Humans lost the ability to make vitamin C because of a crippled gene. The gene in question, called GULO, once encoded the enzyme responsible for the final step in vitamin C production. In our primate ancestors, accumulated mutations rendered that gene non-functional roughly 60 to 70 million years ago, and it has been deteriorating ever since. Today, every human carries a broken copy of GULO on chromosome 8, a molecular fossil of a capability we used to have and can never use again.
The Gene That Broke
Most mammals manufacture their own vitamin C through a chain of chemical reactions that converts glucose into ascorbic acid, primarily in the liver. The last step in that chain requires an enzyme called L-gulono-γ-lactone oxidase, encoded by the GULO gene. In humans, this gene exists only as a pseudogene, often labeled GULOP, meaning it is still physically present in the genome but riddled with mutations that prevent it from producing a working enzyme. Only a handful of the original exons are still recognizable. A 2024 study comparing primate GULOP sequences found that just six of the original twelve exons retain enough identity to be identified, with the rest having degraded beyond recognition over millions of years.1PubMed Central. Conservation of a Chromosome 8 Inversion and Exon Mutations Confirm Common Gulonolactone Oxidase Gene Evolution Among Primates, Including H. Neanderthalensis
This is not a subtle defect. The gene is so thoroughly broken that no amount of diet change or lifestyle adjustment could coax it back into working. And the damage is shared across all anthropoid primates, from spider monkeys to gorillas to Neanderthals, which tells us the loss happened once, deep in our evolutionary past, and has been inherited by every descendant species since. A comparative genomics study confirmed GULO as one of the long-established genes definitively lost on the human lineage.2PLOS Computational Biology. Comparative Genomics Search for Losses of Long-Established Genes on the Human Lineage
Humans Are Not the Only Ones
Losing the ability to synthesize vitamin C sounds like it should be a catastrophic evolutionary mistake, and yet it has happened repeatedly across the animal kingdom. Guinea pigs cannot make vitamin C. Neither can certain species of bats or some songbirds in the order Passeriformes. Teleost fish, a vast group that includes most bony fish, have also lost the capacity. In every case studied so far, the underlying cause is the same: mutations in the GULO gene that knock out that final enzymatic step.3PubMed Central. The genetics of vitamin C loss in vertebrates
The losses go even further outside vertebrates. Research into invertebrate genomes has found that GULO was lost independently in large taxonomic groups including crustaceans, nematodes, flatworms, and bivalves.4PubMed Central. Multiple independent L-gulonolactone oxidase (GULO) gene losses and vitamin C synthesis reacquisition events in non-Deuterostomian animal species The fact that so many unrelated lineages have independently lost the same gene suggests that losing vitamin C synthesis is not as dangerous as it sounds, at least under the right dietary circumstances. Interestingly, some insects and nematodes that lack the GULO gene entirely can still make vitamin C through an alternative biochemical pathway that scientists have not yet fully characterized.5PubMed Central. The evolution of vitamin C biosynthesis and transport in animals
Why Evolution Let It Happen
The most intuitive explanation for why our ancestors survived without a working GULO gene is dietary abundance. Early primates lived in tropical and subtropical forests where fruit was plentiful. If you are eating vitamin-C-rich food every day, the enzyme becomes redundant. Maintaining a functional gene has a real metabolic cost, and mutations that disable unused genes face no selective pressure to be weeded out. Over millions of years, random mutations accumulated until the gene was beyond repair.
That dietary-surplus explanation is widely accepted, but it may not be the whole story. One alternative hypothesis points to a metabolic trade-off. The biochemical pathway that produces vitamin C shares early steps with the pathway that detoxifies foreign chemicals through glucuronate conjugation. If losing vitamin C synthesis freed up metabolic resources for stronger detoxification, the trade-off might have been advantageous in certain ecological niches.6J-STAGE. Ascorbate is a multifunctional micronutrient whose synthesis is lacking in primates This idea is harder to test, and it remains more speculative than the dietary explanation, but it highlights that gene losses are not always purely neutral events.
A more famous hypothesis tried to connect the loss of vitamin C synthesis to another peculiarity of primate biochemistry: elevated uric acid levels. The logic went like this: uric acid is an antioxidant, so as primates lost the ability to make vitamin C (itself an antioxidant), rising uric acid stepped in to fill the gap. The timing seemed to line up on the surface. But a detailed study that resurrected ancient uricase enzymes, the enzymes that break down uric acid, found that the critical loss of uricase activity happened between 20 and 30 million years ago, while the loss of vitamin C synthesis occurred around 67 million years ago. The two events appear to be separated by tens of millions of years and are likely unrelated.7PubMed Central. Phylogenetic Articulation of Uric Acid Evolution in Mammals and How It Inform a Therapeutic Uricase That is a good reminder that evolutionary “just so” stories can sound compelling while being wrong about causation.
How Your Body Makes Do
Living without the ability to synthesize vitamin C does not mean the body treats it carelessly. Humans have evolved several mechanisms to absorb, recycle, and conserve every bit of dietary ascorbic acid they take in.
Absorption from the gut and reclamation by the kidneys depend on specialized transport proteins called sodium-dependent vitamin C transporters, abbreviated SVCT1 and SVCT2. These two transporters have distinct jobs. SVCT1, found mainly in the gut lining and kidney, handles whole-body vitamin C balance by absorbing it from food and reclaiming it from urine before it is excreted.8PubMed. SVCT1 and SVCT2: key proteins for vitamin C uptake Your kidneys are particularly aggressive about this reclamation, using SVCT1 in the proximal tubule to pull vitamin C back out of the fluid that will become urine.9PubMed. Differential distribution of the Sodium-vitamin C cotransporter-1 along the proximal tubule of the mouse and human kidney SVCT2, by contrast, is distributed across metabolically active tissues like the brain and adrenal glands, where it protects cells from oxidative stress.
Perhaps the most striking compensatory adaptation involves red blood cells. Human red blood cells express a glucose transporter called Glut1 on their surface. Glut1 can also transport dehydroascorbic acid, the oxidized and depleted form of vitamin C. Once inside the red blood cell, the dehydroascorbic acid gets chemically reduced back to active ascorbic acid. This recycling system is not something all mammals have. Mice, which make their own vitamin C, express a different glucose transporter (Glut4) on their red blood cells and do not have this recycling ability. The Glut1-based system appears to have evolved specifically in mammals that lost vitamin C synthesis, as a way to squeeze more use out of every molecule they get from food.10Cell Press. Erythrocyte-Specific Glut1 and Stomatin Homologs Cooperate in Mammalian Glucose and Dehydroascorbic Acid Transport
What Vitamin C Actually Does in the Body
Most people associate vitamin C with immune health and cold prevention, but its biological roles go much deeper than that. Its best-understood job is as a cofactor for enzymes that build and maintain collagen, the structural protein that holds connective tissue together. Without adequate vitamin C, collagen synthesis falters, which is why scurvy, the disease of severe vitamin C deficiency, manifests as bleeding gums, loosening teeth, and wounds that refuse to heal.11PubMed Central. Efficacy of Vitamin C Supplementation on Collagen Synthesis and Oxidative Stress After Musculoskeletal Injuries: A Systematic Review
More recently, vitamin C has been recognized as a key player in epigenetics, the system that controls which genes are switched on or off in a given cell. It acts as a cofactor for enzymes called TET proteins, which remove chemical tags from DNA. A study in embryonic stem cells showed that vitamin C promotes TET activity, leading to widespread changes in gene activation patterns. Other antioxidants could not replicate this effect, confirming that vitamin C’s role here is specific and not just a generic antioxidant function.12PubMed Central. Vitamin C induces Tet-dependent DNA demethylation and a blastocyst-like state in ES cells Vitamin C also appears to serve as a cofactor for certain histone-modifying enzymes, adding another layer to its involvement in gene regulation.13PubMed Central. The epigenetic role of vitamin C in health and disease
These findings have reshaped how researchers think about the vitamin. It is not simply a passive antioxidant floating in the bloodstream. It plays active, specific roles in fundamental cellular processes, from building tissue to regulating how DNA is read.
Scurvy Is Not Just a Historical Curiosity
Scurvy killed an estimated two million sailors between the 1500s and 1800s, making it one of the deadliest nutritional diseases in history.14PubMed. The discovery of vitamin C James Lind’s famous 1747 trial aboard HMS Salisbury, in which he tested six different remedies on scurvy-stricken sailors and found that only oranges and lemons worked, is often cited as the first controlled clinical trial in medicine. Yet it took another half century before the British Navy mandated citrus rations, and the actual vitamin responsible was not isolated until 1928, when Albert Szent-Györgyi extracted what he called “hexuronic acid” from adrenal glands.
The classic symptoms of scurvy, including fatigue, purplish skin spots especially on the legs, muscle pain, and gum bleeding, reflect the breakdown of collagen throughout the body.15PubMed. Scurvy: historical review and current diagnostic approach Left untreated, it is fatal, with death coming from infection or sudden cardiac events. While full-blown scurvy is rare today, subclinical deficiency is more common than most people realize, particularly in populations with limited access to fresh produce, people with alcohol use disorders, and older adults in institutional care. The historical devastation of scurvy underscores how precarious our dependence on dietary vitamin C really is.
How Much You Need and How the Body Handles It
Because we cannot synthesize vitamin C, dietary intake is everything. A pharmacokinetics study that carefully measured plasma vitamin C levels at different doses found that the relationship between dose and blood levels follows a curve that plateaus sharply. Between about 30 and 100 milligrams per day, blood levels rise steeply. Beyond 200 milligrams per day, the curve flattens out as the kidneys start excreting the excess. Complete plasma saturation occurs at about 1,000 milligrams per day, meaning that megadoses beyond that point produce no additional increase in blood levels.16PubMed. Vitamin C pharmacokinetics in healthy volunteers: evidence for a recommended dietary allowance
This matters because it sets a hard ceiling on how much vitamin C your body can actually use from oral supplements. The kidney’s reclamation system, described earlier, works to hold onto vitamin C when levels are low, but once plasma is saturated, the kidneys let the surplus go. The body tightly regulates what it keeps. For most adults eating a varied diet with fruits and vegetables, achieving adequate vitamin C levels is straightforward. The challenge has always been what happens when the diet falls short, because unlike most other mammals, we have no internal backup.
What Knockout Mice Have Shown Us
Since guinea pigs were the first animal model for scurvy research, discovered almost by accident in the early 1900s by Norwegian researchers Axel Holst and Theodor Frölich, scientists have wanted a mouse model for vitamin C deficiency. Mice normally make their own vitamin C, so they don’t develop scurvy. But in the last two decades, researchers have created genetically modified “Gulo knockout” mice that have their GULO gene deliberately disabled, mimicking the human condition.
These mice develop scurvy-like symptoms after about four weeks without supplemental vitamin C, a timeline that mirrors the onset of scurvy in humans.17The American Journal of Clinical Nutrition. Dietary ascorbate intake affects steady state tissue concentrations in vitamin C–deficient mice: tissue deficiency after suboptimal intake and superior bioavailability from a food source (kiwifruit) When given supplemental vitamin C in their drinking water, the knockout mice survive and appear outwardly healthy. But behavioral testing has revealed subtler effects. Knockout mice were less physically active than normal mice and swam more slowly in some tests, suggesting a mild motor deficit. Males were more affected than females. Interestingly, the knockout mice showed an exaggerated response to a drug that stimulates the dopamine system, despite being less active at baseline, hinting that vitamin C deficiency may alter brain chemistry in ways that are not immediately obvious.18PubMed Central. Effect of vitamin C deficiency during postnatal development on adult behavior: functional phenotype of Gulo(−/−) knockout mice
Beyond their value for studying deficiency, these knockout mice have opened an unexpected door. Because the GULO gene is broken and non-functional in these animals, the locus where it sits on the chromosome is essentially empty genetic real estate. Researchers have explored using the GULO locus as a “safe harbor” for inserting new genes into mouse and human cell lines, taking advantage of the fact that disrupting a gene that already does nothing should cause no harm.19PubMed Central. Gulo gene locus, a new gene editing locus for mammalian cells It is a peculiar irony: a genetic loss that makes us vulnerable to scurvy may also give gene therapists a convenient parking spot for therapeutic DNA.
High-Dose Vitamin C and Cancer Research
The inability to make our own vitamin C has kept alive a recurring question: are we chronically undersupplied? Linus Pauling famously argued that humans need far more vitamin C than dietary guidelines suggest, a claim that remains controversial. Where the research has gotten genuinely interesting, though, is in the use of high-dose intravenous vitamin C in cancer treatment. Oral supplements hit the absorption ceiling discussed earlier, but intravenous delivery bypasses the gut entirely and can push blood levels far higher than what any pill achieves.
At those pharmacological concentrations, vitamin C behaves differently than it does at nutritional doses. Rather than acting as an antioxidant, it generates hydrogen peroxide in tissues, which can be selectively toxic to cancer cells while sparing healthy ones. A review of the evidence found that high-dose intravenous vitamin C shows promise as a multi-targeting anticancer agent, with roles in promoting cancer-cell-killing oxidative stress, modifying epigenetic markers on cancer cells, boosting immune responses, and acting synergistically with standard chemotherapy while reducing its side effects.20PubMed Central. High-dose intravenous vitamin C, a promising multi-targeting agent in the treatment of cancer The research is still evolving, and high-dose vitamin C is not a standard cancer treatment, but the findings illustrate how our dependence on external vitamin C has shaped an entire branch of medical investigation that would be moot in animals that simply make their own.
The connection back to epigenetics adds another dimension. Because vitamin C is required for the TET enzymes that regulate DNA methylation, some researchers suspect that cancer cells in a vitamin-C-depleted environment may accumulate abnormal methylation patterns that promote tumor growth. If that hypothesis holds up, it would mean that our inability to produce vitamin C does not just make us vulnerable to scurvy but may also shape cancer biology in ways that are only beginning to be understood.
Could the Gene Ever Be Fixed
With modern gene-editing tools, the idea of restoring a functional GULO gene in human cells is not pure fantasy, though it remains far from practical. The gene itself is badly degraded. Only half its exons are even recognizable as sequences, and simply patching one or two mutations would not restore function. You would essentially need to insert a complete working copy of the gene from a species that still has it, along with the regulatory sequences that ensure it is expressed in the right tissue at the right time, predominantly the liver.
Even if that technical challenge were solved, the medical payoff is questionable. Eating an orange is simple, cheap, and effective. Gene therapy carries real risks, including the possibility of the inserted gene landing in the wrong spot and disrupting something important. The condition it would treat, namely dietary vitamin C dependence, is easily managed with food. No serious clinical effort is underway to restore human vitamin C synthesis, and none is likely in the foreseeable future. The broken GULO gene is, for now, a permanent feature of the human genome, one of those evolutionary accidents that stuck because it simply did not matter enough to be fixed by natural selection and is too trivial, clinically, to justify the risks of fixing it ourselves.