When it comes to maximising fat oxidation during exercise, the debate between training in a fasted or fed state has intrigued those of us in the fitness industry for years. Recent research sheds light on how different states of energy availability (fasted vs. fed) affect fat and carbohydrate metabolism, and how the strategic use of caffeine can enhance fat burning. By understanding these findings, we can better tailor exercise programmes to help clients optimise fat loss while maintaining performance and avoiding overtraining. Let’s look into the research and explore practical applications for your clients.
Overview of the Research
1. Fasted vs. Fed Exercise and Fat Oxidation
In a systematic review and meta-analysis conducted by Vieira et al. (2016), researchers examined how aerobic exercise performed in a fasted state compared to exercise in a fed state. The study reviewed 27 trials with 273 participants and found that fat oxidation was significantly higher during fasted exercise compared to fed-state exercise. On the other hand, plasma glucose and insulin concentrations were higher in the fed state, which led to greater reliance on carbohydrates for energy.
Key Finding: Fasted exercise promotes higher fat oxidation, making it an attractive strategy for clients focused on fat loss. However, the higher insulin and glucose levels in the fed state are beneficial for performance and energy during longer or more intense workouts.
2. Effects of Glycogen Depletion on Female Athletes at Altitude
In Stephanie Cotter’s 2024 study on female collegiate distance runners training at moderate altitude, researchers explored how training in a glycogen-depleted state (fasted) compared to a fed state. The study found that glycogen-depleted training increased heart rate and the athletes’ Rating of Perceived Exertion (RPE), meaning they felt they were working harder. Despite this, no significant improvement in performance (running pace) was observed between fasted and fed states. This suggests that while fat oxidation may be increased, training in a glycogen-depleted state can lead to greater perceived effort and potentially increased physiological stress, especially for female athletes.
Key Finding: Fasted training can raise stress markers such as heart rate and RPE, without necessarily improving performance. This is especially important for female athletes who may be at greater risk of overtraining or low energy availability.
3. Caffeine as a Fat Oxidation Booster
Another relevant study comes from Fernández-Sánchez et al. (2024), which reviewed the effects of caffeine on fat oxidation during exercise in a fed state. The meta-analysis showed that caffeine intake (between 3 to 6 mg/kg) increased fat oxidation even when a meal had been consumed within 5 hours before exercise. However, this effect was less pronounced in those who regularly consumed caffeine or those who were highly trained. The research highlighted that caffeine can be a useful tool for enhancing fat metabolism, particularly for clients who are not habitual caffeine users or are less aerobically trained.
Key Finding: Caffeine can enhance fat oxidation during fed-state exercise, making it a valuable addition for clients who prefer to train after eating. However, its effectiveness may diminish for habitual caffeine users or highly trained athletes.
Practical Tips for Fitness Professionals
1. Incorporate Fasted Training for Fat Loss Clients
For clients looking to increase fat loss, fasted-state training can be a useful tool. Since fat oxidation is higher when training in a fasted state, clients who engage in moderate-intensity aerobic exercise (e.g., walking, jogging, or cycling) before breakfast may see greater fat-burning benefits. Keep in mind, though, that fasted training may not be suitable for everyone. Clients with low energy availability or those at risk of overtraining, may experience greater fatigue or stress if fasted sessions are overused.
Tip: Start with 1–2 fasted sessions per week for clients aiming for fat loss, and gradually monitor their energy levels and recovery. Consider using fasted training for lower-intensity sessions to avoid excessive stress on the body.
2. Balance Fasted and Fed Training for Performance-Focused Clients
While fasted training is excellent for fat oxidation, it may not be the best approach for clients focused on performance, particularly for high-intensity or long-duration sessions. The higher glucose and insulin levels in the fed state help fuel performance, which is crucial for maintaining energy during more challenging workouts.
Tip: Use fed-state training when clients need to perform at their best, such as during strength training, interval sessions, or endurance workouts. This ensures they have the necessary energy to maintain intensity and avoid early fatigue.
3. Use Caffeine Strategically
For clients who prefer to train in a fed state but still want to enhance fat oxidation, caffeine can be a powerful tool. The Fernández-Sánchez study showed that caffeine can increase fat oxidation even when a meal is consumed before exercise. However, its benefits are more pronounced in clients who are less trained or who don’t regularly consume caffeine.
Tip: Recommend a moderate dose of caffeine (3–6 mg/kg) about 60 minutes before training. This can be especially effective for clients working on fat loss but who prefer to eat before their workouts. Caffeine intake can be from coffee, tea, or supplements, but always consider individual tolerance and potential side effects.
4. Monitor Female Athletes for Overtraining Risks
Female athletes, particularly those training at high intensities or at altitude, are at greater risk of overtraining when glycogen levels are low. The Cotter study showed that fasted training increased heart rate and perceived exertion without improving performance, suggesting that long-term fasted training could lead to burnout.
Tip: For female clients, balance fasted and fed-state sessions carefully. Make sure they’re getting enough energy from carbohydrates to support their training and recovery, especially if they’re training frequently or at higher intensities. Encourage regular monitoring of energy levels, sleep, and mood to catch early signs of overtraining.
Fasted training can be a highly effective tool for maximising fat oxidation, but it’s not a one-size-fits-all solution. By understanding the differences between fasted and fed exercise, and incorporating strategic caffeine use, we can create tailored programmes that help clients achieve their fat loss and performance goals without compromising recovery. Remember to keep the individual’s needs, goals, and risk of overtraining in mind when designing their training plan.
References
Vieira, A.F., Costa, R.R., Macedo, R.C.O., Coconcelli, L., & Kruel, L.F.M. (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. Click here to review the full research article
Cotter, S. (2024). The Impact of Relative Glycogen Depletion on the Training-Induced Stress Response in Female Collegiate Distance Runners at Moderate Altitude. Master’s Thesis, Adams State University.
Fernández-Sánchez, J., Trujillo-Colmena, D., Rodríguez-Castaño, A., Lavín-Pérez, A.M., Del Coso, J., Casado, A., & Collado-Mateo, D. (2024). Effect of acute caffeine intake on fat oxidation rate during fed-state exercise: a systematic review and meta-analysis. Nutrients, 16(207). Click here to review the full research article
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