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Many endurance athletes are familiar with this phenomenon: fitness is good, training is on track, yet performance suddenly drops. Often, the limiting factor is not the cardiovascular system, but rather how the body supplies energy and how effectively it can switch between different energy sources. This is precisely where metabolic flexibility comes into play.

Metabolic flexibility describes the body’s ability to dynamically adapt energy metabolism to the current exercise intensity and energy availability. In endurance sports, this primarily means being able to switch efficiently between fat and carbohydrate metabolism, depending on what the situation requires.

A metabolically flexible system relies predominantly on fat as an energy source at low and moderate intensities and increasingly turns to carbohydrates as intensity rises. The key factor is not an ‘either/or’ approach, but the ability to use both systems in parallel and prioritise them according to demand.

Why is metabolic flexibility crucial for performance?

Carbohydrates are only available to the body in limited amounts. Once glycogen stores are depleted, performance declines – even if the heart, lungs and muscles are still capable of working. A well-developed fat metabolism helps conserve these stores and maintain performance over longer periods.

At the same time, endurance performance is rarely constant: climbs, changes of pace, race situations or a final sprint require a short-term, high energy supply from carbohydrates. Metabolically flexible athletes are better able to cope with these intensity changes without subsequently falling into an energy deficit.

Another advantage is that, as metabolic flexibility improves, the intensity at which the body becomes predominantly dependent on carbohydrates shifts to higher levels. This makes metabolism more economical, reduces fatigue and enhances overall endurance performance.

Metabolic flexibility and nutrient timing

In addition to training, the timing of nutrient intake also plays an important role. Metabolic flexibility does not mean training permanently with low carbohydrate availability (low carb), but rather using carbohydrates strategically when they are needed for quality, intensity and recovery.

Long, easy training sessions can be completed with a low carbohydrate intake to promote fat metabolism. In contrast, intensive sessions, competitions and key workouts should be deliberately supported with a high carbohydrate intake to avoid performance losses and to optimise training adaptations.

A sustained energy deficit or restrictive dietary strategies impair both performance and recovery in the long term. Metabolic flexibility develops through adaptability, not through deprivation.

How can metabolic flexibility be improved in a targeted way?

The foundation is a high training volume at low to moderate intensity with little or no carbohydrate intake. This promotes fat metabolism and supports structural adaptations within the muscles. In addition, intensive training stimuli such as interval sessions or threshold workouts with high energy availability are required. These improve the efficient use of carbohydrates and enhance the ability to rapidly supply energy during changes in intensity. Equally important as the training itself are sufficient sleep, an energy intake matched to requirements and sensible periodisation. Metabolic adaptations do not occur in the short term, but develop over the course of several months.