The Great Carb Con: Scientists Break Down a Shocking New Study
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No athlete's health should ever be sacrificed at the altar of performance. Not like six plates of pasta. There was something in those studies that was ignored: you have this willpower and you're trying to wring all the metabolic juices out of your muscles and just push yourself. More reliable than any other biomarker that you can look at. This really brings a new meaning to mind over matter. And that's how we got to where we are on sports nutrition guidelines. 4,000 calories from just carbs alone. You can have your cake and eat it too.
Myth: You should eat carbs for peak performance. As an ex-marathon runner, host always lived by this mantra, maybe with confirmation bias because of love for Nutella, candy, and marshmallows. People accept it due to inertia of status quo, not rigorous challenge. Dr. Andrew Cutnik recently challenged this with a meta-analysis of randomized control trials showing you don't need to carb load for peak performance.
Dr. Andrew Cutnik is a leading researcher in metabolic health, especially blood sugar control and diabetes. He approaches scientific research and communication with ferocity and integrity.
New paper: Meta-analysis of over 100 years of evidence, 600 citations, analyzing over 160 sports performance studies. Goal: Understand if carbohydrates improve sports performance and the underlying metabolism. Findings starkly contrast current sports nutrition guidelines.
Cutnik's team did randomized control trials over last 5 years on very low carbohydrate diets and sports performance. Historically, carbs deemed virtually essential. Seen in Gatorade bottles, gel packs at marathons and cycling events, part of endurance sport religion.
Team studied athletes, including Ironman competitors, adapting to very low carb diets for longer durations than prior short-term studies, which assumed low carb impairs performance. Results challenged this belief.
In 1800s, Olympic athletes consumed protein and fat-focused diets, seen in cafeteria foods and diet plans.
1960s: Physician Jonas Bergstrom developed Bergstrom muscle biopsy technique. Found glucose stored as glycogen in muscles. Assumption: Stored glucose essential for performance. Higher glycogen levels correlated with longer exercise performance.
20 years later: Techniques assessing fat and carb burn via oxygen consumption and CO2 exhalation ratios. Athletes consuming more carbs burned more carbs and performed better.
This led to belief (still in textbooks and exercise science classes) that carbs essential, especially for high-intensity performance.
Problem: Associations between glycogen/performance and carb oxidation/performance, never causal.
Athletes consuming carbs often saw performance boost, precipitating belief carbs needed for total sugar burned and glycogen levels.
Average male stores 300-500g glucose as glycogen (females ~300g). Athletes need daily replenishment, especially high-volume exercisers.
Recommendations: Up to 300-500g carbs/day, or north of 1,000g for high-end average bodyweight male. 4,000 calories from carbs alone.
~500g carbs in adult male: Liver (small bucket), muscle (bigger bucket). Muscle glycogen specific, does not defend blood sugar—muscle greedy, keeps carbs to itself.
Alternative model to carb logic (quick-burning fuel for high-intensity, bigger tank via glycogen): Leaps where associations (muscle glycogen, carb burn) not causal for performance.
Non-causal: Link between muscle glucose storage/carb burn and performance.
In pivotal 1960s Bergstrom studies and 1970s/80s Coyle studies: Glycogen drops and lower carb oxidation associated with earlier fatigue, but ignored concurrent blood sugar drops at fatigue.
Low blood glucose causes brain energy deficit, reliable mechanistic link to fight-or-flight and impaired performance, known since 1920s. Causally linked to physical and cognitive performance impairment—brain key tissue; deprivation triggers survival mode, diverting energy from movement.
Two pools: Muscle (doesn't feed blood), blood/liver (feeds brain).
Alternative: Central nervous system (not muscle) limits performance.
Blood sugar total for average male: ~5g (100-fold less than glycogen stores, ~1/3 tablespoon honey or tiny bit orange juice).
In meta-analysis (600 citations, 160 studies using carbs): 88% of studies showing carb improvement had placebo/control group with persistent blood glucose reduction (more reliable than any other biomarker).
Started 90-100 mg/dL, dropped to near hypoglycemic (~60-70 mg/dL, 20-40 mg/dL drop). Subtle change induced dramatic functional shifts.
Visual: Muscle carb load = plates of pasta (400-500g). Blood sugar change = 1-2g (~two grapes, or one grape—body protects blood glucose 70-120 mg/dL tightly via regulatory mechanisms, even zero dietary glucose).
Drop triggers brain energy deficit signal, counterregulatory response: Elevated adrenaline (catecholamines, epinephrine, norepinephrine), cortisol, glucagon to liberate tissues and restore glucose. These impair muscle functional capacity.
Where no blood sugar drop in control, little performance improvement with carbs.
Causality for blood glucose: Animal studies genetically modified enzymes for hyper-saturated muscle glycogen—utilizable but no performance improvement vs. controls. Liver glycogen does extend performance (liver restores blood glucose).
Human evidence: 1924-1926 Harvard studies (Nick's school) on Boston Marathon. Many finishers had slurred vision, pale skin, speech loss; only meaningful blood change was glucose drop. Next year, carbs improved all.
Type 1 diabetes (Cutnik's condition since age 16): Manual blood glucose alteration to hypoglycemia reliably impairs physical/cognitive performance by 5-20%.
RCTs: Minimal carbs to restore blood glucose matched performance improvements seen with deficits.
Key: Performance limited by blood sugar drop (~1-2 grapes sugar), not muscle glycogen.
Nuance: Carbs can improve performance, but mechanism differs from expected; practical for carb amount/type.
Small nectarine or half apple = performance boost of giant pasta bowl.
Ironman competitors: Provided ~10g carbs/hour—enough to maintain blood glucose, not spike insulin, shift glycogen, or dramatically shift carb burn.
Result: 22% average performance improvement across high-carb and very low-carb ketogenic athletes.
Contrast: Guidelines (American College of Sports Nutrition, Dietitians of Canada/US, ISSN, Gatorade Sports Science Institute) recommend 5-12g/kg bodyweight daily, 60-90g/hour during exercise (6-9x more than 10g/hour).
Ironman triathletes at 70% max aerobic capacity cycling for >2 hours until "hit the wall" (unable to maintain 70% output). Chosen to represent prolonged high-end performance; athletes sufficiently trained.
Cycling was chosen because it is one of the cleanest ways to test the question. Unlike running or resistance exercise, where you lift weights, take breaks, and repeat, cycling involves sitting on a bike and consistently putting out a consistent amount of power and percentage of VO2 max. This control removes confounding variables to cleanly ask if carbohydrates significantly impact performance. In the study, they did impact performance.
Historical meta-analyses in various sports, including resistance and anaerobic, show carbohydrates do not consistently improve performance unless exercise is sufficiently strenuous and over 45-60 minutes. Blood glucose drops typically occur after that point, consistent with findings across sports. The model used high-end aerobic exercise athletes in a controlled lab setting, but results align with studies where carbohydrates improve or do not improve performance, typically over 60 minutes, in aerobic (cycling, running) or anaerobic (resistance) exercise.
For workouts under an hour, carbs are not going to help. Over an hour, even in high-level Ironman triathletes on a bike at ~70% VO2 max, outputting over 300 watts consistently (try holding 300-350 watts to gauge intensity), they needed the equivalent of a third of a banana per hour for peak performance.
The study used two diets: a ketogenic diet (lowers muscle glycogen) and high-carb. Keto athletes burn higher fat and lower carbs. Both performed identically, disavowing links between sugar burned, glycogen levels, and performance. Both groups experienced hypoglycemia.
Low-carb athletes had the same performance but higher hypoglycemia incidence. Calculating glucose, ketones, and lactate (other brain fuels), total circulating brain energy metabolites were equivalent at fatigue: low-carb had lower glucose but higher ketones and lactate; high-carb had higher glucose, lower ketones/lactate. Feelings of fatigue matched throughout.
Review of 100+ years, 600 citations, 160+ exercise studies: blood glucose (linked to brain energy) consistently predicts performance. RCTs show lower glucose but higher ketones/lactate provide sufficient brain energy for equivalent performance across diets. Key: not just hypoglycemia degree (worse in low-carb), but total circulating brain fuel dictates outcomes.
Paradigm shift: focus from muscle nutrients to brain compartment. Brain dysfunction impairs performance. Placebo effect (perception of better performance) is a consistent benefit.
Sports events/gyms are filled with sugar water due to historical carb focus. Rethink to precision nutrition: precise carbs only when appropriate.
Mind over matter or mind over muscle—it's brain metabolism, not willpower or muscles, dictating performance threshold via central nervous system checks and balances, primarily blood sugar plus ketones/lactate.
Insulin prioritizes carbs: first to liver (hepatic portal vein) for brain glucose storage, then suppresses fat breakdown, finally excess to muscle. Survival priority: low blood glucose/low liver glycogen impairs performance; low muscle glycogen does not (often correlated with high carbs but not causal).
Sports nutrition guidelines recommend excessive carbs, leading to pre-diabetes in some RCTs (driven by food environment). Can use 6-9x less carbs consistent with performance biomarkers.
Health matters: no athlete's health should be sacrificed for performance. High-carb diets cause issues, e.g., Ironman champion Lionel Sanders (exercises 4+ hours/day) developed pre-diabetes from refined carbs; it resolved on reduction.
High-performance athletes can look elite but have internal damage from diets compatible with performance yet harmful. Research shows max performance possible with a third of a banana (not cake), on health-promoting diets.
Most athletes don't understand: at events, carbs seen as essential. Post-publication (30% athletes pre-diabetic), high-level athletes/clinicians report elevated glucose, avoid low-carb fearing performance loss despite evidence.
Ketogenic diets have most evidence for normalizing glucose/insulin from pre-diabetes/diabetes, yet athletes forego due to misinformation.
Most important: dose-response study on graded carbs (10g vs 60-90-120g+/hour, some report 200g+/hr) with tracer analysis for metabolism. Does it impact performance? Evidence suggests no, but check metabolic effects.
Studies (4+ weeks keto) show no performance impairment. Muscle glycogen doesn't normalize up to 12 weeks, other changes incomplete at 4 weeks (minimum for no impairment). Year-long (e.g., strict keto/carnivore) untested; expect greater fat/ketone adaptation. Hard RCT: athletes unlikely to randomize to non-preferred diet for a year.
andrewkutnik.com (free resources, contact form). Instagram/YouTube: @AndrewKutnikPhD; LinkedIn/TikTok: search Andrew Kutnik.
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