Weighing up fasted training

Fasted training, aka exercising without prior food intake, has gained attention in the realm of sports nutrition for its possible benefits and drawbacks. While some people swear by this approach for fat loss and metabolic adaptations, in reality it carries many risks, particularly concerning performance and under-fueling.

What are some of the positives of fasted training?

1. Enhanced fat oxidation: fasted training may promote greater fat oxidation during exercise. A study by Achten and Jeukendrup (2004) found that exercising in a fasted state can stimulate higher rates of fat oxidation, potentially leading to improved body composition. The study indicated that participants in a fasted state burned approximately 20-30% more fat during exercise compared to those who had carbohydrates before exercising.

2. Improved insulin sensitivity: a review by Longo and Mattson (2014) highlighted that intermittent fasting, which includes periods of fasting, improves insulin sensitivity, helping better manage blood glucose levels. Increased insulin sensitivity may lead to more efficient glycogen synthesis post-exercise, critical for muscle recovery.

3. Mental strength: training in a fasted state often requires extra mental toughness. A study by Hargreaves et al. (2008) noted that training without readily available fuel required athletes to adapt mentally which could enhance psychological resilience.

4. Hormonal benefits: exercise performed in a fasted state may stimulate the release of growth hormone (GH), which plays a role in fat metabolism and muscle growth. A controlled study showed that fasted exercise could result in a three-fold increase in GH levels after high-intensity workouts. Additionally, fasted training may promote mitochondrial biogenesis, improving aerobic capacity (Little et al. (2010)).

BUT, what are some of the risks of fasted training?

1. Decreased performance: a major drawback of fasted training is the potential impact on performance. While low-intensity exercise may not suffer too much, high-intensity or longer efforts often do. A study by Coyle (1997) demonstrated that glycogen-depleted athletes exhibited performance decrements of up to 20% during high-intensity activities. The lack of available glucose can impair muscular power and endurance, particularly in efforts requiring short bursts of power.

2. Increased risk of injury: training in a fasted state can heighten the risk of injury. Insufficient energy intake can lead to muscle fatigue, impaired motor skills, and a lack of optimal mental focus. According to a 2016 study published in the British Journal of Sports Medicine, inadequate fueling was associated with a 45% increase in injury risk among collegiate athletes.

3. Under-fueling: chronic fasted training can increase the risk of under-fueling, manifesting as low energy availability (LEA). A study by Mountjoy et al. (2018) emphasized the importance of adequate caloric intake for athletes, citing that energy deficits can lead to an array of health issues like Relative Energy Deficiency in Sport (REDs). Under-fueling significantly impairs performance and recovery.

4. Delayed recovery: recovery is crucial for athletes (it's where the magic happens!), and fasted training can interfere with this process. Insufficient nutrient intake post-exercise can delay glycogen synthesis and muscle repair. A study conducted by Ivy et al. (2002) showed that athletes who consumed carbohydrates immediately after training repleted glycogen levels significantly faster—by up to 50% within the first 30 minutes—compared to those who delayed ingestion.

5. Reduced immune function: athletes not consuming adequate nutrients to support the demands of training risk a compromised immune response. According to a study by Gleeson et al. (2004), a consistent energy shortage can depress the immune system, making athletes more susceptible to illnesses and infections.

Conclusion

Fasted training may enhance fat oxidation and build additional mental toughness. However, the associated risks including decreased performance, increased injury risk, delayed recovery, and increased risk of LEA to name a few—cannot be overlooked. Athletes must adopt a personalised approach, weighing up their goals against the potential risks and consequences of fasted training. For optimal performance and recovery, adequate fueling through a well-balanced food intake remains crucial. As research continues to evolve, athletes should base their training strategies on scientific evidence while closely monitoring their health and performance outcomes.

References

• Achten, J., & Jeukendrup, A. E. (2004). Optimizing fat oxidation through exercise and diet. The Journal of Sports Sciences, 22(9), 885-897.

• Coyle, E. F. (1997). Carbohydrate ingestion during exercise. Journal of Sports Sciences, 15(3), 213-220.

• Coyle, E. F. (2005). Role of dietary carbohydrate in muscle glycogen storage and exercise performance. Journal of Sports Sciences, 23(3), 267-272.

• Gleeson, M., Bishop, N. C., Dunn, M., & Foley, J. (2004). Training, diet and immunity. Journal of Sports Sciences, 22(6), 553-566.

• Halson, S. L. (2014). Sleep training and recovery. Journal of Sports Sciences, 32(4), 317-335.

• Ivy, J. L., Goforth, H. W., Damon, B. M., McCauley, T. R., & O'Brien, J. (2002). Early postexercise muscle glycogen recovery is enhanced with a carbohydrate-protein supplement. Journal of Applied Physiology, 93(4), 1337-1344.

• Little, J. P., Safdar, A., Wilkin, G. P., & Tarnopolsky, M. A. (2010). A single bout of high-intensity interval training increases the expression of PGC-1α and other genes associated with mitochondrial biogenesis in human skeletal muscle. Journal of Physiology, 588(6), 1011-1022.

• Longo, V. D., & Mattson, M. P. (2014). Fasting: Molecular mechanisms and clinical applications. Cell Metabolism, 19(2), 181-192.

• Mountjoy, M., Sundgot-Borgen, J., Burke, L., & et al. (2018). Relative energy deficiency in sport: A consensus statement. International Journal of Sports Nutrition and Exercise Metabolism, 28(4), 359-369.