Evidence Level
Strong
The nitric oxide pathway is one of the most well-characterized in vascular physiology — the 1998 Nobel Prize was awarded for its discovery. L-citrulline has emerged as the preferred oral NO precursor after Schwedhelm et al. (2008) demonstrated that citrulline raises plasma arginine levels more effectively than arginine itself, due to arginine's extensive first-pass hepatic metabolism. A 2019 meta-analysis by Trexler et al. confirmed that citrulline supplementation improved exercise performance across multiple modalities. For the nitrate pathway, Siervo et al. (2013) established that dietary nitrate from beetroot significantly reduces blood pressure through the sequential conversion of nitrate to nitrite (by oral bacteria) to nitric oxide.