Inside AJCN’s Great Debates in Nutrition
Few topics in sports nutrition generate as much discussion as the role of carbohydrates in endurance performance. Should athletes prioritize a high carbohydrate diet to maximize speed and efficiency? Or can a well-formulated low-carbohydrate, high-fat (or ketogenic) diet support performance just as well?
The American Journal of Clinical Nutrition (AJCN), a journal of the American Society for Nutrition (ASN), examines this question in its Great Debates in Nutrition series. This installment features two opposing papers and a consensus paper focused on whether a low-carbohydrate diet impairs endurance sports performance.
Why Does Fuel Choice Matter?
Endurance performance depends on how well the body produces energy during exercise.
Muscles rely mainly on two fuels:
- Carbohydrates (or carbs), stored as glycogen in muscles and the liver
- Fat, stored in larger amounts throughout the body
Both fuels can support endurance exercise. The debate centers on whether one supports high-level performance better than the other.
The Case For: Yes, It Can Impede Performance
Louise M. Burke, PhD, Mary MacKillop Institute for Health Research, Australian Catholic University, maintains that while ketogenic low-carbohydrate diets dramatically increase fat oxidation (fat burning), they may reduce performance during high-intensity endurance events.
She explains that carbohydrates produce more energy than fat for the same amount of oxygen consumed, making them a more oxygen-efficient fuel during high-intensity exercise. If fat requires more oxygen to produce energy, athletes competing near their maximum capacity may need to slow down.
In studies of elite race walkers, Burke’s team observed higher fat burning, but also a decline in race performance following ketogenic adaptation. She notes that carbohydrate intake should be adjusted based on training intensity and competition demands. In her view, strict long-term carbohydrate restriction may not be ideal for high-intensity endurance events. Nutrition should be personalized and flexible.
The Case Against: No, It Does Not Impede Performance
Timothy Noakes, MBChB, MD, DSc, University of Cape Town, South Africa, maintains that long-held assumptions about carbohydrate dependence for endurance performance may need reevaluation.
After reviewing randomized controlled trials comparing high-carbohydrate and low-carbohydrate diets, he concludes that after proper adaptation of four to six weeks, athletes perform similarly on both diets.
Noakes contends that athletes can adapt to burn fat at very high rates, that running low on muscle glycogen may not directly cause fatigue, and that hypoglycemia (low blood glucose), rather than low muscle glycogen, may be the main cause of performance decline during long events. In one study cited, consuming just 10 grams of carbohydrate per hour to prevent low blood glucose improved performance, regardless of the athletes’ usual diet.
For Noakes, the key question is: after athletes adapt to their diet, does the balance of carbs and fat they regularly eat actually affect performance? Based on current research in trained athletes, he concludes it does not.
Points of Agreement and Open Questions
Despite opposing perspectives, the authors agree that:
- Eating carbohydrates during prolonged exercise can improve performance.
- A low-carb diet increases fat burning, although a period of physiological adaptation of several weeks is required to achieve optimal results.
- Fat requires more oxygen than carbohydrates to produce the same amount of energy.
- Fuel use during high-intensity exercise is complex.
But some key questions remain:
- Does burning fat, which uses more oxygen than carbs, really limit performance for elite athletes?
- Is running out of muscle glycogen actually what causes fatigue?
- Are high carbohydrate intakes during competition necessary, or just a long-standing habit?
- Do elite athletes react differently to diet compared with recreational athletes?
Why This Debate Matters
The AJCN Great Debates in Nutrition series, edited by David Ludwig, MD, PhD, was created to spark robust yet collegial discussion on major controversies in nutrition.
By presenting two evidence-based perspectives along with a consensus summary, The AJCN encourages the nutrition field to:
- Reexamine long-held beliefs
- Identify gaps in the evidence
- Design stronger studies
- Raise public awareness of the importance of nutrition science to health
Whether you work with elite endurance athletes, recreational competitors, or study metabolism, this debate provides valuable insight.
As nutrition science continues to evolve, productive disagreement remains one of its greatest strengths.
Read the Full Debate
Explore the yes and no perspectives as well as the consensus paper in The American Journal of Clinical Nutrition and join the conversation shaping the future of sports nutrition research.
