Forget Carbo-loading – Sports Nutrition is Getting a Shake Up
A new study is food for thought for those into eating better for exercise and sports. The future of performance isn’t high-carb dogma—it’s metabolic flexibility.
In a landmark review published in Endocrine Reviews, Andrew Koutnik, PhD, and colleagues synthesized nearly 600 studies spanning over 100 years of research on carbohydrate intake, metabolism, and performance. What they found – the limiting factor in endurance is not muscle glycogen.
For most of modern sports science, endurance nutrition has revolved around one central idea: carbohydrates fuel performance. From pasta dinners before races to energy gels during marathons, athletes have long been taught that success depends on filling muscles with glycogen and preventing those stores from running empty.
But emerging research is challenging that narrative.
Koutnik et al. shows a growing body of evidence suggestong that endurance performance may depend less on muscle glycogen levels than previously believed — and more on the brain’s access to energy. This shift does not mean carbohydrates are unimportant. Rather, it reframes their role within a broader metabolic system that includes fat oxidation, energy availability, and something scientists call metabolic flexibility.
This evolving perspective has implications not only for elite athletes but also for recreational exercisers, coaches, and anyone interested in metabolism and long-term health.
The rise of the carbohydrate paradigm
The carbohydrate-centric model of endurance performance took shape in the mid-20th century when physiologists began measuring muscle glycogen directly. Researchers observed that endurance exercise depleted glycogen stores and that exhaustion often coincided with low muscle glycogen levels.
From these observations came a practical conclusion:
If glycogen depletion leads to fatigue, then maximizing glycogen should improve endurance.
This logic drove decades of sports nutrition advice. Carbohydrate loading protocols were developed in the 1960s and 1970s, and by the late 20th century, carbohydrate supplementation during exercise became standard practice.
The strategy worked — at least in some contexts. Carbohydrate ingestion during prolonged exercise can maintain blood glucose levels and delay fatigue, particularly in events lasting longer than an hour.
But as physiology research advanced, scientists began to realize that the glycogen-depletion model might be incomplete.
Fat: the overlooked endurance fuel
Even the leanest athlete carries enormous energy reserves in body fat. While muscle glycogen stores typically contain roughly 1,500–2,000 kcal, fat stores can hold tens of thousands of calories.
During low-to-moderate intensity exercise, fat is already the dominant fuel source. Endurance training further increases the body’s ability to oxidize fat by enhancing mitochondrial function and enzyme activity.
Studies have shown that training with reduced carbohydrate availability — for example, exercising in a fasted state — can stimulate metabolic adaptations that increase fat oxidation capacity.
More recent research confirms that dietary strategies, including low-carbohydrate or periodized carbohydrate intake, can substantially alter substrate utilization during exercise without necessarily improving or impairing performance outcomes.
These findings raise an important question:
If the body can rely heavily on fat for energy, why would glycogen depletion alone determine endurance limits?
The brain’s role in endurance
One emerging explanation centers on the nervous system.
The brain consumes a disproportionate share of the body’s energy relative to its size. Although it represents only about 2% of body mass, it uses roughly 20% of resting energy expenditure. Maintaining stable energy supply to the brain is therefore critical for survival — and potentially for athletic performance.
Modern exercise physiology increasingly recognizes that fatigue is not purely muscular. Instead, endurance performance is influenced by central regulation, meaning the brain integrates signals about energy status, temperature, stress, and metabolic reserves before allowing continued effort.
In this model, fatigue may occur not because muscles are incapable of contracting, but because the brain reduces motor output to protect the organism from metabolic danger.
Research on energy availability in athletes supports this idea. Studies show that insufficient energy availability can affect physiological regulation and performance-related processes even when measurable performance changes are small or inconsistent.
From this perspective, maintaining adequate circulating fuel — especially blood glucose — may be critical not only for muscles but for brain function during prolonged exertion.
Rethinking carbohydrate intake during exercise
A recent synthesis of hundreds of studies led by researcher Andrew Koutnik and colleagues argues that carbohydrate intake during endurance exercise may primarily support brain energy availability rather than muscle glycogen preservation.
Their analysis suggests several provocative ideas:
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High carbohydrate intake can suppress fat oxidation during exercise.
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Rapid carbohydrate availability may accelerate glycogen utilization.
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Small amounts of carbohydrate may restore performance by maintaining blood glucose.
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Central energy regulation may play a larger role in endurance fatigue than muscle glycogen depletion.
While these claims remain debated within sports science, they align with broader trends in metabolism research emphasizing the importance of energy regulation at the whole-body level.
Importantly, this interpretation does not argue against carbohydrates entirely. Instead, it suggests that more is not always better, and that strategic intake may be more effective than maximal intake.
The concept of metabolic flexibility
At the center of this discussion is metabolic flexibility, the ability to shift between fuel sources depending on availability and demand.
Metabolic flexibility allows the body to:
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Burn fat efficiently during lower-intensity exercise
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Use carbohydrates rapidly during high-intensity efforts
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Maintain stable energy supply during changing conditions
Researchers define metabolic flexibility as the capacity to adapt substrate utilization to energy availability and physiological requirements.
This adaptability is fundamental to energy homeostasis and is closely linked to mitochondrial function and overall metabolic health.
When metabolic flexibility is impaired — as in insulin resistance or metabolic disease — the body becomes less efficient at switching fuels.
For endurance athletes, strong metabolic flexibility may allow sustained performance without dependence on constant carbohydrate intake.
Training the metabolism, not just the muscles
Endurance training already promotes metabolic flexibility. Over time, athletes develop:
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Increased mitochondrial density
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Improved fat oxidation capacity
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Enhanced glycogen storage efficiency
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Greater insulin sensitivity
Some training strategies intentionally manipulate carbohydrate availability to amplify these adaptations.
For example:
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Fasted training sessions
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Low-carbohydrate training blocks
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Periodized nutrition strategies
These approaches aim to increase fat oxidation while preserving the ability to use carbohydrates when necessary.
However, the evidence is mixed. Some studies show metabolic benefits without performance changes, suggesting that physiological adaptation does not always translate directly into faster race times.
This highlights an important distinction between metabolic efficiency and competitive performance, which may depend on multiple interacting factors.
The brain–body energy partnership
If endurance performance depends on both muscular and neural energy supply, then fueling strategies must support both systems.
The brain can use multiple fuels:
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Glucose
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Lactate
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Ketones (during prolonged exercise or carbohydrate restriction)
Maintaining stable blood glucose appears especially important during long-duration exercise. When glucose levels fall too low, fatigue can occur rapidly — even when muscle energy reserves remain.
This observation helps explain why small carbohydrate doses during exercise often improve performance.
Instead of “filling the tank,” these smaller amounts may function more like maintaining system stability.
Moving beyond “carb dependency”
The traditional sports nutrition message often implied that endurance athletes must consume large quantities of carbohydrates to perform well.
But modern research suggests a more nuanced view:
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Carbohydrates are useful but not always limiting.
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Fat oxidation can support a large portion of endurance energy needs.
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The brain plays a central regulatory role in fatigue.
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Fuel availability, not just fuel quantity, matters.
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Metabolic flexibility may be a key determinant of performance and health.
This reframing does not eliminate carbohydrate loading or sports drinks. Instead, it positions them as tools rather than necessities.
Athletes may benefit from developing the ability to perform across a range of fuel conditions rather than relying exclusively on high-carbohydrate intake.
Implications beyond sport
The idea of metabolic flexibility extends far beyond endurance athletics.
Modern lifestyles often involve constant carbohydrate availability and limited metabolic variability. Over time, this can contribute to reduced metabolic flexibility and increased risk of chronic disease.
Exercise — especially endurance training — acts as a powerful stimulus for restoring metabolic adaptability.
This connection between performance physiology and public health is increasingly recognized in metabolic research.
The same mechanisms that help athletes sustain long efforts may also help protect against:
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Type 2 diabetes
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Obesity
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Cardiovascular disease
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Age-related metabolic decline
In this sense, endurance metabolism research offers insight into human health more broadly.
A more integrated model of endurance performance
The emerging picture of endurance performance is more complex than earlier models suggested.
Rather than focusing on a single limiting factor, performance appears to depend on the interaction of multiple systems:
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Muscular metabolism
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Brain energy availability
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Cardiovascular capacity
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Thermoregulation
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Hormonal signaling
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Fuel availability
Muscle glycogen remains important, but it is no longer viewed as the sole determinant of endurance.
Instead, endurance performance may be best understood as a system-level.
Athletes who can switch efficiently between fuels — and maintain stable brain energy supply — may have an advantage not only in endurance performance but also in lifelong metabolic health.
Conclusion
For decades, endurance nutrition centered on a simple idea: prevent glycogen depletion at all costs.
Today, research is revealing a more sophisticated picture.
Endurance performance may depend less on how much carbohydrate is stored in muscle and more on how effectively the body manages energy overall — especially for the brain.
Carbohydrates still matter, but metabolic flexibility, fat oxidation, and energy availability may matter just as much.
The emerging lesson is not to abandon carbohydrates, but to understand them within the broader physiology of human energy regulation. The future of endurance performance may lie not in maximizing one fuel, but in mastering them all.
References
Burke, L. M., Hawley, J. A., Wong, S. H., & Jeukendrup, A. E. (2011). Carbohydrates for training and competition. Journal of Sports Sciences, 29(sup1), S17–S27.
Goodpaster, B. H., & Sparks, L. M. (2017). Metabolic flexibility in health and disease. Cell Metabolism, 25(5), 1027–1036.
Hawley, J. A., Leckey, J. J. (2015). Carbohydrate dependence during prolonged, intense endurance exercise. Sports Medicine, 45(Suppl 1), S5–S12.
Jeukendrup, A. E. (2014). A step towards personalized sports nutrition: Carbohydrate intake during exercise. Sports Medicine, 44(Suppl 1), S25–S33.
Koutnik, A. P., D’Agostino, D. P., & Volek, J. S. (2024). Carbohydrate intake, metabolism, and endurance performance: Revisiting the glycogen-centric model. Endocrine Reviews.
Loucks, A. B., Kiens, B., & Wright, H. H. (2011). Energy availability in athletes. Journal of Sports Sciences, 29(Suppl 1), S7–S15.
Noakes, T. D. (2012). Fatigue is a brain-derived emotion that regulates exercise performance to ensure the protection of whole-body homeostasis. Frontiers in Physiology, 3, 82.
Spriet, L. L. (2014). New insights into the interaction of carbohydrate and fat metabolism during exercise. Sports Medicine, 44(Suppl 1), S87–S96.
Stellingwerff, T., Morton, J. P., & Burke, L. M. (2019). A framework for periodized nutrition for endurance sport. International Journal of Sport Nutrition and Exercise Metabolism, 29(2), 141–151.
About the author: John O’Sullivan is CEO and co-founder (with Dr Tim Ball among 45 scientists) of Principia Scientific International (PSI). He is a seasoned science writer, retired teacher and legal analyst who assisted skeptic climatologist Dr Ball in defeating UN climate expert, Michael ‘hockey stick’ Mann in the multi-million-dollar ‘science trial of the century‘. From 2010 O’Sullivan led the original ‘Slayers’ group of scientists who compiled the book ‘Slaying the Sky Dragon: Death of the Greenhouse Gas Theory’ debunking alarmist lies about carbon dioxide plus their follow-up climate book. His most recent publication, ‘Slaying the Virus and Vaccine Dragon’ broadens PSI’s critiques of mainstream medical group think and junk science.