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Vitamin C has long been considered the ultimate cold vitamin — but the evidence shows that it doesn’t prevent cold in most people. Even its benefit in terms of the course of a flu-like infection is minimal. Vitamin C is primarily relevant for high-risk groups and those with certain chronic illnesses.
Vitamin C Is Biochemically Essential
L-ascorbic acid, better known as vitamin C, is a water‑soluble vitamin essential to humans. The human body cannot synthesize vitamin C, so regular intake through diet is necessary.
Physiologically, vitamin C primarily acts as a powerful reducing agent and antioxidant. It helps protect cells from oxidative stress — cell damage caused by normal metabolism processes, inflammatory processes, or external stresses such as tobacco smoke.
Vitamin C is also involved in numerous biochemical reactions. This is precisely why the recommended reference values are not based on arbitrary assumptions, but rather on measurable functional parameters — for example, vitamin C-dependent effects in immune cells such as leukocytes and the development of antioxidant defense mechanisms in the body.
Recommendations for Daily Intake
Vitamin C is primarily obtained through fruits and vegetables in our daily diet. The German Nutrition Society (DGE) data show that juices, vegetables, fruits, and potatoes are among the most important sources of vitamin C for children and adolescents. The German National Nutrition Survey II (NVS II, 2005-2006) confirms this pattern for adults: Mean vitamin C intake is roughly 100 mg/d. This highlights that a diet rich in fruits and vegetables is the central foundation for adequate vitamin C status.
Vitamin C is sensitive to light, oxygen, water, and especially heat. Storage and preparation can therefore significantly reduce actual vitamin C content. To minimize losses, nutrient‑preserving methods such as short cooking with little water, steaming, or blanching are recommended.
The DGE recommends age‑ and sex‑specific reference intakes:
- Adult men: 110 mg/d
- Adult women: 95 mg/d
- During pregnancy: 105 mg/d
- During lactation: 125 mg/d
Smokers represent a particularly high-risk group. The DGE recommends significantly higher intakes for them — 135 mg/d for women and 155 mg/d for men. This adjustment reflects clinical observations that smoking is associated with lower vitamin C levels and increased oxidative stress in the body.
Vitamin C Deficiency
The prevalence of vitamin C deficiency depends heavily on how “deficiency” is defined. Some analyses refer to dietary intake below reference values, while others use biochemical parameters such as serum or plasma concentrations. Internationally, cutoff values such as ≤ 11 µmol/L for deficiency and ≤ 23-28 µmol/L for hypovitaminosis are discussed. Because these thresholds vary, prevalence rates from different studies are not directly comparable.
For Germany, evidence comes from several perspectives. According to NVS II, recommended intakes are met or exceeded across age groups. At the same time, individual variation is high, meaning that subgroups may still fall below target values.
Laboratory measurement data are particularly interesting. In a German cross‑sectional study using high-performance liquid chromatography to measure vitamin C, 17.4% of participants had levels below 5 mg/L which was classified as “insufficiency,” and 3.3% were below 1.5 mg/L, which is considered a potential scorbutogenic deficiency.
This means that even though average values in large nutrition studies often have a reassuring effect, a biochemically low vitamin C status is by no means rare in clinically relevant subgroups.
Scurvy: A Historic Deficiency
The importance of vitamin C is particularly evident in cases of severe deficiency: Scurvy has been considered a classic vitamin C deficiency disease for centuries. It often affected sailors who had no access to fresh food for weeks. Citrus fruits later became the primary remedy.
Clinically, scurvy can manifest itself through symptoms including bleeding gums, an increased bleeding tendency, skin changes such as perifollicular hemorrhages or ecchymoses, joint‑pain-like complaints and impaired wound healing. In Western countries, scurvy has become extremely rare.
Risk Situations for Deficiency
Risk for vitamin C deficiency increases when:
- Intake is too low
- Intestinal absorption is impaired (malabsorption)
- Needs rise because of illness
- Increased losses occur
Increased consumption during oxidative stress and inflammatory processes is particularly important — precisely the situations common in many chronic diseases that can slowly deplete the body’s reserves. This applies especially to smokers.
Increased Need Among Smokers
Studies show that smokers have lower vitamin C levels than nonsmokers. The main reason is the significantly increased oxidative stress caused by tobacco smoke: vitamin C, as an antioxidant, is consumed in greater quantities, leading to faster depletion of stores. Accordingly, professional societies recommend higher daily intakes for smokers than for nonsmokers.
Deficiency in Patients With Kidney Disease
Patients with chronic kidney disease, particularly those undergoing dialysis, represent a particularly clinically relevant risk group. Several studies report low plasma vitamin C levels in these patients. In one large cohort, about one third exhibited vitamin C deficiency that was about 33%, whereas another 31% had insufficiency — although cutoff values differ by study.Key causes include dialysis-related losses of water-soluble vitamins and dietary restrictions, which are frequently necessary in this patient group and further complicate access to vitamin-rich foods.
Deficiency After Stomach Surgery and Intestinal Disease
The risk after bariatric surgery is similarly significant. In one cohort, 36% of the individuals examined showed conspicuously low levels. Overall, these data show that standard recommendations often are insufficient: After bariatric procedures, structured supplementation and consistent monitoring are essential to detect and treat deficiencies early.
Chronic inflammatory bowel diseases are also associated with higher risk for vitamin C deficiency. One study found deficiency in about 21.6% of all patients. The correlation with inflammatory activity was striking: the more pronounced the disease, higher the risk for deficiency — presumably as a combination of reduced intake, altered diet, and increased consumption due to chronic inflammation.
Other common situations in which vitamin C deficiency can become clinically relevant include alcohol use disorders, geriatric risk factors, and social factors such as poverty or isolation, which can significantly restrict access to fresh foods. Similarly, highly selective diets with little fresh food can quickly lead to an insufficient vitamin C intake, classically described as a “tea and toast” diet.
Timing Matters: Pre, Not Post
Beyond chronic illnesses, many people take vitamin C to prevent or treat colds. The most robust and clinically cited evidence comes from a Cochrane review on vitamin Cfor preventing and treating the common cold, which included placebo‑controlled studies with doses of at least 200 mg/d. This review provides a reliable, practical guide for medical advice.
The result: In the general population, regular vitamin C supplementation does not lead to a clinically relevant reduction in the incidence of cold. Analyses show no consistent effect on the frequency of colds.Based on their data, the authors conclude that routine supplementation for cold prevention in the general population is not justified.
Recommendation for Extreme Athletes
The situation is different in clearly defined extreme situations. In studies involving short periods of intense physical exertion and/or cold stress — such as marathon runners or soldiers in subarctic exercises — the incidence of cold decreased. In the pooled analysis, the relative risk in the reported studies was approximately 0.48.
Effects of Regular Intake
Regarding the influence on the course and duration of the symptom, regular vitamin C intake shows a small but relatively consistent effect. Evaluations have shown that the duration of cold was reduced by about 8% in adults and by approximately 14% in children. Analysis with children indicated that doses in the range 1 to 2 g per day can shorten the duration of colds even further— roughly 18% — although effects are population and dose-dependent.
No Benefit After Symptom Onset
Timing is also crucial. When vitamin C is taken only after the onset of symptoms, the few therapeutic studies show no consistent benefit in terms of duration or severity of the disease.
This leads to the conclusion that vitamin C is not a comprehensive cold remedy, as often suggested in marketing. Regular intake can slightly reduce the duration and sometimes the severity of cold, but according to the best available evidence, taking it only at the first symptoms is not reliably effective.
Vitamin C Is Not Universally Harmless
In this context, it’s also important to note that many people consider vitamin C “natural” and therefore harmless, which isn’t true. Ascorbic acid is also a pharmacologically active substance with potential adverse effects.
At high doses of vitamin C, gastrointestinal side effects are the most predominant. Typical problems include diarrhea and abdominal discomfort such as cramps or a general feeling of unwellness in the gastrointestinal tract.
Internationally, a so-called “Tolerable Upper Intake Level” for adults is 2000 mg/d. Above this threshold, the risk for adverse effects increases significantly. In certain risk situations — for example, in patients with a predisposition to kidney stones (nephrolithiasis) — an individual benefit-risk assessment is advisable.
In the area of dietary supplements, the German Federal Institute for Risk Assessment takes a significantly more conservative approach. It proposes a maximum daily dose of 200 mg of vitamin C per day for a single dietary supplement dose.
Conclusion: Weigh Benefits Against Harms
Studies show that supplementation primarily benefits high-risk groups. For individuals without a deficiency, the situation is more complex. Vitamin C doses that show measurable effects in studies, such as on the course of colds often exceed 200 mg/d and frequently even in the gram range. These doses significantly exceed the reference values; patients must expect side effects — or, with lower doses, no effects at all.
This story was translated from Medscape’s German edition.
