Female Athlete Cyclists Taking A Break During A Ride To Fuel

Boost Your FTP by Mastering Metabolic Flexibility

BY Dr. Sam Impey

Zone 2 training improves your FTP by building mitochondria. Mastering metabolic flexibility can help you make the most of your Zone 2 training. Here's how.

Zone 2 training is the hot topic in endurance sports right now. This steady-state style of training focuses on maximizing energy production through aerobic pathways in the mitochondria—the powerhouses of our cells. By repeatedly engaging in Zone 2 training, your body builds more mitochondria, enhancing your ability to produce energy efficiently and resulting in higher Functional Threshold Power (FTP).

How you fuel beforehand directly impacts Zone 2 training at the cellular level. A mix of fats and carbohydrates is always used as fuel, but your body uses different amounts depending on when you train, and what you eat before. Training your body’s ability to switch between fuel sources is known as metabolic flexibility. Mastering metabolic flexibility during your Zone 2 training ensures you’re using energy efficiently and getting the most out of your workout. 

Fueling Your Rides: The Metabolic Dimmer Switch

When we exercise, our bodies use a combination of fats and carbohydrates for energy. Think of your body’s fuel utilization like a dimmer switch on a light. You can adjust this “dimmer switch” to either burn more fat or more carbohydrates depending on how you fuel beforehand and the time of day you train.

The ability to effectively move this dimmer switch up and down is known as metabolic flexibility. Improving your metabolic flexibility can significantly boost your FTP and enhance your endurance during workouts. To maximize your FTP and build more mitochondria, aim to do 70-80% of your Zone 2 rides with the dimmer switch turned towards burning carbohydrates, and 20-30% towards burning fat. 

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How to Burn More Fat

If you want to burn more fat during Zone 2 training, focus on what you eat before your ride and when you train. Eating foods low in carbohydrates before training reduces the glycogen stored in your liver. This prompts your body to break down fats from fat stores, making them available in your blood as fuel. 

Another effective strategy is to train first thing in the morning. Overnight, your liver maintains your blood glucose levels and lowers glycogen stores, signaling your body to release more fat for energy. Using either of these approaches allows you to burn approximately 45 – 50% fat as fuel during Zone 2 training. 

How to Burn More Carbs

On the other hand, to burn more carbohydrates, consume foods high in carbohydrates 2-4 hours before your workout. This ensures that both your liver and muscle glycogen stores are topped up, allowing you to burn approximately 75% carbohydrates and 25% fat as fuel. It’s important to remember that these carbohydrates are still being burned aerobically, driving energy production through the mitochondria.

Balancing Your Training

Understanding how to use metabolic flexibility during your Zone 2 sessions allows you to strategically plan your workouts to burn either more fat or more carbohydrates. If your goal is to increase mitochondria and boost FTP, then do most of your training fuelled by carbohydrates and some of your training fuelled by fats. This balance gives your body the best opportunity to increase FTP, enhance aerobic metabolic processes, and ultimately boost performance.


Coye, E., et al. (1985, Aug 1). Substrate usage during prolonged exercise following a preexercise meal. Retrieved from https://journals.physiology.org/doi/abs/10.1152/jappl.1985.59.2.429 

De Bock, K. et al. (2008, Apr 1). Effect of training in the fasted state on metabolic responses during exercise with carbohydrate intake. Retrieved from https://journals.physiology.org/doi/full/10.1152/japplphysiol.01195.2007 

Impey, S., et al. (2014, Dec). Leucine-enriched protein feeding does not impair exercise-induced free fatty acid availability and lipid oxidation: beneficial implications for training in carbohydrate-restricted states. Retrieved from https://link.springer.com/article/10.1007/s00726-014-1876-y  

Rothschild, J., et al. (2021, Apr). Pre-exercise carbohydrate or protein ingestion influences substrate oxidation but not performance or hunger compared with cycling in the fasted state. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8070691/ 

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Dr. Sam Impey
About Dr. Sam Impey

Dr. Sam Impey’s journey into the world of sports nutrition began with his research into the cellular and molecular adaptations to exercise-nutrition interactions. He led the research around carbohydrate periodisation that developed the Fuel for the Work Required paradigm. Driven by a desire to address challenges faced in professional sports and the understanding that every athlete is unique, Dr Impey co-founded Hexis to empower athletes and coaches with innovative nutrition-tech solutions.

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