What is a fasted workout and how does it affect your body?
A fasted workout is exercise performed after 8-12 hours without eating, typically in the morning before breakfast. While it may enhance fat burning and metabolic flexibility, it can also reduce performance intensity and isn't suitable for everyone.
What is a fasted workout?
A fasted workout refers to exercising in a fasted state, meaning you haven't consumed any calories for at least 8-12 hours before your training session. Most commonly, this involves working out first thing in the morning before breakfast, after your overnight fast. During this time, your body has depleted much of its readily available glucose stores and shifts toward using stored fat as a primary fuel source.
The concept of fasted training has gained popularity among fitness enthusiasts and biohackers looking to optimize their metabolic health and body composition. However, the practice dates back centuries, as many cultures and religions have incorporated fasting and physical activity as part of their traditions.
How your body fuels exercise in a fasted state
Understanding how your body responds to fasted exercise requires a basic knowledge of energy metabolism. When you eat, your body breaks down carbohydrates into glucose, which enters your bloodstream and can be used immediately for energy or stored as glycogen in your muscles and liver. During exercise, your body typically uses a combination of glucose and fat for fuel, with the ratio depending on exercise intensity and duration.
Metabolic shifts during fasting
After an overnight fast, several metabolic changes occur that affect how your body fuels exercise:
- Liver glycogen stores are partially depleted (muscle glycogen remains relatively preserved)
- Blood glucose levels are at their lowest point of the day
- Insulin levels are low, promoting fat mobilization
- Growth hormone levels are elevated, supporting fat burning and muscle preservation
- Your body increases its reliance on fat oxidation for energy
These metabolic adaptations mean that during fasted exercise, your body becomes more efficient at tapping into fat stores for fuel. However, this doesn't necessarily translate to greater overall fat loss, as total caloric balance throughout the day remains the primary determinant of weight change.
The role of exercise intensity
The intensity of your workout significantly impacts how your body responds to fasted training. Low to moderate-intensity exercise (like walking, easy cycling, or yoga) relies heavily on fat oxidation and can be performed effectively in a fasted state. High-intensity exercise, however, depends more on glucose for quick energy, which may be limited during fasting, potentially affecting performance.
Benefits of fasted workouts
Enhanced fat oxidation
Research shows that exercising in a fasted state can increase fat oxidation during exercise by 20-30% compared to fed exercise. A study published in the British Journal of Nutrition found that participants who performed aerobic exercise before breakfast burned up to 20% more fat than those who ate beforehand. This enhanced fat burning occurs because low insulin levels and depleted glycogen stores force your body to rely more heavily on fat for fuel.
Improved metabolic flexibility
Regular fasted training can improve your metabolic flexibility - your body's ability to switch between using carbohydrates and fats for fuel. This adaptation can benefit overall metabolic health and may help with blood sugar regulation. If you're interested in understanding your glucose patterns and metabolic health, comprehensive testing can provide valuable insights into how your body responds to different nutritional strategies.
Potential hormonal benefits
Fasted exercise may optimize certain hormonal responses:
- Increased growth hormone production, which supports fat burning and muscle maintenance
- Enhanced insulin sensitivity, improving your body's ability to manage blood sugar
- Elevated norepinephrine levels, which can boost alertness and fat mobilization
Potential drawbacks and considerations
Reduced performance capacity
While fasted workouts can be effective for low to moderate-intensity exercise, they may compromise performance during high-intensity training. Studies have shown that athletes performing high-intensity interval training (HIIT) or heavy resistance training in a fasted state often experience:
- Decreased power output
- Reduced training volume
- Earlier onset of fatigue
- Impaired recovery between sets or intervals
Muscle protein considerations
Extended fasting combined with exercise may increase muscle protein breakdown, particularly during longer or more intense sessions. While this doesn't necessarily lead to muscle loss if your overall nutrition is adequate, it may not be optimal for those focused on muscle growth or strength gains.
Individual variability
Response to fasted training varies significantly between individuals. Factors that influence how well you tolerate fasted workouts include:
- Training experience and fitness level
- Metabolic health and insulin sensitivity
- Sleep quality and stress levels
- Gender (women may be more sensitive to fasting due to hormonal considerations)
- Genetic factors affecting fat oxidation capacity
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Who should try fasted workouts?
Fasted workouts may be beneficial for:
- Individuals looking to improve metabolic flexibility and fat oxidation
- Those performing low to moderate-intensity cardio or yoga
- People with good metabolic health and stable blood sugar
- Experienced exercisers who have adapted to training with varying fuel availability
- Anyone interested in the potential cognitive benefits of combining fasting with exercise
However, certain groups should approach fasted training with caution or avoid it altogether:
- Individuals with diabetes or blood sugar regulation issues
- Pregnant or breastfeeding women
- Those with a history of disordered eating
- Athletes preparing for competition or focusing on performance gains
- People new to exercise who are still building foundational fitness
How to implement fasted workouts safely
Start gradually
If you're new to fasted training, begin with short, low-intensity sessions and gradually increase duration and intensity as your body adapts. Start with 20-30 minutes of light cardio or yoga before progressing to longer or more challenging workouts.
Stay hydrated
Proper hydration is crucial during fasted workouts. Drink water before, during, and after exercise. You can also add electrolytes to your water, especially for longer sessions or if you're sweating heavily.
Listen to your body
Pay attention to signs that fasted training may not be working for you:
- Dizziness or lightheadedness
- Excessive fatigue or weakness
- Nausea or digestive discomfort
- Persistent decrease in performance
- Mood changes or irritability
Time your post-workout nutrition
While the anabolic window isn't as narrow as once believed, consuming protein and carbohydrates within 2-3 hours after your fasted workout can support recovery and muscle protein synthesis. This is especially important after resistance training or high-intensity sessions.
Monitoring your metabolic response
Understanding how your body responds to fasted workouts can help you optimize your approach. Key biomarkers to monitor include:
- Fasting glucose levels to assess baseline metabolic health
- HbA1c for long-term blood sugar control
- Cortisol levels to evaluate stress response
- Thyroid hormones to ensure metabolic rate isn't negatively affected
- Testosterone and other hormones that may be impacted by training and nutrition
Regular monitoring of these biomarkers can help you determine whether fasted training is supporting or hindering your health goals. For a comprehensive analysis of your metabolic health markers, you can also upload your existing blood test results to SiPhox Health's free analysis service for personalized insights and recommendations.
Making fasted workouts work for you
Fasted workouts can be a valuable tool for improving metabolic flexibility and potentially enhancing fat oxidation, but they're not a magic solution for weight loss or fitness gains. The effectiveness of fasted training depends on numerous factors including your individual physiology, fitness goals, and overall lifestyle.
The key is to experiment mindfully and pay attention to how your body responds. Some people thrive on fasted morning workouts, feeling energized and focused, while others perform better with some fuel in their system. There's no one-size-fits-all approach, and the best workout timing is the one that allows you to train consistently and effectively while supporting your overall health and well-being.
Remember that whether you train fasted or fed, the fundamentals of fitness remain the same: progressive overload, adequate recovery, proper nutrition throughout the day, and consistency over time. Fasted workouts are simply one strategy among many that may help you reach your health and fitness goals.
References
- Gonzalez JT, Veasey RC, Rumbold PL, Stevenson EJ. (2013). Breakfast and exercise contingently affect postprandial metabolism and energy balance in physically active males. British Journal of Nutrition, 110(4), 721-732.[PubMed][DOI]
- Vieira AF, Costa RR, Macedo RC, Coconcelli L, Kruel LF. (2016). Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis. British Journal of Nutrition, 116(7), 1153-1164.[PubMed][DOI]
- Aird TP, Davies RW, Carson BP. (2018). Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis. Scandinavian Journal of Medicine & Science in Sports, 28(5), 1476-1493.[PubMed][DOI]
- Zouhal H, Saeidi A, Salhi A, et al. (2020). Exercise Training and Fasting: Current Insights. Open Access Journal of Sports Medicine, 11, 1-28.[PubMed][DOI]
- Rothschild JA, Kilding AE, Plews DJ. (2020). What Should I Eat before Exercise? Pre-Exercise Nutrition and the Response to Endurance Exercise: Current Prospective and Future Directions. Nutrients, 12(11), 3473.[PubMed][DOI]
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Frequently Asked Questions
How can I test my glucose and metabolic health at home?
You can test your glucose and metabolic health at home with SiPhox Health's Heart & Metabolic Program. This CLIA-certified program includes HbA1c, fasting glucose, and other key metabolic markers, providing lab-quality results from the comfort of your home.
How long should I fast before a fasted workout?
Most experts recommend fasting for 8-12 hours before a fasted workout, which typically means exercising first thing in the morning before breakfast. Some people extend this to 14-16 hours, but beginners should start with the shorter duration and adjust based on how they feel.
Will fasted workouts help me lose weight faster?
While fasted workouts can increase fat oxidation during exercise, research shows that total daily caloric balance is still the primary factor for weight loss. Fasted training may offer metabolic benefits, but it won't necessarily accelerate weight loss compared to fed exercise if total calories remain the same.
Can I drink coffee before a fasted workout?
Yes, black coffee is generally acceptable before a fasted workout as it contains minimal calories and won't significantly impact your fasted state. The caffeine may even enhance fat oxidation and exercise performance. However, avoid adding cream, sugar, or other caloric additions.
What types of exercise are best for fasted training?
Low to moderate-intensity activities work best for fasted training, including walking, light jogging, cycling, yoga, or moderate resistance training. High-intensity interval training or heavy weightlifting may be compromised in a fasted state due to reduced glycogen availability.
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