Feeding Tolerance, Glucose Availability, and Whole-Body Total Carbohydrate and Fat Oxidation in Male Endurance and Ultra-Endurance Runners in Response to Prolonged Exercise, Consuming a Habitual Mixed Macronutrient Diet and Carbohydrate Feeding During Exercise

Using metadata from previously published research, this investigation sought to explore: (1) whole-body total carbohydrate and fat oxidation rates of endurance (e.g., half and full marathon) and ultra-endurance runners during an incremental exercise test to volitional exhaustion and steady-state exercise while consuming a mixed macronutrient diet and consuming carbohydrate during steady-state running and (2) feeding tolerance and glucose availability while consuming different carbohydrate regimes during steady-state running. Competitively trained male endurance and ultra-endurance runners (n = 28) consuming a balanced macronutrient diet (57 +/- 6% carbohydrate, 21 +/- 16% protein, and 22 +/- 9% fat) performed an incremental exercise test to exhaustion and one of three 3 h steady-state running protocols involving a carbohydrate feeding regime (76-90 g/h). Indirect calorimetry was used to determine maximum fat oxidation (MFO) in the incremental exercise and carbohydrate and fat oxidation rates during steady-state running. Gastrointestinal symptoms (GIS), breath hydrogen (H-2), and blood glucose responses were measured throughout the steady-state running protocols. Despite high variability between participants, high rates of MFO [mean (range): 0.66 (0.22-1.89) g/min], Fat(max) [63 (40-94) % V ? O-2max], and Fat(min) [94 (77-100) % V ? O-2max] were observed in the majority of participants in response to the incremental exercise test to volitional exhaustion. Whole-body total fat oxidation rate was 0.8 +/- 0.3 g/min at the end of steady-state exercise, with 43% of participants presenting rates of >= 1.0 g/min, despite the state of hyperglycemia above resting homeostatic range [mean (95%CI): 6.9 (6.7-7.2) mmol/L]. In response to the carbohydrate feeding interventions of 90 g/h 2:1 glucose-fructose formulation, 38% of participants showed breath H-2 responses indicative of carbohydrate malabsorption. Greater gastrointestinal symptom severity and feeding intolerance was observed with higher carbohydrate intakes (90 vs. 76 g/h) during steady-state exercise and was greatest when high exercise intensity was performed (i.e., performance test). Endurance and ultra-endurance runners can attain relatively high rates of whole-body fat oxidation during exercise in a post-prandial state and with carbohydrate provisions during exercise, despite consuming a mixed macronutrient diet. Higher carbohydrate intake during exercise may lead to greater gastrointestinal symptom severity and feeding intolerance.

Automated summary

This study aimed to investigate the rates of carbohydrate and fat oxidation in endurance and ultra-endurance runners in different exercise conditions, as well as the feeding tolerance and glucose availability during different carbohydrate diets. The results showed that most participants were able to achieve high rates of fat oxidation before reaching exhaustion in the incremental exercise test. At the end of steady-state exercise, the overall fat oxidation rate was around 0.8 g/min, despite having blood glucose levels above the resting state. Higher carbohydrate intake was associated with greater gastrointestinal symptoms and feeding intolerance during exercise.