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 13C-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 ± 7.3%). l-Arginine also led to very likely small reductions in endogenous fat oxidation rate relative to glucose (12 ± 4%) and l-glutamine (14 ± 4%), and relative to no glucose, likely reductions in exercise oxygen consumption (2.6 ± 1.5%) and plasma lactate concentration (0.20 ± 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.
KeywordsStable isotope Carbohydrate metabolism Exercise economy Gastrointestinal distress Nitric oxide
We thank Alexander Rossi, Kristen Goele, Megan Thorburn, and Rhys Thorp for their laboratory assistance and the cyclists for their valued contribution. This research was funded by Massey University.
Conflict of interest
The authors have nothing to disclose.
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