The Reason Your Body Cannot Make Vitamin C Anymore

Summary

The video explains that humans cannot synthesize vitamin C because the gene for the final step in its production, GULO, is no longer functional. The gene remains on chromosome 8 as the pseudogene GULOP: it is missing five of the 12 exons found in working versions, and the remaining sequence contains mutations that disrupt its instructions. Many mammals—including goats, rats, and dogs—still make vitamin C in their livers.

Vitamin C and scurvy

• Vitamin C, or ascorbic acid, is chemically similar to glucose. In animals that make it, a series of enzymes converts sugar into vitamin C; GULO performs the final step.
• A vitamin is a substance an animal needs but cannot make for itself. Vitamin C is therefore a vitamin for humans, but not for animals that synthesize it.
• The body uses vitamin C to support enzymes involved in making stable collagen, which strengthens skin, blood vessels, gums, tendons, and other tissues. It also contributes to the production of carnitine and norepinephrine.
• When vitamin C is depleted, collagen becomes weak. This helps explain scurvy’s symptoms: bleeding under the skin and gums, poor wound healing, joint pain, and other hemorrhages. The video also describes fatigue and low mood as earlier symptoms.
• The video cites an experiment in which volunteers were first given a baseline amount of vitamin C and then placed on a vitamin-C-deficient diet. Symptoms appeared as body stores fell; after vitamin C was restored, blood levels recovered. It says that roughly 10 mg a day can prevent scurvy, though recommended daily allowances are higher.

Historical evidence

• The video estimates that scurvy killed more than two million sailors during the Age of Sail, when long voyages and preserved foods often meant little dietary vitamin C.
• It recounts a 1747 trial by a Scottish naval surgeon: 12 sailors with scurvy were divided into pairs and given different proposed treatments. The pair given oranges and lemons improved most rapidly.
• The British Admiralty later required lemon juice for sailors. The video notes that heat could destroy vitamin C, making preserved juice less effective, and that the vitamin itself was not isolated until 1928.

How and when the gene was lost

• The video says that some primates—apes, common monkeys, and tarsiers—cannot make vitamin C, while lemurs, lorises, and galagos can. This pattern suggests the loss occurred after those lineages diverged.
• Researchers estimate that the primate GULO gene became inactive about 61 million years ago, though estimates vary. Guinea pigs and some bats also lost the ability independently; some birds and fish are mentioned as further examples.
• Shared mutations in humans, chimpanzees, and orangutans support the idea that the primate loss occurred once in a common ancestor. The video also says Neanderthal DNA is consistent with this history.
• The conventional explanation is that vitamin-C-rich diets made the gene unnecessary, allowing mutations to accumulate through relaxed natural selection. The video argues this may not be the whole story: many animals with vitamin-C-rich diets kept the gene, and animals that synthesize vitamin C may also generate hydrogen peroxide as a by-product.

A possible benefit: reduced parasite reproduction

The video describes a 2025 mouse study suggesting that low vitamin C might sometimes hinder parasites that depend on their host for the nutrient:

• Researchers infected ordinary mice and genetically modified mice that could not make vitamin C with schistosome parasites.
• The modified mice were given a low-vitamin-C diet during the period when the worms produce eggs, then received vitamin C again.
• According to the video, the low-vitamin-C group had much lower mortality and less egg-related disease, even though the number of worms was similar. The worms’ eggs were fewer and malformed.
• The proposed explanation is that the parasites need vitamin C for processes involved in reproduction. Temporarily reducing vitamin C could impair egg production before the host develops scurvy.

The video stresses that this experiment demonstrates a possible advantage in mice infected with a particular parasite; it does not prove that parasite protection caused the gene loss in primate ancestors. The evolutionary explanation remains uncertain, and diet, parasite pressure, or both may have contributed.

Vitamin C recycling and practical points

• The video describes a recycling process in human red blood cells: the transporter GLUT1, working with stomatin, can take up oxidized vitamin C and help convert it back into active vitamin C. It says this recycling system was described in a 2008 paper.
• The kidneys also conserve vitamin C when levels are low and excrete more when levels are high. These mechanisms help explain why humans rely on smaller quantities than animals that manufacture it.
• The video says most people obtain vitamin C from fruits and vegetables, and that the nutrient is vulnerable to water, air, and heat. Boiling vegetables and discarding the water can reduce the amount retained.
• It cautions against assuming that humans should take the gram-level quantities produced by some animals. It reports that high-dose supplements have limited effects on colds for most people, with some reduction in cold duration and possible benefit under extreme physical exertion. It also notes that scurvy still occurs among people with very restricted diets. The video advises consulting a health professional about supplements.

Researchers and sources mentioned

• National Center for Biotechnology Information (NCBI) — cited for the human GULO pseudogene record.
• Proceedings of the National Academy of Sciences (PNAS) — the video says the schistosome and vitamin-C-deficient mouse study was published there in December 2025; no authors are named in the subtitles.
• A 2008 paper on vitamin C recycling in red blood cells — authors are not named.
• Researchers in Iowa who conducted the vitamin-C-deprivation experiment — not named.
• A Scottish naval surgeon who conducted the 1747 citrus trial — not named in the subtitles.
• Researchers studying GULO loss, primate evolution, and schistosome infection — discussed but not individually named.
• Historical sailors’ records and ship logs — cited as evidence of scurvy’s symptoms and impact.