Inside the AJCN Great debates in food
Few topics in sports nutrition generate as much debate as the role of carbohydrates in endurance performance. Should athletes prioritize a high-carbohydrate diet to maximize speed and performance? Or can a well-designed low-carb, high-fat (or ketogenic) diet also support performance?
American Journal of Clinical Nutrition (AJCN), the journal of the American Society for Nutrition (ASN), addresses this question Great debates in food series This section includes two contrasting papers and one consensus paper on whether a low-carbohydrate diet impairs athletic performance.
Why is fuel selection important?
Endurance performance depends on how much energy the body produces during exercise.
Muscles rely mainly on two fuels:
- Carbohydrates (or carbohydrates) stored as glycogen in the muscles and liver
- Fat that is stored in large quantities throughout the body
Both fuels can support sustained exercise. The debate is whether one supports high-level performance better than the other.
Case in point: Yes, it can interfere with performance
Louise M. Burke, PhD, Mary MacKillop Institute of Health Research, Australian Catholic University, maintains that while low-carb ketogenic diets significantly increase fat oxidation (fat burning), they can reduce performance during extreme endurance events.
He explains that carbohydrates produce more energy than fat for the same amount of oxygen consumed, making them a more efficient fuel during intense exercise. If fat requires more oxygen to produce energy, athletes who are competing near their maximum capacity may slow down.
In a study of elite race walkers, Burke’s team observed greater fat burning but decreased race performance after ketogenic adaptation. He notes that carbohydrate intake should be adjusted based on training intensity and competition requirements. In his view, strict long-term carb restriction for high-intensity events may not be ideal. Food should be individual and flexible.
Case Against: No, it does not prevent performance
Timothy Noakes, MBChB, MD, DSc, University of Cape Town, South Africa, states that long-held assumptions about carbohydrate dependence for endurance performance may need to be reconsidered.
After reviewing randomized controlled trials comparing high-carbohydrate and low-carbohydrate diets, he concluded that after four to six weeks of proper adaptation, athletes perform the same on both diets.
Noakes claims that athletes can adapt to burning fat at a much faster rate, that depletion of muscle glycogen may not directly cause fatigue, and that hypoglycemia (low blood glucose), rather than low muscle glycogen, may be the primary cause of reduced performance during prolonged events. One study found that consuming just 10 grams of carbohydrate per hour to prevent low blood glucose levels improved athletes’ performance, regardless of their regular diet.
For Noakes, the big question is: After athletes adjust their diet, does the balance of carbohydrates and fats they eat on a regular basis actually affect performance? Based on current research in trained athletes, he concluded that this is not the case.
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 physiological adaptation period of several weeks is required to achieve optimal results.
- To produce the same amount of energy, fat requires more oxygen than carbohydrates.
- Fuel utilization during intense exercise is complex.
But some key questions remain:
- Does burning fat, which uses more oxygen than carbohydrates, really limit the performance of elite athletes?
- Does depletion of muscle glycogen actually cause fatigue?
- Is high carb intake necessary during competition or just a long term habit?
- Do elite athletes approach diet differently than recreational athletes?
Why is this discussion important?
AJCN Great debates in food The series, edited by David Ludwig, MD, PhD, is designed to generate robust yet collegial discussion of key controversies in nutrition.
By presenting two evidence-based perspectives along with a consensus opinion, the AJCN encourages the nutrition industry to:
- Rethinking long-held beliefs
- Identify gaps in evidence
- Design a stronger study
- Increase public awareness of the importance of food science for health
Whether you work with elite endurance athletes, recreational competitors, or metabolic training, this discussion provides valuable insight.
As the science of nutrition continues, the manufacturing controversy remains one of its greatest strengths.
Read the full discussion
learn yes and no prospects as well consensus paper in American Journal of Clinical Nutrition and join the conversation shaping the future of sports nutrition research.
