The amino acid citrulline is a popular supplement for health and exercise performance. This amino acid is produced naturally in the body and can also be found in food. Citrulline is an amino acid that was first found in watermelon. This amino acid is considered nonessential, meaning that your body can naturally produce some on its own. However, you can increase your levels by eating certain foods containing citrulline or by taking a supplement. These higher levels may produce beneficial effects on health and exercise performance. Citrulline can also be used to help widen your blood vessels and may play a significant role in muscle building. One major way citrulline works is by increasing vasodilation which refers to the widening of arteries or veins. Vasodilation is also associated with lowering blood pressure and increasing blood flow.