Effects of Citrulline or Watermelon Supplementation on Body Composition: A Systematic Review and Dose–Response Meta-Analysis

Background/Objectives: L-Citrulline (CIT) is a non-essential amino acid abundant in watermelon and commonly used as a dietary supplement to enhance exercise performance. Although its benefits for endurance and resistance training are well documented, its effects on body composition remain uncertain. This systematic review and dose–response meta-analysis aimed to assess the impact of CIT supplementation on anthropometric parameters. Methods: A comprehensive search of major databases identified relevant randomized controlled trials (RCTs) published until March 2025. A random-effects model was used to synthesize the data. Results: Twenty-one RCTs were included. Overall, CIT supplementation had no substantial effects on body mass index (BMI), body weight, fat mass (FM), waist circumference (WC), body fat percentage (BFP), and fat-free mass (FFM). Subgroup analyses revealed reductions in FM among participants over 40 years of age and in those administered more than 6 g/day of CIT. Interventions lasting 3 to 8 weeks were associated with a significant increase in FFM. Dose–response analyses suggested a non-linear association between CIT supplementation duration and changes in FM and FFM. Conclusions: CIT supplementation appears to have no overall effect on body composition. However, exploratory findings indicated potential benefits at higher doses or shorter durations. Rigorous trials controlling for dietary intake and training variables are needed to clarify its long-term effects.