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Carbohydrate Ingestion on Exercise Metabolism and Physical Performance.

Timothy D Noakes1, Philip J Prins2, Alex Buga3

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Summary

Carbohydrate ingestion during exercise prevents exercise-induced hypoglycemia (EIH), a key factor in fatigue. This strategy enhances athletic performance by maintaining blood glucose levels, not solely through glycogen replenishment.

Keywords:
carbohydratesexerciseglycogenhypoglycemiametabolismperformance

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Area of Science:

  • Exercise Physiology
  • Sports Nutrition
  • Metabolic Regulation

Background:

  • Historically, carbohydrate (CHO) intake during exercise was linked to performance improvements, primarily through preventing hypoglycemia and brain damage.
  • Subsequent research focused on muscle glycogen depletion as the primary fatigue cause, often overlooking exercise-induced hypoglycemia (EIH).
  • Emerging evidence suggests EIH plays a critical role in exercise fatigue, challenging the established paradigm.

Purpose of the Study:

  • To comprehensively review over a century of research on carbohydrate ingestion, exercise metabolism, and athletic performance.
  • To elucidate the primary mechanism by which carbohydrate ingestion enhances exercise performance.
  • To re-evaluate the role of EIH and glycogenolysis in exercise termination and fatigue.

Main Methods:

  • Systematic review and meta-analysis of over 160 studies spanning more than 100 years.
  • Analysis of data on carbohydrate ingestion, exercise metabolism, and physical performance.
  • Examination of neuroendocrine mechanisms and metabolic responses during exercise.

Main Results:

  • Exercise-induced hypoglycemia (EIH) strongly correlates with exercise cessation, while muscle glycogen depletion alone does not cause fatigue.
  • Carbohydrate ingestion preserves blood glucose by reducing liver glycogenolysis but can paradoxically increase muscle glycogen breakdown.
  • High-fat-adapted athletes perform comparably with lower carbohydrate oxidation, indicating CHO is not always obligatory.
  • Carbohydrate ingestion enhances performance, even in glycogen-depleted states, by preventing EIH.

Conclusions:

  • The primary benefit of carbohydrate ingestion during prolonged exercise (>2-3 hours) is the prevention of EIH.
  • This is particularly crucial for individuals with impaired hepatic gluconeogenesis.
  • Findings necessitate revisions to sports nutrition guidelines regarding CHO intake timing and quantity for optimal athletic performance.