Humans can’t make vitamin c

Most living things today make their own Vitamin C and do not have to obtain it through food sources. They do this by converting glucuronic acid (C6H10O7), derived from glucose (C6H12O6), into ascorbic acid (C6H8O6). The exceptions that cannot make their own Vitamin C are guinea pigs, fruit eating bats, the red-vented bulbul bird, and higher primates, such as gorillas and humans.

All humans are born with a genetic defect. Our livers are missing a key enzyme, xl-gulono-§-lactone oxidase, which is required to synthesize ascorbic acid from glucose. The loss of this single gene prevents humans from making their own Vitamin C. The negative mutation of the so-called GULO gene has been well studied in both human and primate genomes. If this negative mutation could be corrected, most people would no longer need to supplement Vitamin C in their diets.