Fuelled or Fooled? Examining the Evidence and Mechanisms Behind Ultra-high Carbohydrate Intake in Endurance Athletes

Abstract

Ultra-high carbohydrate (U-HC) ingestion strategies, typically > 90 g·h⁻¹, have gained remarkable traction among elite endurance athletes in recent years. This rise has been fuelled by anecdotal reports linking aggressive carbohydrate fuelling with superior race execution, and record-breaking performances. However, the scientific literature supporting these practices remains limited, and current frameworks used to explain carbohydrate-mediated performance effects, such as muscle glycogen sparing, maintenance of blood glucose or maximisation of exogenous carbohydrate oxidation, do not fully account for the reported U-HC benefits. Indeed, available studies reveal a disconnect between metabolic data and the real-world adoption of intakes far beyond established oxidation ceilings. We examine the existing research on carbohydrate ingestion during endurance exercise, highlighting the metabolic responses and performance outcomes. We identify how traditional mechanisms have been interpreted to justify escalating carbohydrate intake and discuss the constraints of current methodologies in detecting meaningful performance benefits at ultra-high ingestion rates. The article then introduces emerging, yet largely untested, possible mechanistic models that may underpin the perceived utility of U-HC fuelling, including shifts in substrate selection, enhanced lactate shuttling and potential central effects associated with carbohydrate availability. Finally, we explore the relevance of inter-individual variation in carbohydrate oxidation capacity and the implications for developing practical guidelines for athletes. By integrating physiological, metabolic and perceptual perspectives, this current opinion outlines why U-HC strategies have gained prominence and identifies key areas where further research is required to clarify their role within contemporary endurance performance for the masses.