The reason is extensive first pass metabolism where a large amount of the supplemental Arginine is destroyed during initial digestion. However, supplementing with a citrulline does not suffer this same fate with all of it metabolising into arginine!
This is why we chose to produce a Citrulline Supplement as the best way to raise plasma arginine levels and in turn to raise Nitric Oxide and NOT sell you an Arginine Supplement.
Note also that CItrulline not only gives you a better nitric oxide effect, it can, unlike arginine, additionally Simulates Muscle Protein Synthesis and assist in maintaining Positive Nitrogen Balance – important for both athletes and those recovering from physical trauma or age related muscle wasting (sarcopenia).
So with Citrulline you are getting a much better Bang for your Buck!!!