Vitamin C: Benefits, Deficiency, and Proper Use Guide

What Is Vitamin C?

Hello, I am Dr. Emre Gecer. Vitamin C, chemically known as ascorbic acid (L-ascorbic acid), is a water-soluble essential vitamin. "Essential" means that our body cannot synthesize this vitamin and we must obtain it externally — through food or supplements. Humans, monkeys, guinea pigs, and some bat species are among the rare creatures that cannot synthesize vitamin C; this is due to a mutation in the gene encoding the enzyme L-gulonolactone oxidase.

Vitamin C acts as a cofactor in countless biochemical reactions in the body. Collagen synthesis, immune function, iron absorption, antioxidant defense, and neurotransmitter synthesis are among its principal roles. The discovery in the 18th century that scurvy, a disease often seen in sailors throughout history, could be prevented by consuming citrus fruits is one of the most important nutritional discoveries in the history of medicine.

Roles of Vitamin C in the Body

1. Collagen Synthesis

The most fundamental biochemical function of vitamin C is its role in collagen synthesis. Collagen is the most abundant protein in the body and provides structural integrity to skin, bone, cartilage, tendons, blood vessels, and gums. Vitamin C is the cofactor for the enzymes prolyl hydroxylase and lysyl hydroxylase, which act in collagen synthesis. These enzymes hydroxylate proline and lysine amino acids, giving collagen structural stability. In vitamin C deficiency, collagen synthesis is impaired and scurvy develops.

2. Antioxidant Function

Vitamin C is the body's most potent water-soluble antioxidant. It neutralizes free radicals (reactive oxygen species) and protects cells against oxidative stress. It also helps regenerate vitamin E (a fat-soluble antioxidant), maintaining the integrity of the antioxidant network. By preventing damage to DNA, proteins, and lipids, it has a protective effect on cellular aging and on the development of chronic disease.

3. Immune System Support

Vitamin C has multifaceted effects on the immune system:

• Enhances chemotaxis, phagocytosis, and the oxidative burst of neutrophils
• Supports lymphocyte proliferation and function
• Strengthens natural killer (NK) cell activity
• Stimulates interferon production
• Maintains epithelial barrier function

During an infection, vitamin C concentration in immune cells drops rapidly, indicating that vitamin C requirements increase during periods of infection.

4. Increasing Iron Absorption

Vitamin C significantly enhances the absorption of plant-source non-heme iron. It reduces ferric iron (Fe3+) to ferrous iron (Fe2+); this form is absorbed much more easily from the intestine. Taking 100 mg of vitamin C with a meal can increase iron absorption up to 4-fold. Therefore, in iron-deficiency anemia, iron supplements are recommended to be taken together with vitamin C.

5. Other Roles

• Neurotransmitter synthesis: Acts as a cofactor in the synthesis of norepinephrine and serotonin
• Carnitine synthesis: Participates in the synthesis of carnitine, which is required for the transport of fatty acids into mitochondria for energy production
• Tyrosine metabolism
• Amidation of peptide hormones

Daily Vitamin C Requirement (RDA)

The daily recommended amount of vitamin C varies with age, sex, and physiologic status:

• Adult men: 90 mg/day
• Adult women: 75 mg/day
• Pregnant women: 85 mg/day
• Breastfeeding women: 120 mg/day
• Smokers: +35 mg in addition to daily requirement (smoking increases oxidative stress and accelerates vitamin C consumption)
• Children (4-8 years): 25 mg/day
• Adolescents (14-18 years): Boys 75 mg, girls 65 mg/day

Tolerable Upper Intake Level (UL): 2000 mg/day for adults. Above this dose the risk of side effects increases.

Foods Rich in Vitamin C

A balanced diet can easily meet the daily vitamin C requirement. The richest sources:

• Red bell pepper (sweet): 1 piece (~150 g) — approximately 190 mg
• Kiwi: 1 piece (~75 g) — approximately 70 mg
• Broccoli (cooked): 1 serving (~150 g) — approximately 80 mg
• Strawberry: 1 serving (~150 g) — approximately 90 mg
• Orange: 1 piece (~130 g) — approximately 70 mg
• Grapefruit: Half a piece — approximately 45 mg
• Tomato: 1 piece (~150 g) — approximately 20 mg
• Brussels sprouts (cooked): 1 serving — approximately 75 mg
• Cantaloupe: 1 serving — approximately 60 mg
• Parsley: 1 bunch (~50 g) — approximately 65 mg

Important note: Vitamin C is sensitive to heat, light, and air. Losses occur during cooking, storage, and processing. Cooking vegetables briefly in little water or steaming them minimizes vitamin C loss. Fresh consumption provides the highest vitamin C intake.

Vitamin C Deficiency: Scurvy

Severe vitamin C deficiency leads to scurvy. Clinical findings appear when plasma vitamin C levels fall below 11 micromol/L. Historically it was common in sailors on long sea voyages who could not obtain fresh fruits and vegetables. Today it is rare in developed countries but is still seen in risk groups.

Symptoms of Scurvy

• Bleeding and swelling of the gums: One of the earliest and most prominent findings. The gums swell, take on a reddish-purple color, and bleed easily. In advanced stages the teeth loosen and fall out.
• Petechiae and ecchymoses: Pinhead-sized (petechiae) or larger (ecchymotic) hemorrhagic spots on the skin. The vessel wall weakens because collagen synthesis is impaired.
• Poor wound healing: Because of impaired collagen synthesis, wounds heal very slowly or healed wounds can reopen.
• Corkscrew hairs: Keratosis around hair follicles and curling of the hairs into corkscrew shapes. A specific finding of scurvy.
• Perifollicular hemorrhage: Petechial hemorrhages around hair follicles
• Joint and muscle pain: Especially in the lower extremities
• Fatigue and weakness
• Depression and irritability

Risk Groups

• Individuals who do not eat fruits and vegetables
• Chronic alcoholics
• Smokers (increased vitamin C consumption)
• Older adults (inadequate nutrition)
• Patients with malabsorption (Crohn's, celiac)
• Hemodialysis patients
• Individuals with eating disorders

Risks of Excessive Vitamin C Intake

Because vitamin C is water-soluble, the excess is excreted by the kidneys and the risk of toxicity is low. However, at very high doses (>2000 mg/day) some side effects may occur:

• Kidney stones: High doses of vitamin C can increase oxalate production and raise the risk of calcium oxalate kidney stones. Particular caution is needed in individuals with a history of kidney stones.
• Gastrointestinal complaints: Nausea, diarrhea, abdominal cramps. Osmotic diarrhea is the most common side effect.
• Incorrect laboratory results: High doses of vitamin C can give false-negative fecal occult blood tests (guaiac method). They can also affect glucometer readings.
• Risk of iron overload: In individuals with hemochromatosis (iron storage disease), it can trigger excessive iron absorption

Vitamin C Supplements

Supplement Forms

• Ascorbic acid: The most common and cheapest form. May cause discomfort in people with sensitive stomachs.
• Sodium ascorbate: A buffered form. Suitable for those with stomach sensitivity because it is less acidic. Caution is needed in patients on sodium restriction because of its sodium content.
• Calcium ascorbate (Ester-C): Another buffered form.
• Liposomal vitamin C: Claimed to have higher bioavailability thanks to its lipid coating. Some studies have shown higher plasma levels, but clinical superiority has not been conclusively proven. It is more expensive.

Bioavailability note: Up to doses of 200 mg, vitamin C absorption is approximately 70-90%. As the dose increases, the absorption rate decreases; at 1000 mg and above it falls below 50%. Therefore divided doses (2-3 times during the day) are more effective than a single high dose.

Common Claims and the Scientific Evidence

Vitamin C and the Common Cold

The claim popularized by Linus Pauling in the 1970s — that "megadose vitamin C prevents the common cold" — is one of the most debated topics. Current meta-analyses show:

• Regular vitamin C supplementation (200 mg/day and above) can shorten the duration of the common cold by 8% in adults and 14% in children
• No significant effect has been shown on prevention of the common cold in the general population
• In people doing intense physical activity (marathon runners, soldiers), regular intake can reduce the incidence of the common cold by 50%
• Vitamin C taken after symptoms have begun does not provide a meaningful benefit

Vitamin C and Cancer Prevention

In epidemiologic studies, a diet rich in fruits and vegetables (high vitamin C intake) has been associated with a lower risk of some cancer types. However, there is no strong evidence that vitamin C supplements are effective in preventing or treating cancer. High-dose intravenous vitamin C is being investigated as complementary therapy in some oncology centers, but evidence is not yet sufficient for routine use.

Recommendations for Proper Use

• Try to obtain vitamin C primarily from foods. Consuming 5 servings of fruit and vegetables per day is generally sufficient.
• If supplementation is needed, 250-500 mg/day is usually enough; avoid megadoses.
• Take in divided doses (morning and evening).
• You can increase iron absorption by taking it together with iron supplements.
• If you have a history of kidney stones, strictly avoid high doses.
• If you smoke, remember that your daily requirement is higher.

Conclusion

Vitamin C is an indispensable nutrient for the healthy functioning of our body. Immune support, collagen synthesis, antioxidant defense, and iron absorption are among its main roles. Daily requirements can easily be met with a balanced diet. When supplementation is needed it is important to use reasonable doses and to weigh megadose claims against the scientific evidence. For those at risk of deficiency, regular medical check-ups and using supplements when needed under a doctor's guidance is the correct approach.

Wishing you healthy days.
Dr. Emre Gecer

References

• Harrison's Principles of Internal Medicine, 22nd Edition — Chapter: Vitamin and Trace Mineral Deficiency and Excess
• Dietary Reference Intakes for Vitamin C — Institute of Medicine (IOM), National Academies Press
• Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev, 2013
• Turkish Nutrition Guide (TÜBER), 2022