Two studies were conducted to investigate gastrointestinal (GI) tolerance of high carbohydrate (CHO) intakes during intense running. The first study investigated tolerance of a CHO gel delivering glucose plus fructose (GLU+FRC) at different rates. The second study investigated tolerance of high intakes of glucose (GLU) vs. GLU+FRC gel. Both studies used a randomized, 2-treatment, 2-period crossover design: Endurance-trained men and women (Study 1: 26 men, 8 women; 37 ± 11 yr; 73 ± 9 kg; 1.76 ± 0.07 m. Study 2: 34 men, 14 women; 35 ± 10 yr; 70 ± 9 kg; 1.75 ± 0.09 m) completed two 16-km outdoor-runs. In Study 1 gels were administered to provide 1.0 or 1.4 g CHO/min with ad libitum water intake every 3.2 km. In Study 2 GLU or GLU+FRC gels were given in a double-blind manner to provide 1.4 g CHO/min. In both studies a postexercise questionnaire assessed 17 symptoms on a 10-point scale (from 0 to 9). For all treatments, GI complaints were mainly scored at the low end of the scale. In Study 1 mean scores ranged from 0.00 ± 0.00 to 1.12 ± 1.90, and in Study 2, from 0.00 ± 0.0 to 1.27 ± 1.78. GI symptoms were grouped into upper abdominal, lower abdominal, and systemic problems. There were no significant treatment differences in these categories in either study. In conclusion, despite high CHOgel intake, and regardless of the blend (GLU vs. GLU+FRC), average scores for GI symptoms were at the low end of the scale, indicating predominantly good tolerance during a 16-km run. Nevertheless, some runners (~10–20%) experienced serious problems, and individualized feeding strategies might be required.
Beate Pfeiffer, Alexandra Cotterill, Dominik Grathwohl, Trent Stellingwerff, and Asker E. Jeukendrup
Trent Stellingwerff, Jean-Philippe Godin, Maurice Beaumont, Aude Tavenard, Dominik Grathwohl, Peter J. van Bladeren, Anne-France Kapp, Johannes le Coutre, and Sami Damak
Recent studies have demonstrated a direct link between increased exogenous CHO oxidation (CHOexog) and enhanced performance. The limiting factor for CHOexog appears to be at the level of intestinal transporters, with sodium/glucose cotransporter 1 (SGLT1) and glucose transporter Type 5 (GLUT5) responsible for glucose and fructose transport, respectively. Studies in animal models have shown that SGLT1 and intestinal glucose uptake are up-regulated by high carbohydrate diets or noncaloric sweeteners. The aim of this study was to determine the effect of preexercise ingestion of noncaloric sweeteners on CHOexog during exercise in athletes. In a randomized, crossover, double-blind fashion twenty-three healthy male cyclists (age = 29 ± 7yrs, mass = 73.6 ± 7.4kg, VO2peak = 68.3 ± 9.3 ml/kg/min) consumed 8 × 50ml doses of either placebo (CON) or 1mM sucralose (SUCRA) every 15 min starting 120 min before the onset of exercise. This was followed by 2h of cycling at 48.5 ± 8.6% of VO2peak with continual ingestion of a maltodextrin drink (1.2g/min; 828ml/hr). Average CHOexog during the first hour of exercise did not differ between SUCRA and CON conditions (0.226 ± 0.081 g/min vs. 0.212 ± 0.076 g/min, Δ =0.015 g/min, 95%CI -0.008 g/min, 0.038 g/min, p = .178). Blood glucose, plasma insulin and lactate, CHO and fat substrate utilization, heart rate, ratings of perceived exertion, and gastrointestinal symptoms did not differ between conditions. Our data suggest that consumption of noncaloric sweeteners in the immediate period before exercise does not lead to a significant increase in CHOexog during exercise.