Practicing exercise on an empty stomach, also known as fasted training, has gained popularity in recent years, particularly among those aiming for fat loss. Proponents claim it helps burn more fat, while critics warn of reduced performance and potential muscle loss. So, what does the science say?
This article explores the physiological effects of fasted training, its pros and cons, and how to determine whether training in a fasted state is beneficial or harmful based on your individual goals, fitness level, and health.
What is Fasted Training?
Fasted training typically refers to exercising in the morning after fasting for 8–12 hours, during which the body has not consumed any calories. In this state:
- Insulin levels are low
- Glycogen stores may be reduced
- The body is more likely to use fat as a fuel source
Fasted training can include:
- Cardio (especially low to moderate intensity)
- Resistance training
High-intensity interval training (HIIT)
Potential Benefits of Fasted Training
1. Increased Fat Oxidation
Fasted cardio may enhance the use of fat as a fuel source. A 2016 study published in the British Journal of Nutrition found that fat oxidation during exercise was significantly higher in a fasted state compared to a fed state (Schoenfeld et al., 2014).
However, this does not necessarily translate to greater fat loss over time—the key factor remains maintaining a caloric deficit.
2. Improved Insulin Sensitivity
Some studies suggest that fasted training can improve insulin sensitivity and glucose regulation, particularly in individuals with insulin resistance or metabolic syndrome (van Proeyen et al., 2010).
3. Convenience and Habit Formation
Exercising before breakfast can be easier for those with busy schedules and may help establish consistent habits by eliminating the need to plan meals in advance.
Potential Drawbacks of Fasted Training
1. Reduced Exercise Performance
For many individuals, glycogen depletion in a fasted state can lead to:
- Decreased endurance
- Reduced performance
- Lower training intensity
This is particularly relevant for high-intensity or prolonged workouts.
2. Increased Muscle Protein Breakdown
Without amino acids from food, fasted training—especially resistance training—can increase muscle protein breakdown (Tipton et al., 2001). Over time, this may hinder muscle retention or growth.
To mitigate this, some experts recommend consuming 10–15 grams of essential amino acids or a small protein shake before fasted resistance training.
3. Greater Perceived Effort
Fasted workouts may feel more challenging, which can impact consistency, especially for beginners.
Fasted vs. Fed Training for Fat Loss: What Does the Research Say?
Long-Term Fat Loss
Despite higher fat oxidation during fasted workouts, overall fat loss over time does not significantly differ when calories are matched between fasted and fed states.
Key Study:
Schoenfeld BJ et al. (2014) conducted a randomized trial comparing fasted and fed aerobic training. After 4 weeks, both groups lost similar amounts of fat.
Conclusion: Fasted training may influence fuel utilization during exercise but does not directly impact overall body fat loss without a caloric deficit.
Who Might Benefit from Fasted Training?
- Endurance athletes seeking metabolic adaptations
- Individuals focused on fat loss who feel comfortable without food
- Intermittent fasters maintaining consistency with morning workouts
- Advanced athletes experimenting with fuel utilization
Who Should Avoid Fasted Training?
- Beginners or underweight individuals
- Those focused on muscle hypertrophy or strength gains
- Individuals with low blood sugar sensitivity or prone to dizziness
- High-intensity athletes requiring peak performance
Tips for Safe and Effective Fasted Training
- Stay hydrated (water, black coffee, or electrolytes)
- Opt for low to moderate-intensity cardio when fasted
- For resistance training, consider consuming EAAs or BCAAs pre-workout
- Monitor for dizziness, fatigue, or reduced performance
- Eat a balanced post-workout meal (protein + carbs)
Conclusion
Fasted training has both benefits and limitations. While it may enhance fat oxidation and metabolic flexibility for some, it can also compromise performance or muscle retention, especially during intense or resistance training.
The effectiveness of fasted training depends on:
- Goals (fat loss vs. muscle gain)
- Comfort
- Training style
- Overall caloric and macronutrient intake
There is no one-size-fits-all approach. The best strategy is one that supports your consistency, health, and performance—whether fasted or fed.
References
- Schoenfeld BJ, et al. Changes in body composition associated with fasted vs. fed aerobic exercise. J Int Soc Sports Nutr. 2014;11:54.
- van Proeyen K, et al. Training in the fasted state improves glucose tolerance during a fat-rich diet. J Physiol. 2010;588(Pt 21):4289–4302.
- Tipton KD, et al. Timing of amino acid-carbohydrate ingestion alters anabolic response of muscle to resistance exercise. Am J Physiol Endocrinol Metab. 2001;281(2):E197–E206.
- American College of Sports Medicine. ACSM Guidelines for Exercise Testing and Prescription, 11th Edition.
