted workouts for steady-state, low-to-moderate intensity efforts.

Conclusion: Personal Preference and Overall Consistency Win

Ultimately, the science suggests that for overall fat loss and body composition changes, the timing of your cardio relative to your meals is not the deciding factor. A consistent calorie deficit is what truly drives weight loss. Choosing between fasted or fed cardio should be based on your personal preference, how you feel, and what fits best into your schedule. For some, a pre-workout meal provides the energy needed for a higher-intensity session, while others feel better working out on an empty stomach. The most effective cardio routine is the one you can stick with consistently. Focus on moving your body regularly and fueling it properly for recovery, regardless of whether you've eaten recently or not.

A Note on Consulting a Professional

While this article provides general information, it's not a substitute for professional medical advice. If you have underlying health conditions, such as diabetes, or are an athlete with specific performance goals, consult a healthcare provider or a registered dietitian before making significant changes to your exercise or nutrition routine.

How to Lose Weight By Fasting and Doing Cardio

This information aligns with strategies discussed by Crunch Fitness regarding fasted cardio and intermittent fasting.

Body Composition Changes Associated with Fasted vs. Non-Fasted Aerobic Exercise

For a deeper dive into the specific body composition changes, this research paper offers a focused analysis.

Frequently Asked Questions

For most healthy individuals, it is generally safe to do low-to-moderate intensity cardio on an empty stomach. However, it is not recommended for people with conditions like diabetes, low blood pressure, or for high-intensity, long-duration exercise.

While fasted cardio can lead to a higher rate of fat oxidation during the workout itself, most studies show no significant difference in total fat loss over a 24-hour period compared to exercising after a meal.

Low-to-moderate intensity exercises are best for fasted cardio, such as brisk walking, light jogging, or cycling. High-intensity workouts like HIIT should be done with fuel in your system for optimal performance and safety.

After a fasted workout, it is crucial to eat a balanced meal with protein and carbohydrates within 30-60 minutes to replenish energy stores and aid muscle recovery. Good examples include a protein shake with fruit or Greek yogurt with berries.

If you perform intense or long-duration fasted cardio, there is an increased risk that your body may break down muscle protein for energy. Eating adequate protein throughout the day and after your workout helps mitigate this risk.

Eating before cardio provides your body with readily available fuel (glucose), which can improve your performance and endurance, especially for high-intensity or long-duration workouts. It also reduces the risk of low blood sugar and muscle breakdown.

Eating within 30 to 60 minutes after a fasted workout is generally recommended to maximize recovery and replenish depleted energy stores.