Does Pattern of Drink Intake Affect Exogenous Carbohydrate Oxidation During Prolonged Submaximal Running?

Abstract

Marathons and half-marathons usually position drink stations every 5 km, providing opportunity for fast runners to consume fluid and carbohydrate approximately every 15-20 min. In recent attempts to break the marathon world record, drinks have been provided at more frequent intervals, often in smaller volumes. PURPOSE: To determine how the pattern of carbohydrate ingestion during running affects exogenous carbohydrate oxidation rates and measures of gastrointestinal (GI) comfort. METHODS: Twelve well-trained male runners (27 ± 7 y, 67.9 ± 6.7 kg, V̇O2peak: 68 ± 7 mL/kg/min) completed three exercise trials of 100 min steady state running at 70% V̇O2peak. During the first trial, 200 mL water was consumed every 20 min and results were used for background correction of 13CO2 breath enrichment. In the final two trials a 1 L volume of a 10% dextrose solution, enriched with [U-13C] glucose, was consumed at a rate of either 200 mL every 20 min (CHO-20) or 50 mL every 5 min (CHO-5). Expired breath and venous blood samples were collected at rest and every 20 min during exercise. Subjective scales of GI comfort were recorded at regular intervals. RESULTS: Exogenous carbohydrate oxidation rates were higher after 80 (0.58 ± 0.16 v 0.48 ± 0.16 g/min; P =0.020) and 100 min (0.67 ± 0.14 v 0.58 ± 0.15 g/min; P =0.016) of running in CHO-20 compared to CHO-5. During exercise, total carbohydrate oxidation rates were similar between trials and remained within a range of 2.0 to 2.5 g/min (P>0.168). Oxidation of endogenous carbohydrate was lower in CHO-20 (1.87 ± 0.37 v 2.08 ± 0.44 g/min; P<0.05). Serum glucose concentration increased above 5 mmol/L after drink ingestion, remaining elevated throughout exercise with no difference between trials (P =0.095). There were no differences in reported symptoms of GI comfort (P >0.05), with no subject reporting severe symptoms (all < 6) in either trial. CONCLUSIONS: Ingestion of a larger volume of carbohydrate solution at less frequent intervals increased exogenous carbohydrate oxidation rates, resulting in similar rates of total carbohydrate oxidation but with reduced contribution from endogenous carbohydrate stores.