Citrulline is a non-essential amino acid, meaning that it can be synthesised (produced) within the body. The synthesis of citrulline primarily occurs in the enterocytes (cells) of the small intestine, via an enzymatic reaction of two other amino acids, glutamine and proline. Once formed, citrulline is involved in various metabolic pathways — most notably, the urea cycle — and is a precursor for the amino acid Arginine.
One of the key benefits of arginine is its role in Nitric Oxide (NO) production. NO is a vital signalling molecule, though principally, is a potent vasodilator modulating blood flow and oxygen delivery throughout the body.1 During aerobic exercise, skeletal muscle requires oxygen for energy production, therefore producing more NO leads to greater supply of oxygen to working muscles, and higher aerobic capacity, meaning you can push yourself to higher exercise intensities.2
Now, you may be thinking “why don’t we just supplement with arginine or NO?”. Well, citrulline has a longer half-life than arginine and NO, meaning that it circulates for longer once in our system, and is actually more effective at increasing levels of arginine than arginine supplements themselves. Think of citrulline supplementation as a stepping stone to boosting your body’s NO production.
We also mentioned that citrulline plays a part in the urea cycle — the metabolic process of converting ammonia to urea, which is then excreted as a waste product. Citrulline helps reduce ammonia accumulation within skeletal muscle, which works to reduce muscular fatigue and extend exercise duration.