Optimizing Fat Oxidation for Marathon and Ultramarathon Performance

In endurance sports, the ability to efficiently use fat for fuel is a crucial factor in performance, particularly for marathon and ultramarathon runners. Training adaptations that enhance fat oxidation can help delay glycogen depletion, reduce the risk of bonking, and sustain energy levels over long distances. This article explores the science behind fat metabolism, the differences in fueling strategies for marathons vs. ultramarathons, and how to optimize training for better fat utilization.

The Science of Fat Oxidation in Endurance Running

Fat oxidation refers to the process of breaking down fatty acids to generate energy in the form of ATP. Unlike glycogen, which provides a limited energy reserve (typically lasting 60–90 minutes at marathon pace), fat stores are virtually unlimited, even in lean athletes (Jeukendrup, 2014). However, fat oxidation occurs more efficiently at lower exercise intensities, while higher intensities rely more on glycogen and carbohydrates.

Research indicates that the highest rates of fat oxidation occur in Zone 2 training (around 50–75% of VO2 max) (Venables et al., 2005). This intensity is often referred to as the ‘fat-burning zone’ and is essential for endurance athletes looking to improve metabolic efficiency.

Marathon vs. Ultramarathon Fat Utilization

Marathon Training & Fat Adaptation

• Marathon runners rely more on glycogen due to sustained high-intensity efforts, but improving fat oxidation can help preserve glycogen stores.

• Studies suggest that marathoners benefit from incorporating 20–30% of their weekly training in Zone 2 (low-intensity) to improve fat metabolism (Hawley & Leckey, 2015).

• Fasted runs (e.g., morning runs before breakfast) can stimulate fat oxidation but should be limited to 60–90 minutes to avoid muscle breakdown. Runners also need to consider limiting carbohydrates in their meal the night before, so to start the run with low glycogen reserves.

• Carbohydrate periodization (strategically reducing carb intake before some training sessions) has been shown to enhance fat oxidation without impairing high-intensity performance (Marquet et al., 2016).

• Carbohydrate periodization and fasted runs should be done early on in the training plan. Nearer race day, the focus should shift towards race specific trainings that are well fueled by carbohydrates to maximise performance and train the gut.

Ultramarathon Training & Fat Adaptation

• Ultramarathoners spend a greater portion of their race in lower intensity zones, making fat oxidation a primary energy source.

• Long, slow distance (LSD) runs should be a key component, with at least 50% of training volume in Zone 2 to maximize fat oxidation capacity.

• Longer fasted runs (up to 2 hours) may be beneficial in enhancing fat metabolism but should be balanced with proper recovery and protein intake.

• A higher-fat, lower-carb approach in training (while still using carbs for race-day fueling) has been studied as a way to boost fat utilization (Volek et al., 2016).

Training Strategies to Enhance Fat Oxidation

1. Zone 2 Training – At least 40–50% of endurance training should be in Zone 2 to improve fat oxidation efficiency.

2. Fasted Training – Occasional fasted sessions (especially morning runs) can enhance fat adaptation, but they should not replace key quality workouts.

3. Long Runs at Moderate Effort – For marathoners, long runs at 65–75% of max HR help transition between fat and glycogen metabolism. For ultrarunners, keeping these runs below 70% max HR prioritizes fat oxidation.

4. Carbohydrate Periodization – Training with low glycogen stores at times and refueling strategically improves metabolic flexibility. Be sure to fuel properly afterwards to avoid RED-S

5. Race-Day Nutrition Matching – While training adaptations enhance fat utilization, race-day fueling should still include carbohydrates to sustain race pace.

Conclusion

Fat oxidation is a critical component of endurance performance, particularly for long-distance events like marathons and ultramarathons. While marathoners must balance fat and carbohydrate utilization for sustained speed, ultrarunners rely more heavily on fat metabolism due to prolonged lower-intensity efforts. Training strategies such as Zone 2 training, carbohydrate periodization, and fasted runs can optimize fat oxidation, leading to better endurance and race-day efficiency. By understanding and applying these principles, endurance athletes can maximize their performance and sustain energy levels throughout their events.

References

• Jeukendrup, A. (2014). "A step towards personalized sports nutrition: carbohydrate intake during exercise." Sports Medicine.

• Venables, M. C., Achten, J., & Jeukendrup, A. E. (2005). "Determinants of fat oxidation during exercise in healthy men and women: a cross-sectional study." Journal of Applied Physiology.

• Hawley, J. A., & Leckey, J. J. (2015). "Carbohydrate dependence during prolonged, intense endurance exercise." Sports Medicine.

• Marquet, L. A., et al. (2016). "Periodization of carbohydrate intake: Short-term effect on performance." International Journal of Sports Nutrition and Exercise Metabolism.

• Volek, J. S., et al. (2016). "Metabolic characteristics of keto-adapted ultra-endurance runners." Metabolism.