In athletes with normal blood sugar, it takes at least four weeks on a very-low-carbohydrate diet for glucose levels and ketone levels to stabilize, and for exercise performance to become consistent.
Mechanism
Synthesis from 1 study
After four weeks of eating very few carbs, the body switches from burning sugar to burning fat and ketones for energy. This switch stabilizes blood sugar levels and lets the muscles keep working hard during exercise without needing carbs. The change is complete at four weeks because that’s how long...
Most probable mechanism
When a person eats very little carbohydrate for four weeks, the liver starts making ketones from fat to fuel the brain and muscles. This reduces the need for glucose, so the body stops producing excess glucose and begins using ketones more efficiently. After four weeks, blood sugar levels become stable, ketones stay high, and the body can perform intense exercise without running out of energy.
Low carbohydrate intake depletes liver glycogen stores and reduces insulin secretion, triggering increased fatty acid mobilization from adipose tissue and enhanced hepatic fatty acid β-oxidation.
Hepatic β-oxidation produces ketone bodies, primarily R-β-hydroxybutyrate, which enter the bloodstream and serve as alternative fuels for the brain, heart, and skeletal muscle.
Elevated ketone levels suppress hepatic glucose production and enhance insulin sensitivity in peripheral tissues, reducing glucose variability and stabilizing fasting and postprandial blood glucose concentrations.
Skeletal muscle upregulates fatty acid transport proteins and mitochondrial β-oxidation enzymes, enabling sustained fat oxidation during exercise, while ketone bodies are directly oxidized to generate ATP.
After four weeks, glucose production and utilization reach equilibrium, ketone levels plateau at peak concentrations, and metabolic flexibility allows energy demands during endurance exercise to be met without reliance on glycogen or exogenous carbohydrate.
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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