Metabolic Efficiency vs. Metabolic Flexibility: What Endurance Athletes Need to Know

Endurance athlete burning both carbohydrate and fat for fuel

Endurance athletes are constantly looking for ways to race faster, recover better, and sustain performance over long distances. Two concepts often discussed in sports physiology—metabolic efficiency and metabolic flexibility—both influence how athletes produce and use energy. Although they’re related, they are not the same thing. Understanding these concepts and how to utilize them can help athletes optimize 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 rely almost entirely on fat for fuel at lower intensities (think zone 1 and low zone 2). Then as the intensity increases (high zone 2 and zone 3) they will be able to continue to utilize a slightly higher percentage (not 100%) fat for fuel compared to an athlete who is not metabolically efficient. An athlete training in a sub-maximal effort will still be burning some carbohydrate, but the percentage of fat to carbohydrate being used will be more than a non-metabolic efficient athlete.

Why it matters for endurance athletes

• Improved endurance: Even the leanest athletes store tens of thousands of calories of fat, but only ~2,000 calories of glycogen. More fat use = delayed glycogen depletion.

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

• Better performance at moderate intensities: 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 limitation to being more fat adapted. 

First, let’s review the crossover concept in endurance exercise. This concept refers to the relative balance between fat and carbohydrate metabolism during sustained exercise. At rest and at very easy intensities, our body is burning nearly 100% fat as fuel. As the exercise intensity starts to increase, there is a shift that starts to occur and the percentage of fat being utilized for energy starts to go down and the amount of carbs used for exercise increases. To see a visual of this you can go to this link for more information. The goal of improving fat adaptation is to push that crossover point a little bit further to the right or to increase the amount of fat burned at slightly high intensities before the body “switches” to burning more carbohydrates for fuel. 

The limitation with this concept is that the body will never be able to burn fat for fuel at the highest of intensities (zone 4 into the highest training zones). This is because the ability of the body to break down fat and use it for fuel is too slow to meet the metabolic demands of very high intensity activities. Fat metabolism also requires a large and steady state of oxygen. Higher intensity exercise often outpaces oxygen intake. As intensity increases, the body needs to rely more on carbohydrate for fuel. Carbohydrate can be burned as fuel during high intensity exercise through anaerobic glycolysis. This type of fuel is limited and is why we can only train at these high intensities for relatively short durations.

When we are looking at different types of activities or events, in general, training metabolic efficiency might not make sense for any race in which the intensity is greater than about 70% Vo2 Max or where the athlete is going beyond their zone 2 and into higher zones of heart rate closer to zone 3 or higher. This would include any race of about 2 hours or less such as a half marathon or olympic distance triathlon and faster. There maybe some benefit to training metabolic efficiency for a marathon runner, but if the runner is focusing on fat adaptation, they may be compromising their ability to use carbohydrates when needed (more on this later).

There are not many studies that use the term metabolic efficiency, but there are studies that look at athletes fueling with a low carb approach that can give us some insights into the potential benefits and drawbacks of being metabolically efficient. 

The key thing to consider is that there is an economic cost to being more “fat adapted.” Studies on endurance athletes (race walkers) consuming a high fat, low carb diet found that although they were able to utilize more fat during the 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 in elite race walkers. Link to this study can be found here.

Yes, 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. 

Another issue with becoming more fat adapted is that it limits the ability for the body to use carbs for fuel when needed. A fat-adapted athlete has trouble burning 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 a fat adapted athlete might do just fine when the paces and intensities are stable, but they may struggle to shift to higher intensities. This could be during an uphill portion of a race or trying to surge during a race or kick at the end. The thing to keep in mind here is that this type of adaptation may only occur when carbohydrate availability is limited in the long term. It is possible for an athlete can improve metabolic efficiency without compromising their ability to also burn carbohydrate as fuel when needed. This is why I would not recommend a low carbohydrate approach as a long term solution for most endurance athletes.

Benefits to being more fat adapted or metabolically efficient:

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

• You may notice that your heart rate is lower at your easier paces such as your zone 2 pace, however, at higher paces this may not be the case due to the oxygen cost of burning more fat for fuel

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

• Better late-race resilience- which means you are able to feel slightly better at the end of a longer race

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

The drawbacks to being more fat adapted or metabolically efficient:

• You may struggle to maintain higher intensity exercise 

• It is harder to switch between paces such as during a surge in a race or during a kick at the end of the race

• The transition period may involve a lower carbohydrate approach as well as a period of time doing higher volume, low intensity exercise, which many athletes are not willing to do

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

  
  

Now let’s contrast or compare this to Metabolic Flexibility

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 very important for long-distance performance, especially for marathoners, ultra runners, and triathletes.

A metabolically flexible athlete can:

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

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

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

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

What is the difference? The difference between metabolic efficiency vs flexibility is that metabolic efficiency is training to improve the body’s ability to burn fat during exercise. Being metabolically flexible is the ability for the body to switch back and forth between fat and carbs. The key here is that being metabolically efficient does not mean an athlete is metabolically flexible, but a metabolically flexible athlete can be metabolically efficient. So if an athlete eats and trains only to improve fat utilization they may make it harder for the body to switch to using carbs when the body needs them. An example here might be during a marathon where an athlete has trained their body to be more metabolically efficient using a longer term low carb strategy. They might do very well when their intensity is stable, but what happens if they want to kick at the end of the race or if they have to increase intensity to power up a hill? If they are not also metabolically flexible, they may lack the ability to switch easily to carb burning for fuel once their intensity increases beyond the ability to burn fat. This could potentially limit their ability to perform. 

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 = less muscle fiber fatigue late.

3. Enhances your ability to surge and cover moves- Useful for hills, surges during a race, running in a pack, or sprinting to the finish at the end of the race.

4. Improves fueling tolerance- More efficient carb use → fewer gut issues.

5. Supports higher training volume- Stable energy → better recovery and ability to stack sessions.

The physiology (simple version)

Metabolic flexibility is driven by:

• Mitochondrial density (more = more efficient energy burning)- increasing the amount of mitochondria in the cell can improve endurance performance by increasing the capacity to utilize both fat and carbohydrate for fuel. Think of the mitochondria as the powerhouse of the cell.

• Hormonal signals (insulin, adrenaline, cortisol)- although insulin is not being secreted during exercise, insulin sensitivity is increased. Someone who has a higher level of insulin sensitivity (as opposed to being insulin resistance) will have better metabolic flexibility at both rest and during exercise. Remember, insulin is the signal to let glucose into the cell for fuel and the quicker and easier this process can occur, the better from an energy standpoint. Cortisol and adrenaline are released during exercise to help mobilize both fat and carbohydrate for fuel.

• Glycogen availability- When the body is able to burn more fat at lower intensities the amount of glycogen used at that same intensity will be less which means you can run longer before glycogen is depleted.

• Enzyme adaptation (PDH, CPT-1, AMPK)- when someone is metabolically flexible they can up-regulate enzymes that help with fat utilization and glucose utilization. The enzymes can also help the body more quickly switch fuel sources.

• Fat oxidation rate ("FatMax")- This is how fast the body is able to burn fat for fuel. When we train or eat in a certain way, we can increase the "FatMax" which is measured on a percentage our

  our Vo2 max. For most people this may occur at 40-60% Vo2 max. If someone is more metabolically efficient, it might be at the higher end at closer to 60% Vo2 max or slightly higher.

• Ability to up-regulate carbohydrate oxidation under load- The ability for the body to increase the rate of carbohydrate oxidation as the intensity increases or during a specific load such as heat stress. This means that the body can maintain or increase the ability to digest, mobilize, and utilize carbohydrate as the intensity increases or if there is another form of stress or "load" such as heat.

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 HR at the same pace- an increase in mitochondria usually means that there is more efficiency in energy supply and a decreased load on the heart, which can result in a lower heart rate. 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 gets slightly easier and more manageable or you are able to go faster during long runs and recover more quickly afterwards

• Fewer GI issues when fueling- you may find that you need fewer carbs per hour or that you are able to tolerate more carbs during runs

• Faster marathon pace or a lower rate of perceived exertion at the same marathon pace- your marathon pace might feel a bit easier for you or might find that you are able to increase your marathon pace slightly

• Easier time taking gels without crashing- your energy levels during runs might be more stable or you may not notice the same energy surge and subsequent "crash" when taking gels during a run

• Better late-race strength- You may notice that during the final portion of a race, you are better able to hang on to your goal pace. This is critical for a marathon when many runners start to experience a slow down of the pace during the final 10k.

I would argue 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, but we also want to be able to utilize carbohydrate both during exercise and after meals.

Hopefully, this was a helpful overview of both metabolic efficiency and flexibility and how they relate. If you are interested in fueling and training strategies to enhance metabolic efficiency and metabolic flexibility please check out this article.