Most animals make their own vitamin C—humans lost the ability
Almost all mammals produce their own vitamin C in their liver or kidneys, making scurvy an impossibility for them. However, roughly 61 million years ago, an ancestor of humans suffered a genetic mutation that disabled the gene responsible for synthesizing the enzyme GULO. As a result, humans, apes, and a few other species must get all their vitamin C from food.
The Internal Factory: How Animals Make Vitamin C
For the vast majority of vertebrates, vitamin C is not a dietary requirement but a routine internal metabolite. In amphibians, reptiles, and older avian lineages, the biochemical machinery for producing ascorbic acid is housed primarily in the kidneys. In most mammals and more recently evolved perching birds, the site of production shifted to the liver. Using simple blood glucose as a starting material, these animals run a multi-step enzymatic pathway that converts glucose into galactose derivatives, intermediate lactones, and ultimately L-ascorbic acid.
The final, decisive step in this conversion relies on the enzyme L-gulonolactone oxidase, commonly abbreviated as GULO. The enzyme catalyzes the oxidation of L-gulono-1,4-lactone into ascorbic acid, releasing hydrogen peroxide as a byproduct. Because their bodies continually manufacture this compound, animals such as dogs, cats, cows, and mice produce the equivalent of several grams of vitamin C per day when scaled to human body mass, and many can ramp up their internal synthesis during times of physiological stress, injury, or infection.
The Broken Gene Across Different Species
In humans and our close relatives, the biochemical pathway is intact right up until the final hurdle. The human genome contains the sequence for the GULO enzyme on chromosome 8, but it exists only as a non-functional pseudogene, often designated GULOP. Several key coding exons have been deleted or mutated over evolutionary time, leaving cells unable to manufacture the functional protein. The precursor molecule L-gulonolactone is still produced, but without GULO, it cannot be converted into ascorbic acid and is instead metabolized through alternative pathways or excreted.