Citrulline enhances urea cycle activity in the liver, leading to accelerated ammonia clearance and prolonged aerobic metabolism during physical exertion.

Study: Biochemical, physiological, and performance response of a functional watermelon juice enriched in L-citrulline during a half-marathon race

Citrulline in watermelon juice helped runners produce less lactic acid and more arginine after running, which means their bodies were better at clearing waste and using oxygen longer — just like the claim says.

Study: Ammonia detoxification promotes CD8^+ T cell memory development by urea and citrulline cycles

This study shows that citrulline helps cells remove a toxic waste product called ammonia, which is exactly what the claim says citrulline does. Even though it was tested in immune cells, not muscles or liver during exercise, the same cleanup system is involved.

Study: The Effects of L-Citrulline on the Apparent Digestion and Metabolism of Nutrients, Blood Hormone Levels, Amino Acid Metabolism and the Diversity of Faecal Microbiota in Mares in the Later Stage of Pregnancy

This study found that giving horses a lot of citrulline helped their bodies process waste nitrogen better, which means their livers were likely removing ammonia more efficiently. While it didn’t test exercise, the same liver process is what the claim says citrulline does during workouts.