What is the inability to synthesize vitamin C?

December 16, 2025 •

4 min read

Approximately 7.1% of people in the United States have a vitamin C deficiency, a condition that stems from our body's inability to produce this vital nutrient. This genetic trait, known as hypoascorbemia, results from a mutation that renders a key enzyme non-functional, making dietary intake essential for survival.

Quick Summary

A genetic mutation prevents humans and some other animals from producing vitamin C, which is required for bodily functions. This dietary dependence means a lack of fresh produce can lead to severe deficiency, causing the disease scurvy. The non-functional gene for the enzyme L-gulonolactone oxidase is the root cause.

Key Points

Genetic Cause: A mutated GULO gene prevents humans and certain other animals from producing their own vitamin C, making them dependent on dietary sources.
Scurvy Risk: The primary disease resulting from this genetic trait is scurvy, caused by a severe deficiency of vitamin C due to insufficient dietary intake.
Collagen Disruption: Scurvy's symptoms, including bleeding gums, poor wound healing, and joint pain, are a direct result of impaired collagen synthesis, a process that requires vitamin C.
Dietary Solution: Preventing vitamin C deficiency involves consuming fresh fruits and vegetables, as well as considering supplements for those with inadequate dietary intake.
Evolutionary Ancestry: This inability to synthesize vitamin C is an evolutionary trait inherited from our ancestors, occurring millions of years ago when vitamin C-rich foods were abundant.

A Genetic Trait: The Non-Functional GULO Gene

The inability to synthesize vitamin C, or ascorbic acid, is a genetic trait shared by humans and a few other animals, not a disease. A mutation in the L-gulonolactone oxidase (GULO) gene is the underlying cause, as this gene codes for the enzyme needed for the final step of vitamin C synthesis. Without a functional GULO enzyme, the body cannot convert glucose into ascorbic acid, requiring reliance on dietary sources.

This genetic change is believed to have occurred in anthropoid primate ancestors around 63 million years ago. In an environment where vitamin C-rich foods were abundant, the ability to synthesize it became less critical for survival. The non-functional GULO gene eventually became a 'pseudogene' in human DNA. This trait is also found in guinea pigs, certain fish, and some bat species, resulting from independent mutations in their evolutionary paths.

The Role of Vitamin C in the Body

Vitamin C is essential for various bodily functions, acting as a cofactor in numerous enzymatic reactions necessary for growth and repair. Key roles include:

Collagen synthesis: It is vital for producing collagen, supporting the structure of skin, bones, cartilage, and blood vessels.
Antioxidant protection: Vitamin C is a potent antioxidant, safeguarding cells from damage by free radicals.
Iron absorption: It enhances the absorption of nonheme iron from plant-based foods.
Neurotransmitter synthesis: The vitamin is involved in the creation of certain neurotransmitters like norepinephrine.
Immune function: It supports the immune system by aiding various cellular functions.

Scurvy: The Consequence of Severe Deficiency

Insufficient vitamin C intake in individuals with hypoascorbemia can lead to scurvy, especially after weeks or months of deficiency. Initial symptoms may include fatigue, irritability, and muscle aches. As vitamin C stores drop significantly (below 350 mg), the characteristic signs of scurvy emerge.

Symptoms of severe scurvy include:

Oral health: Swollen, bleeding gums and potential tooth loosening.
Skin: Dry, rough, scaly skin, hemorrhages around hair follicles, and coiled hair.
Wound healing: Poor healing of new wounds and reopening of old ones due to impaired collagen.
Pain: Joint and muscle pain from bleeding into tissues.
Other effects: Anemia, and potentially jaundice, neuropathy, seizures, and death if untreated.

Dietary Solutions for Non-Synthesizers

Preventing vitamin C deficiency relies entirely on consistent dietary intake. Many common foods are excellent sources. The recommended daily allowance (RDA) for adults is typically 90 mg for men and 75 mg for women, with increased needs for smokers and lactating women.

Comparison of Vitamin C Sources


Food Source	Serving Size	Approximate Vitamin C (mg)	Notes
Red Bell Pepper	½ cup, raw	95	A powerful source, often overlooked.
Orange Juice	¾ cup	93	A classic source, but choose cartons over clear bottles.
Orange	1 medium	70	Provides a solid portion of the daily requirement.
Kiwifruit	1 medium	64	A small but potent source.
Strawberries	½ cup, sliced	49	Can be added easily to many dishes.
Broccoli	½ cup, cooked	51	Cooking can reduce content, but still valuable.
Baked Potato	1 medium	17	Provides vitamin C alongside other nutrients.

To maximize intake, consume fruits and vegetables raw when possible, as heat and storage can degrade vitamin C. Supplements containing ascorbic acid are a viable option for those who cannot get enough from their diet.

Conclusion

The inability to synthesize vitamin C is an evolutionary trait in humans, stemming from our ancestry. This genetic characteristic makes us dependent on dietary vitamin C to prevent scurvy, a disease caused by severe deficiency. While scurvy is now rare in many parts of the world due to improved nutrition, understanding the genetic basis for our dependence highlights the ongoing importance of a vitamin C-rich diet for overall health.

The Genetic and Nutritional Implications of Hypoascorbemia

The GULO gene is mutated: Humans possess an inactive GULO gene, which means they cannot produce the enzyme needed for vitamin C synthesis.

Vitamin C is an essential nutrient: Due to the inability to synthesize it, vitamin C must be acquired through diet.

Scurvy is the result of deficiency: Insufficient dietary vitamin C leads to scurvy, impacting collagen production and immune function.

Dietary intervention is critical: Eating fresh produce is the main way to prevent deficiency and treat scurvy.

Cooking reduces vitamin C content: Heat degrades vitamin C, so raw fruits and vegetables retain more of the nutrient.

Supplementation is an effective backup: Vitamin C supplements are a reliable way to ensure adequate intake, especially with dietary restrictions.

Other animals share this trait: Species like guinea pigs, certain bats, and some fish also lack a functional GULO gene.

Evolutionary changes favored recycling: The loss of the GULO gene may have been evolutionarily neutral, potentially linked to more efficient mechanisms for recycling dietary vitamin C.

Frequently Asked Questions

The inability to synthesize vitamin C is caused by mutations in the GULO (L-gulonolactone oxidase) gene. This gene is responsible for producing an enzyme that performs the final step in the vitamin C synthesis pathway.

Humans and certain other animals, including guinea pigs and some primates, do not produce vitamin C because of an ancient genetic mutation that inactivated the GULO gene. Over time, this non-functional gene was passed down through generations.

In addition to humans, other species that cannot synthesize vitamin C include anthropoid primates, guinea pigs, fruit-eating bats, and some fish and bird species. Most other mammals, such as cats and dogs, can produce their own.

The medical term for the inability to synthesize vitamin C is sometimes referred to as hypoascorbemia. The resulting severe deficiency disease is called scurvy.

Humans must obtain vitamin C entirely through their diet. Consuming a variety of fresh fruits and vegetables is the primary source, while supplements can also be used to meet daily requirements.

In environments with consistent access to vitamin C-rich foods, this genetic trait is not a disadvantage. However, a lack of dietary intake, historically common during long sea voyages, can become lethal without intervention, leading to scurvy.

Yes, vitamin C is a delicate nutrient that can be degraded by heat, oxygen, and light. Cooking fruits and vegetables can reduce their vitamin C content, so consuming some raw produce is beneficial.