1,4-D-Gulonolactone (D-Gulonic acid γ-lactone) (also known as reduced ascorbic acid, RAA) is the substrate of the enzyme L-gulono-1,4-lactone oxidoreductase (EC 1.1.3.8), which catalyzes the last step of the biosynthesis of L-ascorbic acid (vitamin C) in plants and animals. The enzyme L-Gulono-1,4-lactone oxidase is missing in scurvy-prone, vitamin C-deficient animals, such as humans. 1,4-D-Gulonolactone is present in human blood and has been used as one of the markers to compare changes in exercise-induced oxidative stress.