Unlocking Your Potential: Understanding Metabolic Efficiency and Flexibility for Endurance Athletes

Endurance athletes are always searching for ways to race faster, recover better, and sustain performance over long distances. Two key concepts in sports physiology—metabolic efficiency and metabolic flexibility—play a significant role in how athletes produce and use energy. While these concepts are related, they are not the same. Understanding them can help you optimize your fueling strategies, avoid mid-race crashes, and train smarter.

Let’s break down the differences and why both matter for your performance.

What Is Metabolic Efficiency?

Metabolic efficiency refers to how effectively your body uses fat as a fuel source during exercise, especially at lower intensities when fat oxidation should be high. An athlete with high metabolic efficiency can rely more on stored fat and less on glycogen at a given sub-maximal intensity. This means that a metabolically efficient athlete might depend almost entirely on fat for fuel at lower intensities (think zone 1 and low zone 2). As the intensity increases (high zone 2 and zone 3), they can still utilize a higher percentage of fat for fuel compared to an athlete who is not metabolically efficient.

An athlete training at a sub-maximal effort will still burn some carbohydrates, but the ratio of fat to carbohydrate used will be more favorable than that of a non-metabolically efficient athlete.

Why It Matters for Endurance Athletes

• Improved Endurance: Even the leanest athletes store tens of thousands of calories of fat, but only about 2,000 calories of glycogen. More fat use leads to delayed glycogen depletion.

• Stable Energy: Less reliance on constant carb intake reduces the risk of gastrointestinal issues and mid-race energy crashes.

• Better Performance at Moderate Intensities: This is ideal for ultra runners, long-distance cyclists, and Ironman triathletes who need steady, economical energy production.

  
  

Fat adaptation is key to metabolic efficiency, but there is a physiological limit to being more fat-adapted.

The Crossover Concept

First, let’s review the crossover concept in endurance exercise. This concept refers to the balance between fat and carbohydrate metabolism during sustained exercise. At rest and at very easy intensities, our body burns nearly 100% fat as fuel. As exercise intensity increases, the percentage of fat utilized for energy decreases, while carbohydrate use increases.

To see a visual of this, you can check out this link for more information. The goal of improving fat adaptation is to push that crossover point further to the right, increasing the amount of fat burned at slightly higher intensities before the body “switches” to burning more carbohydrates for fuel.

Limitations of Metabolic Efficiency

The body will never be able to burn fat for fuel at the highest intensities (zone 4 and above). This is because the ability to break down fat and use it for fuel is too slow to meet the demands of very high-intensity activities. Fat metabolism also requires a steady supply of oxygen. Higher intensity exercise often outpaces oxygen intake, necessitating a greater reliance on carbohydrates for fuel. Carbohydrates can be burned as fuel during high-intensity exercise through anaerobic glycolysis, but this type of fuel is limited. That’s why we can only train at these high intensities for relatively short durations.

When considering different types of activities or events, training for metabolic efficiency may not make sense for races where the intensity exceeds about 70% Vo2 Max or where the athlete is going beyond their zone 2 into higher heart rate zones. This includes races lasting about two hours or less, such as a half marathon or Olympic distance triathlon. There may be some benefit to training for metabolic efficiency for marathon runners, but focusing solely on fat adaptation may compromise their ability to use carbohydrates when needed.

Research Insights

While there aren’t many studies that specifically use the term metabolic efficiency, research on athletes fueling with a low-carb approach provides insights into the potential benefits and drawbacks of being metabolically efficient.

One key consideration is the economic cost of being more “fat-adapted.” Studies on endurance athletes (like race walkers) consuming a high-fat, low-carb diet found that while they could utilize more fat during exercise, the increased rates of fat oxidation resulted in reduced economy (increased oxygen demand for a given speed) at velocities that translate to real-life race performance. You can find the link to this study here.

High-fat diets increase the oxygen (O2) cost of exercise because fat oxidation requires more oxygen than carbohydrate oxidation to produce the same amount of energy. This higher O2 demand can reduce exercise economy and potentially impair performance, particularly at high intensities.

Drawbacks of Fat Adaptation

Another issue with becoming more fat-adapted is that it limits the body’s ability to use carbohydrates for fuel when needed. A fat-adapted athlete may struggle to burn carbs because their body can down-regulate the enzymes and metabolic pathways needed for efficient carbohydrate utilization while enhancing fat-burning systems. This means that while they may perform well at stable paces and intensities, they could struggle to shift to higher intensities, such as during an uphill portion of a race or when trying to surge at the end.

Keep in mind that this adaptation may only occur when carbohydrate availability is limited over the long term. However, it is possible for an athlete to improve metabolic efficiency without compromising their ability to also burn carbohydrates as fuel when needed. This is why I wouldn’t recommend a low-carbohydrate approach as a long-term solution for most endurance athletes.

Benefits of Being More Fat Adapted or Metabolically Efficient

• Marathon pace feels more “aerobic” and may feel like less effort.

• You may notice a lower heart rate at your easier paces, such as your zone 2 pace; however, this may not hold true at higher paces due to the oxygen cost of burning more fat for fuel.

• Long runs and workouts may feel easier at the same effort level.

• Better late-race resilience means you can maintain your performance better at the end of a longer race.

• You need fewer gels or carb-containing fuel for the same paces in training.

  
  

Drawbacks of Being More Fat Adapted or Metabolically Efficient

• You may struggle to maintain higher-intensity exercise.

• It can be challenging to switch between paces, such as during a surge in a race or a kick at the end.

• The transition period may involve a lower carbohydrate approach and a period of higher volume, low-intensity exercise, which many athletes may not be willing to undertake.

• Fueling adequately for training could become an issue, leaving an athlete with low energy availability and an inability to recover from workouts.

  
  

What is Metabolic Flexibility?

Metabolic flexibility is the endurance athlete’s ability to switch efficiently between fat and carbohydrate depending on intensity and fuel availability. This is crucial for long-distance performance, especially for marathoners, ultra runners, and triathletes.

A metabolically flexible athlete can:

• Burn a larger percentage of fat at low to moderate intensities, thus preserving precious muscle and liver glycogen.

• Shift quickly to carbohydrates at high intensities, powering hills, surges, tempo efforts, or the final 10K.

• Switch back and forth efficiently—no bonking, smoother energy, less GI distress, and better pacing control.

  
  

It’s not about being “fat-adapted” full-time or adhering to a strict low-carb diet; it’s about using both training and nutrition strategies that enhance the body's ability to utilize both fuel systems.

The Difference Between Metabolic Efficiency and Flexibility

The difference between metabolic efficiency and flexibility lies in their focus. Metabolic efficiency is about training to improve the body’s ability to burn fat during exercise. In contrast, metabolic flexibility is the ability to switch back and forth between fat and carbohydrates.

Being metabolically efficient does not necessarily mean an athlete is metabolically flexible. However, a metabolically flexible athlete can be metabolically efficient. If an athlete trains solely to improve fat utilization, they may hinder their body’s ability to switch to carbohydrates when needed. For example, during a marathon, an athlete who has trained their body to be more metabolically efficient using a long-term low-carb strategy might perform well at stable intensities. But what happens if they need to kick at the end of the race or increase intensity to power up a hill? If they lack metabolic flexibility, they may struggle to switch to carbohydrate burning when their intensity increases beyond the capacity to burn fat. This could limit their performance.

Why Metabolic Flexibility Matters for Endurance Performance

1. Delays Hitting the Wall: When your body can burn more fat at marathon pace, you save glycogen for the late race.

2. Improves Durability: Less glycogen drain early means less muscle fiber fatigue later.

3. Enhances Your Ability to Surge and Cover Moves: This is useful for hills, surges during a race, running in a pack, or sprinting to the finish.

4. Improves Fueling Tolerance: More efficient carbohydrate use leads to fewer gut issues.

5. Supports Higher Training Volume: Stable energy allows for better recovery and the ability to stack training sessions.

   
   

The Physiology (Simple Version)

Metabolic flexibility is driven by:

• Mitochondrial Density: More mitochondria mean more efficient energy burning. Increasing the number of mitochondria in cells can improve endurance performance by enhancing the capacity to utilize both fat and carbohydrates for fuel.

• Hormonal Signals: Insulin, adrenaline, and cortisol play roles here. Although insulin is not secreted during exercise, insulin sensitivity increases. A higher level of insulin sensitivity (as opposed to insulin resistance) will enhance metabolic flexibility at both rest and during exercise. Remember, insulin signals glucose to enter cells for fuel. The quicker and easier this process occurs, the better from an energy standpoint. Cortisol and adrenaline are released during exercise to help mobilize both fat and carbohydrates for fuel.

• Glycogen Availability: When the body can burn more fat at lower intensities, the amount of glycogen used at that intensity will be less, allowing you to run longer before glycogen depletion.

• Enzyme Adaptation: When someone is metabolically flexible, they can up-regulate enzymes that assist with fat and glucose utilization, allowing for quicker fuel source transitions.

• Fat Oxidation Rate ("FatMax"): This measures how fast the body can burn fat for fuel. Training or eating in a certain way can increase "FatMax," typically occurring at 40-60% Vo2 max. More metabolically efficient individuals may reach this at the higher end, closer to 60% Vo2 max or slightly higher.

• Ability to Up-Regulate Carbohydrate Oxidation Under Load: This refers to the body’s capacity to increase carbohydrate oxidation rates as intensity increases or during specific loads, such as heat stress. This means the body can maintain or increase its ability to digest, mobilize, and utilize carbohydrates as intensity rises or under stress.

  
  

You are flexible when your system can adapt fuel choice quickly and appropriately for the intensity.

How to Know You're Becoming More Metabolically Flexible

You’ll notice:

• Lower Heart Rate at the Same Pace: An increase in mitochondria usually means more efficiency in energy supply, resulting in a decreased load on the heart. Some athletes report a reduction of 5-10 beats per minute during their zone 2 pace runs.

• Easier Transitions Between Easy and Fast Paces: Going from an easier pace to a faster pace might feel effortless or smooth.

• Improved Long-Run Durability: Your long run becomes slightly easier and more manageable, or you can run faster during long runs and recover more quickly afterward.

• Fewer GI Issues When Fueling: You may find that you need fewer carbs per hour or that you can tolerate more carbs during runs.

• Faster Marathon Pace or Lower Rate of Perceived Exertion: Your marathon pace might feel easier, or you may be able to increase your marathon pace slightly.

• Easier Time Taking Gels Without Crashing: Your energy levels during runs may stabilize, and you might not experience the same energy surge and subsequent "crash" when taking gels.

• Better Late-Race Strength: You may notice that during the final portion of a race, you can better maintain your goal pace. This is critical for a marathon, where many runners start to slow down during the final 10K.

  
  

I believe that most athletes would benefit from being more metabolically flexible, both during training and at rest (in between training sessions). Burning fat efficiently for fuel at rest and during lower training intensities can be helpful from a metabolic and fueling standpoint. However, we also want to utilize carbohydrates both during exercise and after meals.

Hopefully, this overview of metabolic efficiency and flexibility has been helpful. If you are interested in fueling and training strategies to enhance both metabolic efficiency and metabolic flexibility, please check out this article.