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

## TL;DR

Seven days of L-citrulline supplementation does not improve running performance in hot conditions when runners are mildly dehydrated.

## Contribution

This study is the first to investigate the effects of L-citrulline on running performance in heat under hypohydration.

## Key findings

- L-citrulline had no effect on 3 km running performance in hot conditions.
- Thermoregulation and gastrointestinal damage were unaffected by L-citrulline.
- No differences in physiological or perceptual responses were observed between L-citrulline and placebo.

## Abstract

7 days L-citrulline supplementation has been reported to improve blood pressure, \documentclass[12pt]{minimal}
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\begin{document}$$\mathop {\text{V}}\limits^{.}$$\end{document}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, \documentclass[12pt]{minimal}
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\begin{document}$$\mathop {\text{V}}\limits^{.}$$\end{document}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% \documentclass[12pt]{minimal}
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\begin{document}$$\mathop {\text{V}}\limits^{.}$$\end{document}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.

The online version contains supplementary material available at 10.1007/s00421-024-05671-4.