Metabolic flexibility: what is it and how to improve it as an athlete? | DEEP DIVE

The majority of what I do as a sports nutritionist coach is hands-on. What do you eat before training? How many carbohydrates do you need? When does a protein shake make sense? I try to make the science as concrete and applicable as possible, because ultimately, that's what's useful.

But sometimes it is also valuable to take a step back and understand how and why something works. Not as a replacement for practice, but as the foundation beneath it. Because if you understand how your body deals with energy, you will naturally make more conscious choices. Even without me giving you a list for it.

This blog is about metabolic flexibility. A term you may have seen before, but whose meaning isn't always clear. I'll.

Metabolic flexibility is the body's ability to switch between burning carbohydrates and fats for energy as needed.

Your body has two main energy sources: carbohydrates and fats. Carbohydrates are readily available and provide energy quickly: ideal for higher exertion. Fats provide energy more slowly, but the supply is enormous and they are particularly suitable as fuel when your body isn't working at full throttle.

A metabolically flexible body uses resources precisely as the situation demands. At rest or during gentle movement, you primarily burn fats. As soon as you exercise harder or demand more of yourself, your body smoothly switches to carbohydrates. And when the exertion then decreases, it switches back just as smoothly. Researchers Goodpaster and Sparks describe this as the ability to respond to and adapt to changing metabolic conditions. (Source: Goodpaster & Sparks, Cell Metabolism, 2017 – https://doi.org/10.1016/j.cmet.2017.04.015)

When this flexibility is lacking, your body continues to rely too heavily on carbohydrates. This happens even when it's not actually necessary. You then deplete your energy stores more quickly, experience more energy dips, and have more difficulty maintaining your level during longer periods of exertion.

How does it work in your body?

To understand why metabolic flexibility is so important, it helps to take a brief look at what happens in your body.

Carbohydrates are stored as glycogen, primarily in your muscles and liver. This supply is limited. Depending on your fitness level and body size, you'll have enough for roughly an hour to an hour and a half of intensive exercise. Fats are practically inexhaustible as an energy source; even a lean, trained athlete has more than enough fat reserves to draw upon for hours.

The key to switching between these two sources is insulin: a hormone that your pancreas produces after eating. Insulin ensures that glucose (the energy from carbohydrates) is absorbed by your cells. The more sensitive your cells are to insulin, the more efficient this process becomes and the faster you then switch back to burning fat.

In people with reduced insulin sensitivity, this switchover is slower and more difficult. The body continues to run on carbohydrates for too long, even in situations where fat burning would actually be more logical. That is, at its core, what metabolic inflexibility is.

What Asker Jeukendrup says about it

Asker Jeukendrup is one of the world's leading sports nutrition scientists. He studied and obtained his PhD at Maastricht University in the field of carbohydrate and fat metabolism during exercise, and has published over 200 scientific articles in the field of sports nutrition and energy metabolism.

One concept he describes in this context is periodisation of nutrition: the idea that you adjust your carbohydrate intake in line with your training. Easy sessions can be done fasted or with low carbohydrate intake to give fat metabolism an extra boost. Hard sessions deserve good fuel. Training with low carbohydrate availability demonstrably leads to better adaptations in fat metabolism, but it is a targeted training stimulus, not a daily strategy. (Source: Jeukendrup, A.E. Periodised Nutrition for Athletes. Sports Med, 2017 – https://doi.org/10.1007/s40279-017-0694-2)

As a coach, I see it a little differently. My starting point is that most recreational athletes benefit from sufficient carbohydrates, especially around training. Too few carbohydrates lead to poorer performance, less recovery and, in the long run, to fatigue that undermines your training. Occasionally training with low carbohydrate availability can be a useful addition for some athletes, but it is not standard advice that I would simply recommend for everyone. The foundation needs to be solid first.

Which sports does it make the most difference for?

Metabolic flexibility is relevant for virtually every recreational athlete, although the impact varies by sport.

During prolonged, moderately intense sports such as running, cycling, swimming or hiking, the impact is greatest. The better your body uses fats at that moderate intensity, the longer you will conserve carbohydrates for moments when you really need them. Those who are metabolically less flexible burn too many carbohydrates even at a gentle pace and run out of energy sooner. Something many recreational athletes will recognise without knowing the reason why.

In team sports like football, hockey, or basketball, it plays out differently. These sports are characterised by constantly varying intensity: a quiet phase, then a sprint, then recovery again. A flexible metabolism recovers faster between these peaks and ensures that you can still perform in the final phase. Even if you only do it once or twice a week.

In strength sports and fitness, the direct impact is less visible, but insulin sensitivity also plays a role here. Specifically in how efficiently your body absorbs nutrients after training and uses them for recovery. This recovery process is particularly important for athletes who do not train daily, as the time between sessions is longer.

How can you test your break-even point?

The point at which your body switches from predominantly burning fat to predominantly burning carbohydrates is called the crossover point. Related to this is the FATmax point: the intensity at which your fat burning is at its highest. This is a useful measure, as it shows how well your body is at utilising fats and at what intensity that decreases. The concept of FATmax was developed by Jeukendrup and Achten and has since become the standard for measuring fat burning during exercise. (Source: Achten & Jeukendrup. Optimising fat oxidation through exercise and diet. Nutrition, 2004 – https://pubmed.ncbi.nlm.nih.gov/15212756/)

Using a specialised exertion test with breath gas analysis, sometimes combined with lactate measurement, you can precisely map out how efficiently your body switches between fat and carbohydrate metabolism at increasing intensity. During such a test, you run or cycle in steps of increasing intensity on a treadmill or cycle ergometer, while your breathing is measured via a mask. The ratio between the CO2 you exhale and the O2 you inhale tells the laboratory exactly which fuel you are burning at any given moment.

In well-trained athletes, the point of maximum fat burning occurs at a relatively high exercise intensity. In less trained individuals, this point is reached at a much lower intensity, often even at a moderate pace. Therefore, a trained body can operate on fat for longer and at a higher intensity before it has to switch over.

Such a test is available at sports labs, university testing centres, and specialised sports clinics in the Netherlands. Search for terms like “exercise test with respiratory gas analysis”, “VO2max test” or “FATmax measurement” to find a provider near you. Always ask in advance if the test includes respiratory gas analysis, because without that measurement, you cannot objectively determine the turning point.

How can you improve your metabolic flexibility?

Metabolic flexibility isn't a fixed given; you can improve it. The foundation lies in regular exercise at low to moderate intensity: think of a relaxed endurance training session, a bike ride, or a walk where you can still hold a conversation. It's precisely at these lower intensities that you train your body to utilise fats more efficiently. Combine this with a diet that aligns with it: sufficient protein and vegetables with every meal, less processed food, and carbohydrates that you broadly tailor to how hard you're training. On a heavy training day, you'll need more carbohydrates; on a rest day or light training day, considerably fewer. Sleep is an underestimated factor. Lack of sleep disrupts hormone balance and reduces insulin sensitivity, two things that directly affect how well your body switches between fuels.

As a recreational athlete, do you need to be concerned with this daily? No. It's primarily valuable to understand how your body works, so you can make better choices when it matters. The practical application is simpler than the theory might suggest. Small, consistent habits over a longer period have more impact than a radical overhaul that you stick with for two weeks.

Do you want to know how your diet aligns with your sport and energy needs? Take Contact for personal sports nutrition advice or sign up for the free Masterclass Sports Nutrition Basics.