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...
Mechanism
Synthesis from 1 study
During 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...
Most probable mechanism
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
Supporting (1)
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Carbohydrate Ingestion Eliminates Hypoglycemia & Improves Endurance Exercise Performance in Triathletes Adapted to Very Low & High Carbohydrate Isocaloric Diets.
Contradicting (0)
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