Citrulline is a vital amino acid for maintaining metabolic health because it helps regulate nitric oxide (NO) production, which controls blood flow throughout the body.  NO is vital for muscle blood flow, sexual function, fuel and hormone delivery to muscles, tissue oxygenation, and general energy levels.  Spanish researchers found that a single dose of 8 grams of citrulline increased lifting endurance during the bench press, and decreased post exercise muscle soreness.  Citrulline supplements, sold in the form of citrulline malate, might reduce muscle soreness, improve weight training endurance, and prevent blood vessel damage that accompanies intense nitric oxide levels.

(Journal of Strength Conditioning Research, 24:  1215-1222, 2010)