Most animals produce their own vitamin C internally
For most animal species, vitamin C is not a vitamin at all, but rather a compound they produce endogenously. The ability to synthesize ascorbic acid (the chemical name for vitamin C) from glucose is an ancient trait shared by a vast number of animals, from reptiles and fish to most mammals. In most mammals and birds, this synthesis takes place in the liver, while in reptiles and older bird species, it occurs in the kidneys. An adult goat, for instance, can manufacture over 13,000 mg of vitamin C per day under normal health conditions, with production increasing significantly during periods of stress.
The synthesis process culminates with an enzyme called L-gulonolactone oxidase (GULO), which is responsible for the final step in the biochemical pathway. For most animals, this enzyme functions perfectly. However, in several distinct lineages, a mutation occurred in the gene responsible for creating this enzyme, effectively breaking the process. Since these species could still get ample vitamin C from their diet, the non-functional gene was passed down and became a permanent trait.
The genetic basis for the loss of vitamin C synthesis
The inability to synthesize vitamin C is due to mutations in the L-gulonolactone oxidase (GULO) gene. Over millions of years, multiple independent GULO gene loss events occurred in different animal groups. In these species, the GULO gene is now a non-functional pseudogene.
- Haplorhini Primates: This group, which includes humans, monkeys, and apes, lost the ability to produce functional GULO approximately 61 million years ago. Their high-fruit diet meant that the loss of internal production had no negative impact on survival.
- Caviidae Rodents: This lineage includes guinea pigs and capybaras, which lost the gene roughly 14 million years ago. The GULO gene in guinea pigs is highly mutated, making it essential that they obtain vitamin C from their diet, unlike many other rodents such as rats and mice.
- Teleost Fish: The ancestor of the vast majority of modern fish lost its GULO gene around 200 million years ago, making it dependent on dietary vitamin C. Cartilaginous fish like sharks still produce their own, but most bony fish do not.
- Bats: While most bats are unable to synthesize vitamin C, some species have retained or even regained the ability. This suggests a more complex evolutionary history involving multiple losses and reactivations of the GULO gene.
- Passerine Birds: Similarly, certain species of passerine birds, a diverse order, have also independently lost the ability to make vitamin C, though some have reacquired it over time.
Why the loss of synthesis wasn't a survival disadvantage
Evolution is driven by a trade-off of costs and benefits. For many ancestral populations, living in environments with an abundance of vitamin C-rich foods meant that maintaining a functional GULO enzyme was no longer a priority. The energy that the body would have used to produce vitamin C could instead be allocated to other metabolic processes. In an environment where the nutrient was readily available, a mutation disabling the gene was a neutral event that did not affect survival. Only when conditions change, such as during long sea voyages for humans, does this genetic deficiency become a severe problem leading to diseases like scurvy.
Comparison of vitamin C production: Synthesizers vs. Non-Synthesizers
| Feature | Vitamin C Synthesizers (e.g., dogs, cats, goats, rats) | Non-Synthesizers (e.g., humans, guinea pigs, some bats) |
|---|---|---|
| Functional GULO Gene | Yes | No (mutated pseudogene) |
| Dietary Requirement | Not required under normal conditions | Essential; must be obtained through food |
| Synthesis Location | Liver (most mammals) or kidney (some birds/reptiles) | N/A (cannot synthesize) |
| Stress Response | Production increases significantly under stress | No change in internal production; requires higher dietary intake |
| Scurvy Risk | Not at risk under normal conditions | High risk if diet is deficient |
| Typical Daily Production | Thousands of milligrams per day | Zero milligrams internally produced |
Conclusion
Yes, there is vitamin C in animals, but the source and availability differ depending on the species. Most animals have an internal production system, meaning it is not considered a dietary vitamin for them. For others, including humans, guinea pigs, and certain fish and bat species, the loss of a key enzyme means they must rely entirely on external dietary sources. This evolutionary divergence highlights how adaptation to an environment rich in a particular nutrient can render an internal production system redundant, yet critical under circumstances of dietary scarcity. Understanding these genetic differences is vital for animal husbandry and veterinary medicine, ensuring that species with this deficiency receive appropriate nutrition.
