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Understanding Movement Economy in Keto-Adapted Athletes: A Flawed Study and the Need for Common Sense

Coach Bronson

The Study in Question

One of the most frequently cited studies arguing against ketogenic diets for endurance performance is the 2017 study by Burke et al., published in The Journal of Physiology. This study examined the effects of a low-carbohydrate, high-fat (LCHF) diet on elite race walkers during a three-week training intervention. The researchers found that while the LCHF diet significantly increased fat oxidation, it also resulted in a higher oxygen cost at given walking speeds.

The study concluded that this reduced exercise economy negated performance benefits compared to high-carbohydrate (HCHO) and periodized carbohydrate (PCHO) diets, which showed improvements in race times. However, this conclusion is based on flawed assumptions about metabolic efficiency and fails to consider the broader physiological impact of keto-adaptation.

Additionally, the observed performance decrement in the LCHF group is likely due to an inadequate adaptation or washout period in the study’s protocol. This is a common issue in ketogenic studies conducted by researchers who do not fully understand the timeline and complexity of keto-adaptation. I will cover this topic in future blog posts to address these misconceptions.

What is Movement Economy?

Movement economy measures how efficiently an athlete converts energy into movement. Typically assessed by measuring oxygen consumption (VO2) at a given workload, it provides insights into an athlete’s work capacity relative to energy expenditure. In conventional endurance sports science, greater oxygen consumption at a given intensity is often interpreted as decreased efficiency. However, this interpretation is based on a fundamental assumption: glucose is the primary fuel source.

The Flaw in Using Oxygen as the Sole Determinant of Efficiency

One of the primary issues with studies assessing movement economy in keto-adapted athletes is their reliance on oxygen consumption as a metric of efficiency. When an athlete becomes fat-adapted, the amount of fat used to produce ATP aerobically increases, which by definition, requires more oxygen. As part of adaptation, VO2 max increases, allowing for greater oxygen availability to meet the increased demand. This shift is a feature, not a flaw. The entire framework of movement economy must be reconsidered in the context of a fat-adapted athlete, where oxygen is utilized efficiently as a key substrate rather than being a limiting factor.

The assumption that increased oxygen use equates to reduced performance is directly contradicted by research, such as the study by Prins et al. (2019) published in the Journal of Sports Science and Medicine. Their findings demonstrated that athletes following an LCHF diet for six weeks exhibited significantly higher fat oxidation during high-intensity exercise (~82% VO2 max) without any impairment in 5K time trial performance compared to a high-carbohydrate diet. This study directly refutes the notion that increased fat oxidation inherently leads to decreased efficiency or performance decline, reinforcing the need to reconsider how movement economy is assessed in keto-adapted athletes.

The Crossover Point and Fat Utilization at Higher Intensities

In carbohydrate-dependent athletes, the crossover point — the intensity at which the body shifts from primarily fat to primarily carbohydrate metabolism — occurs at a low to moderate intensity (~65% VO2 max). Keto-adapted athletes experience a shift in this crossover point to much higher intensities, meaning they rely on fat for fuel even at efforts where glucose oxidation would dominate in non-keto-adapted individuals. This adaptation allows for greater endurance and sustained performance without constant carbohydrate refueling. How is this a bad thing? If an athlete can perform at a higher intensity for longer while utilizing fat, this should be seen as an advantage, not a detriment.

The review by Noakes et al. (2023) in Frontiers in Physiology further challenges the traditional crossover model, demonstrating that fat-adapted athletes can sustain fat oxidation rates above 1.5 g/min at intensities as high as 85% VO2 max. Their findings show that adaptation to an LCHF diet shifts the crossover point to a higher percentage of VO2 max (>80%), meaning that keto-adapted athletes can rely on fat oxidation at much greater intensities than previously believed. This completely undermines the outdated idea that carbohydrate oxidation is necessary for high-intensity endurance performance. It provides clear evidence that an LCHF diet enhances metabolic flexibility rather than impairing it.

The Holistic View: Beyond Oxygen and Efficiency Misconceptions

When evaluating performance outcomes in keto-adapted athletes, we must take a broader perspective that includes the following:

A Call for Common Sense in Evaluating Nutrition & Performance

The fundamental issue with studies that claim keto-adapted athletes suffer from reduced movement economy is that they are evaluating these athletes through the outdated lens of carbohydrate-based metabolism. Oxygen utilization does not equate to inefficiency in a fat-adapted state. A higher crossover point extends fat utilization into higher intensities, providing a significant endurance advantage. When considering the holistic benefits — greater glycogen preservation, reduced lactate accumulation, and lower muscle breakdown — it becomes clear that keto-adaptation substantially benefits endurance performance.

If we evaluate nutrition protocols, we must do so with an understanding of how they fundamentally change metabolic processes rather than forcing outdated models onto new paradigms. The time for a carbohydrate-centric bias in endurance science is over. Let’s apply some common sense to the process and stop perpetuating a flawed narrative.

References

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