Study analysis · The American Journal of Clinical Nutrition · 2026
The carb wars just got personal: one expert says low-carb diets don't hurt endurance, another says that advice could cost elite athletes medals.
After 4–6 weeks on low-carb or high-carb diets, trained athletes performed the same in lab tests, but taking just 10 grams of carbs per hour during long exercise improved time to exhaustion by 12–22% relative—though the absolute minutes were not reported.
Overview
What the study found
The study in plain English — the bottom line, every takeaway we extracted, and what to do with them.
In simple terms
This study tested two diets on athletes and found that both worked just as well for performance — as long as they ate a little bit of carbs during exercise. It doesn't prove one diet is better, just that neither made them slower under these specific conditions.
What’s the bottom line?
This is a debate article. One expert says no, low-carb diets do not hurt endurance performance after adaptation. Another expert says yes, they can, especially for elite athletes.
How strong is this study?
The study is pretty good because it randomly assigned athletes to diets, which helps make sure the results aren't just luck. But we don't know if the athletes or coaches knew which diet they were on — that could have made them try harder or softer, so we can't be 100% sure the diet alone caused the results.
40 / 100
- COI disclosure+40/40
- Data availabilitydata not shared
- Code availabilitycode not shared
56 / 100
- Randomization+20/20
- Blindingblinding unclear
- Control group+15/15
- Sample sizeno sample size reported
- Follow-up+10/10
100 / 100
54 / 100
- P-values+15/15
- Effect size+20/20
- Confidence intervalsno confidence intervals
- Pre-registrationnot pre-registered
Each component is scored out of 100 and then capped by the study design — a case series cannot reach the ceiling a randomised trial can, however well it is reported.
Where it sits
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 561 / 100
Probability of being correct
Participants are randomly assigned to treatment or control groups, minimizing bias. The gold standard for testing whether an intervention causes an effect.
This design can establish causation. Although this is a randomized controlled trial, blinding was unknown, which introduces potential performance and measurement bias. However, randomization and control group presence allow for causal inference, albeit with moderate confidence due to lack of blinding confirmation.
Major COI
Major conflicts that significantly reduce study credibility
The lead author, Dr. Noakes, has a long-standing personal and professional investment in challenging the high-carbohydrate diet paradigm, and the study's conclusions strongly align with his publicly stated ideological shift, raising concerns about bias despite no explicit funding disclosure.
Conflict Details
N/A: The author has publicly and persistently challenged the established high-carbohydrate diet doctrine for over a decade, and this study serves as a culmination of his ideological shift, creating a strong non-financial conflict of interest.
The study lacks a formal funding statement or COI disclosure. The lead author's intense personal advocacy against the carbohydrate paradigm, combined with the study's definitive conclusions supporting his long-held revisionist view, introduces significant non-financial bias. While no industry funding is indicated, the potential for confirmation bias and selective interpretation of evidence is high due to the author's entrenched position.
Key takeaways
- 01
After 4–6 weeks on low-carb high-fat or high-carb low-fat diets, athletes performed equally well on VO2max tests, 5 km and 1.6 km time trials, 6 × 800 m intervals, and long cycling to exhaustion.
- 02
Taking 10 grams of carbohydrate per hour during long exercise improved time to exhaustion by 12–22% relative on both diets.
- 03
The absolute time improvement was not reported.
- 04
For a trained athlete, the background diet may not matter much for performance if you take a small amount of carbohydrate during long exercise to prevent low blood sugar.
- 05
But the absolute minutes or seconds of improvement were not reported in this study, and the evidence is contested by another expert who studies elite athletes.
Surprising findings
- Muscle glycogen is not obligatory for exercise up to 85% VO2max in adapted athletes.For decades, muscle glycogen depletion was taught as the primary cause of fatigue during prolonged and high-intensity exercise.
- Ingesting just 10 g carbohydrate/hour improved time to exhaustion by 12–22% relative—far less than typical 30–90 g/h recommendations.Conventional sports nutrition often recommends much higher carb intakes during long events, yet this small dose reportedly prevented hypoglycemia and improved performance.
- Exercise-induced hypoglycemia, not muscle glycogen depletion, is proposed as the main metabolic cause of premature fatigue.It reframes fatigue as a brain-driven hypoglycemic symptom rather than a simple fuel shortage in muscle.
- Fat oxidation rates >2 g/min were measured in humans during 6 × 800 m intervals.That intensity was thought to require carbohydrate oxidation; achieving such high fat oxidation challenges textbook metabolism.
- High-carb loading before exercise did not enhance performance in 16 lines of evidence reviewed by Noakes.Carb-loading is a staple pre-race strategy for endurance athletes, yet this analysis argues it fails to improve outcomes.
Practical takeaways
If you're a trained endurance athlete doing prolonged exercise, consider taking about 10 g carbohydrate per hour to prevent low blood sugar—even if you follow a low-carb diet.
This is from a debate article citing RCTs; absolute performance improvement was not reported, and elite competition evidence is contested.
low confidenceDon't assume you need 30–90 g carbohydrate per hour during long exercise if you are fat-adapted; the study argues ~10 g/h may suffice to prevent hypoglycemia.
High carb intakes may still be optimal for many athletes, especially in elite competition; individual needs vary.
low confidenceIf you're considering a low-carb high-fat diet for endurance, allow at least 4–6 weeks for adaptation before judging performance.
Burke's elite data used 3.5 weeks and found suboptimal performance; longer adaptation may not fix all competition-specific issues.
medium confidenceFocus on preventing exercise-induced hypoglycemia during long efforts—this may matter more than muscle glycogen stores.
This is a mechanistic hypothesis from one side of a debate; other experts emphasize muscle glycogen and total carbohydrate availability.
low confidenceWhy this study matters
The 10-gram carb trick that beat diet
In an RCT cited by Noakes, ingesting only 10 g carbohydrate per hour during prolonged cycling at 70% VO2max improved time to exhaustion by 12–22% relative, equally after 4–6 weeks on either a low-carb high-fat or high-carb low-fat diet. The absolute time improvement was not reported.
People think you need huge carb loads during long workouts, but this suggests a tiny amount may prevent the hypoglycemia that actually stops you.
Muscle glycogen may not be the wall
Noakes argues that muscle glycogen is not obligatory up to 85% VO2max. He reports athletes adapted to LCHF achieved fat oxidation rates >2 g/min—highest ever measured—and that the only consistent metabolic marker at exhaustion was low blood glucose.
It challenges the classic 'hit the wall' explanation that muscles run out of glycogen. If true, fatigue may be brain-driven from low blood sugar.
Elite athletes vs regular athletes
Burke's elite-athlete studies found LCHF diets were associated with suboptimal competition performance, but Noakes notes those studies were not randomized and lasted only 3.5 weeks, with performance still improving at termination. Noakes' RCTs in regular trained athletes found no performance difference.
The same question gets different answers depending on who you study and how long you wait for adaptation.
Fat can fuel 85% VO2max
Noakes reports that after LCHF adaptation, fat oxidation exceeded 2 g/min, which he says could provide >90% of the energy required for a 52 kg athlete to run a sub-2-hour marathon. This challenges the idea that high-intensity endurance requires carbohydrate.
It suggests fat is not just for easy pace—it may fuel surprisingly hard efforts in adapted athletes.
The small glucose pool vs the big glycogen pool
Noakes distinguishes the small glucose pool (SGP) in liver/blood from the large glucose pool (LGP) in muscle glycogen. He argues the SGP, not muscle glycogen, determines fatigue during prolonged exercise, and 16 lines of evidence support this.
It flips the focus from muscle fuel to blood sugar control during exercise.
Conflicts of interest on both sides
Noakes profits from low-carb books and Nutrition Network; Burke has sports nutrition industry ties. The article is an Oxford-style debate, not a systematic review or new primary RCT.
Even the experts arguing about evidence have financial and career incentives that may shape their positions.
Burke's counterpunch: lab tests aren't races
Burke argues that VO2max, time trials, and time-to-exhaustion tests are not adequate proxies for real competition performance, and that elite athletes use integrated nutrition strategies. She says her controlled elite studies show LCHF is suboptimal.
It raises the question: if a lab test says equal, but races say worse, which do you trust?
Want the whole report?
Detailed mode opens the full scientific breakdown — every score component, the methodology, conflicts of interest, the evidence analysis behind each claim, and the raw study data.
Overview
What the study found
The study in plain English — the bottom line, every takeaway we extracted, and what to do with them.
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
This is a debate article. One expert says no, low-carb diets do not hurt endurance performance after adaptation. Another expert says yes, they can, especially for elite athletes.
Research results
After 4–6 weeks on low-carb high-fat or high-carb low-fat diets, athletes performed equally well on VO2max tests, 5 km and 1.6 km time trials, 6 × 800 m intervals, and long cycling to exhaustion. Taking 10 grams of carbohydrate per hour during long exercise improved time to exhaustion by 12–22% relative on both diets. The absolute time improvement was not reported.
What this means - more context
For a trained athlete, the background diet may not matter much for performance if you take a small amount of carbohydrate during long exercise to prevent low blood sugar. But the absolute minutes or seconds of improvement were not reported in this study, and the evidence is contested by another expert who studies elite athletes.
Oxford-style debate article asking whether a low-carbohydrate diet impedes endurance sports performance. Noakes argues 'No'; Burke argues 'Yes' and critiques the evidence.
Noakes reports randomized controlled trials showing trained athletes adapted for 4–6 weeks to either a low-carbohydrate, high-fat (LCHF) or high-carbohydrate, low-fat (HCLF) diet performed equally on VO2max tests, 5 km and 1.6 km time trials, 6 × 800 m intervals, and cycling to exhaustion at 70% VO2max. Ingesting 10 g carbohydrate/hour during prolonged exercise improved time to exhaustion by 12–22% relative, equally on both diets, by preventing exercise-induced hypoglycemia. He concludes muscle glycogen is not obligatory up to 85% VO2max and that the small glucose pool in liver/bloodstream, not muscle glycogen, is the main metabolic contributor to fatigue. Burke rebuts, citing elite-athlete data and methodological limitations.
Methods Used
Narrative debate/review of randomized controlled trials, systematic reviews, and meta-analyses comparing LCHF and HCLF diets in trained athletes; performance tests included VO2max, 5 km and 1.6 km treadmill time trials, 6 × 800 m intervals, and cycling to exhaustion at 70% VO2max. The article also includes a rebuttal by Burke and a rebuttal by Noakes. No new primary RCT data are presented in this debate article.
Main Finding
Noakes's main finding is that habitual dietary macronutrient content (LCHF vs HCLF) for 4–6 weeks does not influence endurance performance when 10 g carbohydrate/hour is ingested during exercise to prevent hypoglycemia. Ingesting 10 g carbohydrate/hour improved prolonged cycling time to exhaustion by 12–22% relative, equally after both diets. The absolute performance improvement was not reported in this study. Burke argues that elite athlete evidence and methodological issues invalidate the claim that background diet does not matter.
Confidence Level
Low to moderate. The article is a debate with opposing interpretations, relies on cited RCTs rather than presenting new primary data, and both authors have significant conflicts of interest. Absolute performance effect sizes and competition-specific outcomes are not fully reported.
Study Flags
Red Flags
- •Oxford-style narrative debate, not a systematic review or new primary RCT
- •Major conflicts of interest: Noakes profits from low-carbohydrate books/Nutrition Network; Burke has sports nutrition industry ties
- •Performance measures often laboratory-based, not competition; elite versus regular athlete generalizability contested; absolute performance effects not reported
Surprising Findings
Muscle glycogen is not obligatory for exercise up to 85% VO2max in adapted athletes.
For decades, muscle glycogen depletion was taught as the primary cause of fatigue during prolonged and high-intensity exercise.
Practical Takeaways
If you're a trained endurance athlete doing prolonged exercise, consider taking about 10 g carbohydrate per hour to prevent low blood sugar—even if you follow a low-carb diet.
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 561 / 100
Probability of being correct
Participants are randomly assigned to treatment or control groups, minimizing bias. The gold standard for testing whether an intervention causes an effect.
Human RCT
Subject
Moderate probability
on the GRADE evidence scale
This study tested two diets on athletes and found that both worked just as well for performance — as long as they ate a little bit of carbs during exercise. It doesn't prove one diet is better, just that neither made them slower under these specific conditions.
Major conflicts — industry funding with significant control over the research. Reporting score and overall score cap have been reduced.
Strengths
- Randomized controlled trial design with control group
- Multiple performance outcome measures (VO2max, time trials, interval sessions)
- Direct comparison of two dietary interventions under controlled conditions
Weaknesses
- Blinding status unknown — potential for performance and measurement bias
- Sample size not reported — unknown statistical power
- No details on participant recruitment or inclusion criteria
Methodology
Evidence Keywords
Statistical Reporting
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
This is a debate article. One expert says no, low-carb diets do not hurt endurance performance after adaptation. Another expert says yes, they can, especially for elite athletes.
Research results
After 4–6 weeks on low-carb high-fat or high-carb low-fat diets, athletes performed equally well on VO2max tests, 5 km and 1.6 km time trials, 6 × 800 m intervals, and long cycling to exhaustion. Taking 10 grams of carbohydrate per hour during long exercise improved time to exhaustion by 12–22% relative on both diets. The absolute time improvement was not reported.
What this means - more context
For a trained athlete, the background diet may not matter much for performance if you take a small amount of carbohydrate during long exercise to prevent low blood sugar. But the absolute minutes or seconds of improvement were not reported in this study, and the evidence is contested by another expert who studies elite athletes.
Oxford-style debate article asking whether a low-carbohydrate diet impedes endurance sports performance. Noakes argues 'No'; Burke argues 'Yes' and critiques the evidence.
Noakes reports randomized controlled trials showing trained athletes adapted for 4–6 weeks to either a low-carbohydrate, high-fat (LCHF) or high-carbohydrate, low-fat (HCLF) diet performed equally on VO2max tests, 5 km and 1.6 km time trials, 6 × 800 m intervals, and cycling to exhaustion at 70% VO2max. Ingesting 10 g carbohydrate/hour during prolonged exercise improved time to exhaustion by 12–22% relative, equally on both diets, by preventing exercise-induced hypoglycemia. He concludes muscle glycogen is not obligatory up to 85% VO2max and that the small glucose pool in liver/bloodstream, not muscle glycogen, is the main metabolic contributor to fatigue. Burke rebuts, citing elite-athlete data and methodological limitations.
Methods Used
Narrative debate/review of randomized controlled trials, systematic reviews, and meta-analyses comparing LCHF and HCLF diets in trained athletes; performance tests included VO2max, 5 km and 1.6 km treadmill time trials, 6 × 800 m intervals, and cycling to exhaustion at 70% VO2max. The article also includes a rebuttal by Burke and a rebuttal by Noakes. No new primary RCT data are presented in this debate article.
Main Finding
Noakes's main finding is that habitual dietary macronutrient content (LCHF vs HCLF) for 4–6 weeks does not influence endurance performance when 10 g carbohydrate/hour is ingested during exercise to prevent hypoglycemia. Ingesting 10 g carbohydrate/hour improved prolonged cycling time to exhaustion by 12–22% relative, equally after both diets. The absolute performance improvement was not reported in this study. Burke argues that elite athlete evidence and methodological issues invalidate the claim that background diet does not matter.
Confidence Level
Low to moderate. The article is a debate with opposing interpretations, relies on cited RCTs rather than presenting new primary data, and both authors have significant conflicts of interest. Absolute performance effect sizes and competition-specific outcomes are not fully reported.
Study Flags
Red Flags
- •Oxford-style narrative debate, not a systematic review or new primary RCT
- •Major conflicts of interest: Noakes profits from low-carbohydrate books/Nutrition Network; Burke has sports nutrition industry ties
- •Performance measures often laboratory-based, not competition; elite versus regular athlete generalizability contested; absolute performance effects not reported
Surprising Findings
Muscle glycogen is not obligatory for exercise up to 85% VO2max in adapted athletes.
For decades, muscle glycogen depletion was taught as the primary cause of fatigue during prolonged and high-intensity exercise.
Practical Takeaways
If you're a trained endurance athlete doing prolonged exercise, consider taking about 10 g carbohydrate per hour to prevent low blood sugar—even if you follow a low-carb diet.
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 561 / 100
Probability of being correct
Participants are randomly assigned to treatment or control groups, minimizing bias. The gold standard for testing whether an intervention causes an effect.
Human RCT
Subject
Moderate probability
on the GRADE evidence scale
This study tested two diets on athletes and found that both worked just as well for performance — as long as they ate a little bit of carbs during exercise. It doesn't prove one diet is better, just that neither made them slower under these specific conditions.
Major conflicts — industry funding with significant control over the research. Reporting score and overall score cap have been reduced.
Strengths
- Randomized controlled trial design with control group
- Multiple performance outcome measures (VO2max, time trials, interval sessions)
- Direct comparison of two dietary interventions under controlled conditions
Weaknesses
- Blinding status unknown — potential for performance and measurement bias
- Sample size not reported — unknown statistical power
- No details on participant recruitment or inclusion criteria
Methodology
Evidence Keywords
Statistical Reporting
Scoring
How strong is this study?
The study is pretty good because it randomly assigned athletes to diets, which helps make sure the results aren't just luck. But we don't know if the athletes or coaches knew which diet they were on — that could have made them try harder or softer, so we can't be 100% sure the diet alone caused the results.
40 / 100
- COI disclosure+40/40
- Data availabilitydata not shared
- Code availabilitycode not shared
56 / 100
- Randomization+20/20
- Blindingblinding unclear
- Control group+15/15
- Sample sizeno sample size reported
- Follow-up+10/10
100 / 100
54 / 100
- P-values+15/15
- Effect size+20/20
- Confidence intervalsno confidence intervals
- Pre-registrationnot pre-registered
Each component is scored out of 100 and then capped by the study design — a case series cannot reach the ceiling a randomised trial can, however well it is reported.
Where it sits
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 561 / 100
Probability of being correct
Participants are randomly assigned to treatment or control groups, minimizing bias. The gold standard for testing whether an intervention causes an effect.
This design can establish causation. Although this is a randomized controlled trial, blinding was unknown, which introduces potential performance and measurement bias. However, randomization and control group presence allow for causal inference, albeit with moderate confidence due to lack of blinding confirmation.
Major COI
Major conflicts that significantly reduce study credibility
The lead author, Dr. Noakes, has a long-standing personal and professional investment in challenging the high-carbohydrate diet paradigm, and the study's conclusions strongly align with his publicly stated ideological shift, raising concerns about bias despite no explicit funding disclosure.
Conflict Details
N/A: The author has publicly and persistently challenged the established high-carbohydrate diet doctrine for over a decade, and this study serves as a culmination of his ideological shift, creating a strong non-financial conflict of interest.
The study lacks a formal funding statement or COI disclosure. The lead author's intense personal advocacy against the carbohydrate paradigm, combined with the study's definitive conclusions supporting his long-held revisionist view, introduces significant non-financial bias. While no industry funding is indicated, the potential for confirmation bias and selective interpretation of evidence is high due to the author's entrenched position.
Standing
Who’s using this study?
The videos and claims on this site that lean on this study, and the researchers who wrote it.
1 video from Renaissance Periodization cite this study, drawing 1 claim from it.
- Contradicted
Evidence contradicts this claim.
Evidence
Authored by
2 researchersIf this is your work, this is how we attribute it on Fit Body Science. T Noakes is listed as the lead author.