Short answer: L-Citrulline HCL is converted by the kidneys into arginine, then into nitric oxide — generally a more efficient route than arginine taken directly.
Why "first-pass metabolism" is the specific, checkable reason behind this
Citrulline is generally the more efficient route because less of it is lost to first-pass metabolism — a specific, named pharmacokinetic reason, not a vague efficiency claim.
Why both direct arginine and citrulline are still included together
Including both forms may reflect a genuine attempt to cover multiple entry points into the same nitric oxide pathway, even though they converge on the same end mechanism.
💡 Worth knowing: a second form of citrulline — L-Citrulline Malate — is also included, again feeding the same nitric oxide pathway.