L-citrulline is the natural form of citrulline. Although your body produces its own supply, consuming extra citrulline can have significant performance benefits. Found naturally in fruits such as watermelon as well as on its own in certain supplements, citrulline plays a major part in two important processes. One, it helps boosts nitric-oxide levels, which can help dilate your blood vessels, allowing more oxygen and nutrients to be delivered to your muscles. It also has a role in the removal of ammonia, a compound known to cause exercise-related fatigue, from your blood.