L-Arginine but not L-glutamine likely increases exogenous carbohydrate oxidation during endurance exercise

The addition of l-arginine or l-glutamine to glucose-electrolyte solutions can increase intestinal water, glucose, and sodium absorption in rats and humans. We evaluated the utility of l-arginine and l-glutamine in energy-rehydration beverages through assessment of exogenous glucose oxidation and perceptions of exertion and gastrointestinal distress during endurance exercise. Eight cyclists rode 150 min at 50% of peak power on four occasions while ingesting solutions at a rate of 150 mL 15 min(-1) that contained C-13-enriched glucose (266 mmol L-1) and sodium citrate ([Na+] 60 mmol L-1), and either: 4.25 mmol L-1 l-arginine or 45 mmol L-1 l-glutamine, and as controls glucose only or no glucose. Relative to glucose only, l-arginine invoked a likely 12% increase in exogenous glucose oxidation (90% confidence limits: +/- 8%); however, the effect of l-glutamine was possibly trivial (4.5 +/- A 7.3%). l-Arginine also led to very likely small reductions in endogenous fat oxidation rate relative to glucose (12 +/- A 4%) and l-glutamine (14 +/- A 4%), and relative to no glucose, likely reductions in exercise oxygen consumption (2.6 +/- A 1.5%) and plasma lactate concentration (0.20 +/- A 0.16 mmol L-1). Effects on endogenous and total carbohydrate oxidation were inconsequential. Compared with glucose only, l-arginine and l-glutamine caused likely small-moderate effect size increases in perceptions of stomach fullness, abdominal cramp, exertion, and muscle tiredness during exercise. Addition of l-arginine to a glucose and electrolyte solution increases the oxidation of exogenous glucose and decreases the oxygen cost of exercise, although the mechanisms responsible and impact on endurance performance require further investigation. However, l-arginine also increases subjective feelings of gastrointestinal distress, which may attenuate its other benefits.