Exogenous Carbohydrate Availability
Pre-, Intra- & Post-Exercise Nutrition & Recovery
Exogenous Carbohydrate Availability describes carbohydrate originating from consumed food or drink that becomes available for use, rather than stored endogenous carbohydrate.
Exogenous carbohydrate comes from food or drink, while endogenous carbohydrate comes from existing body stores. Digestion, intestinal absorption, circulation, muscle uptake and oxidation are separate steps. Tracer studies can estimate oxidation of ingested substrate under defined assumptions. The amount consumed is not the amount oxidized immediately, and a high measured oxidation rate does not alone prove better race performance. Specify the substrate source and the process measured before comparing research or planning intake.
Glucose and galactose use intestinal SGLT1 transport, while fructose uses GLUT5. Combining suitable carbohydrate sources can engage different absorption pathways and increase exogenous carbohydrate availability under relevant conditions. Maltodextrin yields glucose units during digestion rather than creating a separate fructose transport pathway. The blend, total intake, fluid provision, exercise and tolerance all matter. No single glucose-to-fructose ratio or hourly intake is a universally optimal prescription for every athlete and event.
Carbohydrate requirements vary with the intensity, duration and importance of the work. Daily intake relative to body mass describes a different quantity from an intake rate during exercise, which is commonly expressed per hour. A larger daily target does not mean the same amount should be consumed in every session. Match supply to the schedule, starting stores and practical tolerance, and distinguish grams eaten from grams absorbed or oxidized when discussing a fueling plan.
For example, a tracer study may show a different oxidation rate after two carbohydrate drinks. Before translating that finding into a race plan, check the amount consumed, exercise duration, fluid provision and the performance outcome. A larger intake or oxidation value is informative without independently proving that the same strategy will suit every athlete.
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