7 days of L-citrulline supplementation does not improve running performance in the heat whilst in a hypohydrated state

PURPOSE: 7 days L-citrulline supplementation has been reported to improve blood pressure, formula presented O2 kinetics, gastrointestinal (GI) perfusion and endurance cycling performance through increasing arterial blood flow. In situations where blood volume is compromised (e.g., hyperthermia/hypohydration), L-citrulline may improve thermoregulation and exercise performance by redistributing blood flow to aid heat loss and/or muscle function. This study assessed 7 days L-citrulline supplementation on running performance in the heat, whilst mildly hypohydrated. METHODS: 13 endurance runners (2 female, 31 ± 8 y, formula presented O2peak 60 ± 6 mL/kg/min) participated in a randomised crossover study with 7 days L-citrulline (CIT; 6 g/d) or placebo (maltodextrin powder; PLA) supplementation. Participants completed a 50 min running 'preload' at 65% formula presented O2peak (32 °C, 50% relative humidity) to induce hyperthermia and hypohydration before a 3 km running time trial (TT). Body mass and blood samples were collected at baseline, pre-preload, post-preload and post-TT, whilst core and skin temperature, heart rate and perceptual responses were collected periodically throughout. RESULTS: TT performance was not different between trials (CIT 865 ± 142 s; PLA 892 ± 154 s; P = 0.437). Core and skin temperature and heart rate (P ≥ 0.270), hydration (sweat rate, plasma volume, osmolality) indices (P ≥ 0.216), GI damage (P ≥ 0.260) and perceptual responses (P ≥ 0.610) were not different between trials during the preload and TT. CONCLUSIONS: 7 days of L-citrulline supplementation had no effect on 3 km running performance in the heat or any effects on thermoregulation or GI damage in trained runners in a hypohydrated state.