Did you know…

Almost every mammal on the planet makes its own vitamin C, in quantities that dwarf the human daily recommended intake. A 155-pound goat produces roughly 13 grams per day. A rat synthesizes 26 to 58 mg per kg of body weight per day. Extrapolated to human body weight, if the enzyme were not broken by a mutation, our own liver would produce something like 2 to 3 grams per day. The current RDA in most countries is 75 to 90 mg. It is calibrated to prevent scurvy, not to match what the body would produce on its own.

The claim, in the reference of comparative physiology

The enzyme that makes vitamin C from glucose in the liver of most mammals is L-gulonolactone oxidase (GLO). In humans, other primates, guinea pigs, and one fruit bat, the gene coding for this enzyme carries a mutation that inactivates it. Nishikimi and Yagi, in their 1957 paper in Nature, identified the missing step. The finding has been reiterated across decades of comparative biochemistry:

“Man, other primates and guinea pig are the only mammals that are known to be unable to synthesize L-ascorbic acid; thus they require vitamin C in their diet to prevent scurvy.”

— Nishikimi & Yagi, and subsequent comparative-biochemistry literature

The reference for what a mammalian liver produces when GLO is intact is not obscure. Rat synthesis at 26-58 mg/kg/day, extrapolated to a 70 kg human, gives an estimate of 2 to 3 grams per day.

The kidney-stones argument, reversed by comparative physiology

The clinical folk-wisdom that high-dose vitamin C causes kidney stones by producing oxalate rests on the biochemistry that ascorbic acid is a precursor to oxalate in the human body. Left aside is that goats, cats, dogs, rats, and every other GLO-intact mammal produces internally, every day, the equivalent of what a human on 10 to 100 g/day supplementation would ingest. If that dose reliably produced kidney stones, no mammal would survive its own liver.

What the RDA is calibrated for

The RDA is a scurvy-prevention threshold. It is not a functional optimization for immune, connective-tissue, or antioxidant demand. Whether the difference matters clinically is a question the field has not resolved, but the framing that treats 90 mg as the physiological target is a framing that ignores what the mammalian liver would say if it could speak.

This entry is a companion piece to fiches on antioxidant status and to the broader argument on why reference ranges are calibrated on populations, not on function.