L-citrulline is a naturally occurring amino acid. It is found in some foods like watermelons and is also produced naturally by the body.
L-citrulline is used for Alzheimer's disease, dementia, fatigue, muscle weakness, sickle cell disease, erectile dysfunction, high blood pressure, and diabetes. It is used for heart disease, body building, increasing energy, and for improving athletic performance.
Our bodies change L-citrulline into another amino acid called L-arginine and also to nitric oxide. L-citrulline might help increase the supply of ingredients the body needs to making certain proteins. It might also help open up veins and arteries to improve blood flow and reduce blood pressure.
L-Citrulline is an amino acid that works to raise both arginine and ornithine plasma levels more efficiently and for longer periods of time than just l-arginine supplementation alone. It has quickly become the leader for nitric oxide increasing supplementation.
As an athletic performance supplement, l-citrulline has been shown to provide the following benefits when dosed properly (4-8g per serving).
- Decreased fatigue 
- Decreased muscle soreness 
- Increased nitric oxide 
- Increased blood flow 
- Increased growth hormone (study showed 66.8% increase post workout) 
L-Citrulline is one of the most promising pre-workout supplement ingredients for the purpose of building muscle.
Derived from watermelon, l-citrulline is considered natural.