During prolonged endurance exercise, trained athletes experience low blood glucose levels below 3.9 mmol/L even without consuming carbohydrates; consuming a small amount of carbohydrates prevents this drop and enhances performance, showing that low blood glucose directly limits performance regardless of muscle glycogen stores.
See the scientific wording
Exercise-induced hypoglycemia (blood glucose <3.9 mmol/L) occurs during prolonged endurance exercise in trained athletes regardless of dietary carbohydrate intake, and minimal carbohydrate ingestion prevents this hypoglycemia and improves performance, indicating that hypoglycemia is a primary limiting factor independent of muscle glycogen levels.
Very strong evidence
We haven't found enough studies to verify this claim yet.
During long endurance exercise, the body uses up glucose faster than it can make it, causing blood sugar to drop. When blood sugar falls too low, the brain doesn't get enough fuel and stops signaling the body to keep going, leading to fatigue. Eating a small amount of sugar during exercise keeps blood sugar stable, so the brain keeps working and the person can keep exercising longer, even if their muscles are already full of stored fuel.
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During prolonged endurance exercise, trained athletes experience low blood glucose levels below 3.9 mmol/L even without consuming carbohydrates; consuming a small amount of carbohydrates prevents this drop and enhances performance, showing that low blood glucose directly limits performance regardless of muscle glycogen stores.
Mechanism
1 studyDuring long exercise, the body runs out of blood sugar faster than it can make more, causing the brain to shut down performance. Eating a little sugar during exercise fixes this by keeping blood sugar up, so the brain keeps telling the body to keep going. Even athletes who burn fat and ketones for fuel still need this sugar to avoid hitting a wall.
During long endurance exercise, the body uses up glucose faster than it can make it, causing blood sugar to drop. When blood sugar falls too low, the brain doesn't get enough fuel and stops signaling the body to keep going, leading to fatigue. Eating a small amount of sugar during exercise keeps blood sugar stable, so the brain keeps working and the person can keep exercising longer, even if their muscles are already full of stored fuel.
Exogenous carbohydrate is ingested and digested into glucose in the gastrointestinal tract
Glucose is absorbed into the bloodstream through intestinal transporters, increasing circulating blood glucose concentration
Maintained blood glucose ensures continuous delivery of glucose to the brain, preventing hypoglycemia-induced central fatigue
Sustained central nervous system function delays volitional exhaustion during prolonged exercise
Less supported by current evidence, but not ruled out
After weeks of low-carbohydrate eating, the body shifts to using ketones as its main fuel, which reduces the need for glucose and helps keep blood sugar stable even during long exercise, preventing it from dropping too low.
Chronic low carbohydrate intake triggers hepatic production of ketone bodies from fatty acids
Elevated ketone bodies replace glucose as a primary fuel for the brain and skeletal muscle
Reduced reliance on glucose lowers systemic glucose demand and stabilizes interstitial glucose concentrations
Metabolic adaptation normalizes glucose variability and prevents early hypoglycemia during prolonged exercise
After long-term low-carbohydrate adaptation, muscles become better at burning fat for energy, allowing the body to keep working at high intensity without needing to rely on stored muscle sugar or blood sugar.
Chronic low carbohydrate intake increases expression of fatty acid transport proteins and mitochondrial enzymes in skeletal muscle
Skeletal muscle oxidizes fatty acids at higher rates to produce ATP during prolonged exercise
Ketone bodies are oxidized in muscle mitochondria to supplement energy production and spare glucose
Sustained ATP production from fat and ketones maintains exercise intensity despite low muscle glycogen stores
Evidence from Studies
No evidence studies found yet.
What Would Prove This
Per GRADE and EBM methodology, here is what ideal scientific evidence would look like to definitively prove or disprove this claim, ordered from strongest to weakest.
Systematic Review of Carbohydrate Intake During Prolonged Endurance Exercise on Blood Glucose and Performance in Trained Athletes
Population: Trained endurance athletes; Intervention: Minimal carbohydrate ingestion (e.g., 15-30g/h) during >2.5h endurance exercise; Comparator: No carbohydrate intake; Outcome: Blood glucose levels <3.9 mmol/L and performance metrics (e.g., time to exhaustion, power output); Duration: Single or repeated exercise sessions across multiple studies.
Double-Blind RCT of Minimal Carbohydrate vs No Carbohydrate on Hypoglycemia and Performance in Trained Athletes During Prolonged Cycling
Population: Trained male and female endurance athletes; Intervention: 20g carbohydrate gel during 3h cycling at 70% VO2max; Comparator: Placebo gel; Outcome: Blood glucose nadir, time to fatigue, and muscle glycogen depletion measured via biopsy; Duration: Two randomized, counterbalanced sessions per participant.
Prospective Cohort Study of Carbohydrate Intake Patterns and Hypoglycemia Incidence During Marathon Running in Trained Athletes
Population: 500 trained marathon runners; Intervention: Self-reported carbohydrate intake during race; Comparator: Low vs. no carbohydrate intake groups; Outcome: Incidence of blood glucose <3.9 mmol/L (measured via fingerstick), race time, and perceived exertion; Duration: Single race event with pre-race glycogen assessment.
Case-Control Study Comparing Athletes With and Without Exercise-Induced Hypoglycemia During Endurance Events
Population: Trained athletes with documented hypoglycemia (<3.9 mmol/L) during endurance events (cases) vs. matched athletes without hypoglycemia (controls); Intervention: Retrospective analysis of pre-race carbohydrate intake and muscle glycogen via biopsy; Outcome: Comparison of glycogen levels, intake patterns, and performance; Duration: Post-event data collection.
In Vitro Study of Glucose Uptake and Metabolic Signaling in Human Skeletal Muscle Cells Under Simulated Exercise and Low Glucose Conditions
Population: Human primary skeletal muscle cells; Intervention: Exposure to 2.5 mmol/L glucose with simulated contraction via electrical stimulation; Comparator: 5.5 mmol/L glucose without stimulation; Outcome: Glucose transporter translocation, AMPK activation, and glycogen synthase activity; Duration: 2–4 hour incubation.