Arginine is one of the basic (positively charged) amino acids, present in most food proteins and classed as conditionally essential in human nutrition: the body normally synthesises enough through the urea cycle, but demand rises during growth, illness, injury, and intense training. Rich dietary sources include meat, dairy products, nuts, seeds, and legumes. Beyond its role as a protein building block, arginine is the direct precursor of nitric oxide, the signalling molecule that regulates blood-vessel dilation, which links it to cardiovascular research and sports-nutrition products. Food analysts also track arginine because its levels reflect protein quality and, in fermented foods, its breakdown can contribute to biogenic amine formation.