Why is vitamin C not produced in our body?

January 10, 2026 •

4 min read

Approximately 40 million years ago, a pivotal genetic mutation occurred in our primate ancestors, inactivating a key gene and setting the stage for one of the most significant metabolic differences between humans and most other mammals: our inability to produce vitamin C. This means that for our body to have this vital nutrient, we must consume it through food or supplements.

Quick Summary

A non-functional GULO gene, a consequence of an ancient genetic mutation, means humans cannot synthesize vitamin C internally. This evolutionary loss necessitates dietary sources of ascorbic acid to prevent deficiency diseases like scurvy. We must consume it daily.

Key Points

Genetic Cause: A specific evolutionary mutation in the GULO (L-gulonolactone oxidase) gene caused humans and certain other primates to lose the ability to produce vitamin C.
Evolutionary Neutrality: This mutation wasn't selected against because our fruit-rich ancestral diet provided a consistent external source of vitamin C, making internal synthesis unnecessary.
Critical Functions: Vitamin C remains vital for health, playing a key role in collagen synthesis, immune function, and acting as a powerful antioxidant.
Dietary Necessity: Because our body does not produce or store vitamin C effectively, we must consume it daily through foods like fruits and vegetables or supplements to prevent deficiency.
Scurvy Risk: The most severe consequence of vitamin C deficiency is scurvy, characterized by fatigue, bleeding gums, and poor wound healing, though it is rare in developed countries today.
Efficient Recycling: Humans have evolved a way to maximize the use of dietary vitamin C by efficiently recycling its oxidized form (DHA) back into active ascorbic acid within cells.

The Genetic Explanation: A Tale of a Lost Gene

At the core of the mystery of why is vitamin C not produced in our body lies a non-functional gene called GULO. This gene, short for L-gulonolactone oxidase, provides the instructions for producing an enzyme of the same name. In most mammals, this enzyme is the final catalyst in the four-step process that converts glucose into vitamin C (ascorbic acid).

Around 63 million years ago, in a common ancestor of modern-day humans, apes, and monkeys, a series of genetic mutations, including insertions and deletions, occurred within the GULO gene. These mutations rendered the gene useless, effectively shutting down the final step of vitamin C synthesis. Today, what remains in the human genome is a non-functional remnant, a pseudogene, of the once-active GULO gene.

The Evolutionary Advantage of Losing GULO

The loss of a crucial metabolic function might seem like a disadvantage, but evolutionary theory suggests the mutation persisted because it was either harmless or even beneficial to our ancestors. This is primarily attributed to a diet rich in fruits and vegetables, which provided a consistent and abundant external source of vitamin C.

No Selective Pressure: With plenty of dietary vitamin C available, there was no evolutionary pressure to maintain the GULO gene. Individuals with the mutation suffered no negative health consequences compared to those without it, so the non-functional gene continued to be passed down through generations.
Energy Conservation: Producing vitamin C is a metabolically demanding process. Losing the ability to synthesize it freed up energy that could be used for other, more critical biological processes.
Potential Side-Effects: The enzymatic pathway for creating vitamin C produces hydrogen peroxide as a byproduct. Some scientists theorize that shutting down this pathway helped reduce oxidative stress in early primates, contributing to their survival in certain environments.

The Consequences of Dietary Dependence

The most well-known consequence of our inability to produce vitamin C is scurvy, a deficiency disease that can be fatal if untreated. Symptoms of scurvy appear after several months of critically low vitamin C intake and include fatigue, bleeding gums, easy bruising, and poor wound healing.

The Critical Role of Vitamin C

Despite our inability to produce it, vitamin C remains indispensable for numerous bodily functions. Its roles are foundational to human health and include:

Collagen Synthesis: Vitamin C is a crucial cofactor for enzymes that produce collagen, a structural protein vital for healthy skin, blood vessels, bones, cartilage, ligaments, and tendons. Without it, these tissues weaken and break down, causing the symptoms of scurvy.
Antioxidant Protection: As a powerful antioxidant, vitamin C helps protect cells from damage caused by free radicals generated during metabolism and exposure to toxins. This protective function is linked to a reduced risk of certain chronic diseases over time.
Immune System Support: Vitamin C contributes significantly to immune function, helping to keep the body healthy and able to fight infection.
Iron Absorption: It enhances the absorption of non-heme iron (the form found in plant-based foods), which is particularly important for individuals on vegetarian or vegan diets.

How Our Body Manages Acquired Vitamin C

Even with the GULO gene turned off, the human body has developed mechanisms to manage the vitamin C it acquires through diet.

Recycling and Transport

When vitamin C is oxidized after acting as an antioxidant, it becomes dehydroascorbic acid (DHA).
Our red blood cells and other cells have a unique ability to transport DHA efficiently using glucose transporter proteins (GLUTs), particularly Glut-1.
Once inside the cell, the DHA is recycled back into active vitamin C, effectively reducing our overall daily requirement and helping maintain intracellular levels.

Dietary Sources and Daily Needs

Since we cannot produce vitamin C, obtaining it from food is a daily necessity. Fortunately, it is widely available in many fruits and vegetables. Here is a list of excellent sources:

Citrus Fruits: Oranges, grapefruits, lemons, and limes.
Berries: Strawberries, blueberries, and blackcurrants.
Peppers: Red and green peppers, especially red.
Cruciferous Vegetables: Broccoli, Brussels sprouts, cabbage, and cauliflower.
Other Sources: Kiwi, pineapple, cantaloupe, and tomatoes.

GULO-Functional vs. GULO-Deficient Species


Feature	GULO-Functional Species (e.g., Rats)	GULO-Deficient Species (e.g., Humans, Guinea Pigs)
Vitamin C Synthesis	Can produce their own vitamin C internally from glucose.	Cannot produce their own vitamin C; dependent on diet.
Genetic Status	Possess a functional GULO gene.	Possess a non-functional GULO pseudogene due to mutation.
Dietary Requirement	Vitamin C is not an essential dietary nutrient.	Vitamin C is an essential dietary nutrient.
Storage Capacity	Generally produce enough to meet their needs and have some reserves.	Limited storage capacity; excess is excreted via urine.
Scurvy Risk	Not susceptible to scurvy under normal conditions.	Susceptible to scurvy with insufficient dietary intake.

Conclusion: A Delicate Balance with Our Diet

The evolutionary path that left us unable to produce our own vitamin C highlights a fascinating aspect of adaptation and natural selection. What was once a redundant metabolic pathway for our ancestors became a lost ability that forever tied our health to our diet. The inactivation of the GULO gene reminds us of our dependence on the nutrients found in our food. As long as we continue to consume a diverse range of fruits and vegetables, we can easily and effectively manage this ancient genetic quirk and maintain our health. For those with limited access or dietary restrictions, supplements offer a reliable way to ensure adequate intake.

For more information on the functions and health effects of vitamin C, visit the NIH Office of Dietary Supplements' fact sheet on the topic.

Frequently Asked Questions

The GULO gene produces the L-gulonolactone oxidase enzyme, which catalyzes the final step of vitamin C synthesis in most animals. Humans have a mutated, non-functional GULO pseudogene, meaning we cannot produce this enzyme or vitamin C internally.

No, our ancient primate ancestors once had a functional GULO gene and could produce their own vitamin C. An ancient mutation, occurring millions of years ago, deactivated this gene.

If a human doesn't consume enough vitamin C for an extended period, they can develop scurvy. Early symptoms include fatigue, joint pain, and irritability, progressing to bleeding gums and poor wound healing in severe cases.

In addition to humans, other primates (monkeys and apes), guinea pigs, some bats, and certain species of fish also have a non-functional GULO gene and cannot synthesize their own vitamin C.

While we cannot produce new vitamin C, the human body has developed a mechanism to recycle it more efficiently. Specifically, our red blood cells use glucose transporters (like Glut-1) to bring oxidized vitamin C (DHA) into the cell, where it is converted back into usable ascorbic acid.

While supplements can provide the necessary vitamin C, the body's absorption is limited. Any excess is simply excreted in the urine, so extremely high doses are generally unnecessary for healthy individuals.

Humans must obtain vitamin C from external dietary sources. Excellent options include citrus fruits, berries, peppers, broccoli, Brussels sprouts, kiwi, and many other fruits and vegetables.