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Citrulline is a non-essential amino acid found naturally in watermelon (Citrullus lanatus), hence its name. It is also synthesized in the body as part of the urea cycle and can be produced synthetically for supplementation, often as L-citrulline or citrulline malate (combined with malic acid to enhance energy production benefits). Traditionally, it has not been used in herbal medicine but gained popularity in modern sports nutrition and vascular health contexts.