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

7 days L-citrulline supplementation has been reported to improve blood pressure, \(\mathop {\text{V}}\limits^{.}\) $\stackrel{.}{\text{V}}$ 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.

13 endurance runners (2 female, 31 ± 8 y, \(\mathop {\text{V}}\limits^{.}\) $\stackrel{.}{\text{V}}$ 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% \(\mathop {\text{V}}\limits^{.}\) $\stackrel{.}{\text{V}}$ 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.

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.

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.