Carbohydrate intake and metabolism during prolonged endurance exercise

Pfeiffer, Beate (2010). Carbohydrate intake and metabolism during prolonged endurance exercise. University of Birmingham. Ph.D.

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Abstract

It is well accepted that CHO ingestion can improve endurance performance. However, a number of questions remain open regarding fine-tuning CHO intake recommendations during prolonged endurance events. A way to measure the bioavailability of ingested CHO is to measure exogenous CHO oxidation with the use of $^{13}$ C or $^{14}$ C tracers. This, however, has been studied only with CHO solutions, predominantly during cycling. In this thesis, we demonstrated that glucose+fructose ingested in the form of gel (1.8 g/min) is as effectively oxidized as an isocarbohydrate solution (1.44±0.29 g/min vs 1.42±0.23 g/min, respectively). Accordingly, the ingestion of glucose+fructose in a solid bar (1.55 g CHO/min) was demonstrated to be oxidized at high rates (1.25±0.15 g/min), comparable to a solution (1.34±0.27 g/min). A comparison of CHO ingestion (1.5 g/min) during cycling and running at the same relative, moderate-intensity (~60% exercise-specific VO $_2$ max) resulted in similar exogenous CHO oxidation rates (1.25±0.10 g/min vs 1.19±0.08 g/min, respectively). The present thesis also tested the gastrointestinal (GI) tolerance of high CHO ingestion rates (1.4 g CHO/min), previously recommended to athletes. High intakes in the form of a glucose+fructose gel were, on average, well tolerated during a 16-km run, and there was no difference between tolerance of glucose and glucose+fructose gel. A questionnaire-based field study of 221 athletes during prolonged endurance events (running, cycling and triathlon) revealed that voluntary CHO intake rates vary greatly between events and individuals (6-136 g/h). High CHO intakes were related to increased scores for nausea and flatulence as well as to better performance. GI distress during all studies was correlated with a reported history of GI distress. Findings from those studies suggest a need for more individualized nutritional advice that optimizes CHO and fluid delivery to enhance performance, while minimizing GI discomfort