As children, we have been taught that veggies and fruits are healthy, and, later on, that vitamin C is important for enhanced immunity when facing a cold. But did you know that, while humans need to get vitamin C via diet, most animals produce this important antioxidant internally? Most species have functional vitamin C genes, but ours have mutated and are now broken! Why is that? And why do we actually need more vitamin C than most of us do?
The dysfunctional vitamin C gene and the evolutionary explanation for it is surprising and has far-reaching implications for understanding the human diet!
For quick readers:
Why do humans not synthesize vitamin C, while most mammals do? To understand this apparently nonsensical evolutionary “mistake” we need to know one well-kept secret: Broken vitamin C genes are unique to species with a fruit-rich diet, also called frugivores. Humans require vitamin C from food because we come from a highly frugivorous ancestry, and the natural human diet is still fruit-based: A frugivore diet! Yes, humans are still frugivores! Not being aware of this, we do not include nearly as much fruit (and therefore vitamin C) in our modern diets as we have adapted to eat! Read here, how much vitamin C we should – naturally – get on a daily basis.
In this article, you will see evidence that lost vitamin C production is a trait unique to frugivores. So let’s dive in:
What? Humans have lost their vitamin C genes?!
Yes, we have! Humans cannot synthesize vitamin C, while most mammals can produce vitamin C from gloucose! We have mutated “broken” genes in this regard. But why? What does this gene loss tell us about the natural human diet? It screams, “we are frugivores!“
Only frugivorous species get sufficient vitamin C from their natural diet! Humans share the Vitamin C-loss characteristic with other frugivores, like frugivorous bats, guinea pigs, and all primates.
Making sense of the vitamin C gene mutation.
With this dietary pattern, the seemingly malfunctioning genetic mismatch starts to make sense – and can even be beneficial, as the organism saves energy by outsourcing energy-intensive metabolic tasks.
Omnivorous, carnivorous, and even herbivorous mammals – like dogs, cats, cows, rats, rabbits, elephants, most bats, and some birds – have functional vitamin C synthesis. Thus they have no need to obtain it from their diet. Only frugivorous species lose their vitamin C production among different mammalian taxa – because their diet contains sufficient amounts of vitamin C. Those animals have a mutation in the GULO gene that renders the gene a dysfunctional “pseudogene”.
The mystery of mutated Vitamin C genes?
In 2010 an article in Nature about the loss of vitamin C genes called “The Mystery of Vitamin C” could not find the answer as to why humans took this evolutionary turn:
“We still do not know why humans lost the capability of synthesizing vitamin C. This event probably had evolutionary significance.”
The author de Tullio calls the loss of such an important gene an “apparent paradox“, as it does not make sense in evolution to loose such a vital function. The answer, however, becomes apparent when looking at the pattern among animals that have also lost their ability to synthesize Vitamin C: Only mammals with a high-fruit diet do not produce Vitamin C internally! This is yet another intriguing sign that humans are still adapted to a high-fruit diet!
Figure: A telling case of convergent evolution: Vitamin C gene loss has occurred independently in species with a high fruit diet (high vitamin C intake). This phylogenetic tree by Drouin et al. (2011) illustrates the ability of different mammals to produce vitamin C. Dark green species have functional genes, light green species do not. Screenshot from scientific american, showing that Lemurs (a primate with functional vitamin C genes) consume far less fruits than other primates”.
Thus, humans, and most other primates, are not entirely alone with their dysfunctional vitamin C genes: This is a mutation that re-emerges in frugivores because frugivores take in large amounts of vitamin C from fruits. Across birds (many birds are frugivores), functional vitamin C genes were lost, and have even reappeared in birds that do not eat fruits.
Outsourcing the production of vitamin C helps the body save energy and invest it in other metabolic tasks. This is why losing a functional gene can be an advantage in evolution. Nature saves energy, and the most energy-efficient genotype is successful, which is why unnecessary genes are lost.
Once again, in so many other topics in health, nutrition and evolution, things just start to make sense when frugivory (humans being specialized fruit-eaters) comes into the picture!
Not convinced yet that we naturally have a high-fruit diet? Check out our article on the adaptive characteristics of frugivorous primates and humans!
Humans compensate for their broken vitamin C production
Species without functional vitamin C production, compensate for their lack not only through a vitamin C-rich diet, but also through effective vitamin C uptake and recycling (source): Our billions of red blood cells take up the oxidized (already used) form of vitamin C (DHA) and transform them back into the active form. Red blood cells carry a large number of transporters (GLUT 1) on their surface, which favor taking in DHA over glucose.
This adaptation is not present in species that produce vitamin C, as their red blood cells hardly take up DHA.
What is a frugivorous diet?
To understand how humans can be categorized alongside fruit eaters, we must understand more about frugivory in primates and our past.
Frugivores are animals whose preferred food is fruits and thus have adaptations for foraging fruits. Highly frugivorous animals eat mainly fruits, but most frugivores also eat other types of foods. Frugivores can be a sub-class of omnivores or herbivores. Chimpanzees, for example, are frugivorous omnivores.
Frugivorous primates eat ripe tropical fruits, greens, nuts, and tubers, but also eggs, insects, and some also eat meat from time to time. Read more on the diet of chimpanzees here.
Frugivorous primates can only exist in tropical arboreal habitats, where fruits are larger, more nutritious, and abundant throughout the year. Humans originated in tropical forests – and we still are adapted to tropical climates – which enabled us to evolve as frugivores. To know more about the “human version” of a frugivorous diet, also visit our guide here.
The natural human diet contains much more fruit than we think…
While this is a topic of debate, with highly varying opinions, the fact remains that we have lost our vitamin C production as a species, because we are frugivores! Our species-specific diet contains high amounts of fruits in nature and, thus, vitamin C.
Due to migration in colder habitats and consequent survival foods, humans, have culturally adopted diets that do not contain nearly as much vitamin C as we naturally would naturally get: Most modern diets, which are “deficient” in fruit! Read more here, on how much vitamin C we optimally should obtain from our diet (it’s 2000 – 3000 mg daily for adults).
This is why vitamin C is such a huge deal in health – and why only a high-fruit diet can really address the root cause! Learn more about the human species-appropriate high-fruit diet here:
Go to How to do the Frugivore Diet
If you are new to this, visit this overview about frugivores and frugivory here!
References
- T. H. Jukes, J. L. King, Evolutionary loss of ascorbic acid synthesizing ability. Journal of Human Evolution. 4, 85–88 (1975), doi:10.1016/0047-2484(75)90002-0. (link)
- G. Drouin, J.-R. Godin, B. Page, The genetics of vitamin C loss in vertebrates. Current Genomics. 12, 371–378 (2011), doi:10.2174/138920211796429736. (link)
- M.C De Tullio, The Mystery of Vitamin C. Nature news (available at https://www.nature.com/scitable/topicpage/the-mystery-of-vitamin-c-14167861/). (link)
- J. G. Goldman, Why lemurs have such strange diets. Scientific American (2018) (available at https://www.scientificamerican.com/article/why-lemurs-have-such-strange-diets/). (link)
- G. Drouin, J.-R. Godin, B. Page, The genetics of vitamin C loss in vertebrates. Current Genomics. 12, 371–378 (2011), doi:10.2174/138920211796429736. (link)
- How humans make up for an ‘inborn’ vitamin C deficiency (2008) ScienceDaily. Available at: https://www.sciencedaily.com/releases/2008/03/080320120726.htm (Accessed: 31 July 2023).
- Scerri, E.M. et al. (2022) ‘Tropical forests in the deep human past’, Philosophical Transactions of the Royal Society B: Biological Sciences, 377(1849). doi:10.1098/rstb.2020.0500.
