Arginine and Citrulline

The source claims substrate shortage is usually not the limiting factor for nitric oxide production. Extra arginine may increase nitric oxide in young, healthy, well-trained individuals, but the source frames this as a narrow condition. Arginine is a substrate that nitric oxide synthase can convert into nitric oxide. Citrulline is described as an indirect and inefficient route because it must be recycled into arginine before contributing to nitric oxide synthesis. High-dose arginine is described as potentially harmful in endothelial dysfunction or vascular disease.