Vitamin C supplementation does not prevent viral respiratory infections, such as those caused by coronaviruses, in the general population
Maintaining an adequate intake of vitamin C is important for staying healthy. However, evidence from clinical trials shows that regular vitamin C supplementation does not prevent viral respiratory tract infections such as the common cold, which can be caused by coronaviruses, in the general population. Although some studies have shown a reduced incidence of the common cold in individuals undergoing severe physical stress, such as marathon runners.
Inadequate support: The bulk of scientific evidence indicates that regular vitamin C supplementation does not reduce the incidence of viral respiratory tract infections such as the common cold, which can be caused by coronaviruses, in the general population.
Lack of context: Vitamin C does appear to reduce the incidence of the common cold, but only in a small, specific population undergoing severe physical stress such as marathon runners. This effect is therefore not generalizable to most people.
The article containing this claim was published in late January 2020 during the novel coronavirus (COVID-19) outbreak and went viral within days, receiving more than 27,000 interactions on Facebook. It states that “Vitamin C protects against coronavirus” and that “the coronavirus pandemic can be dramatically slowed, or stopped, with the immediate widespread use of high doses of vitamin C.”
Vitamin C, also called ascorbic acid, is needed for many biological processes including the repair of damaged tissue and proper immune system function. Its importance was demonstrated in the 18th century when many sailors who spent a long time at sea developed a disease called scurvy—a vitamin C deficiency that causes symptoms of fatigue, gum disease, and poor wound healing. Doctors, notably James Lind, observed that consuming citrus fruits could quickly reverse this condition. Later findings showed that this was due to the high level of vitamin C in these fruits.
In the early 1970s, Nobel laureate and chemist Linus Pauling first popularized the idea of a possible link between vitamin C and protection against the common cold—a viral respiratory tract infection which can be caused by more than 200 viruses, among them coronaviruses—despite little scientific evidence supporting the claim.
Margreet Vissers, a research professor at the University of Otago who has studied vitamin C’s health benefits extensively, explained to Health Feedback:
[See below for Vissers’ full comment.]
“The story of vitamin C and infections, particularly viral infections, is a very hotly contested area, and unfortunately for many years lots of claims have been made both by proponents and skeptical medical doctors that are not founded in fact. Fortunately in the past few years much better scientific data is available and we should concentrate on this and review it carefully.”
Indeed, scientists in recent times have conducted numerous studies to establish whether vitamin C supplementation helps to prevent a cold. A 2013 Cochrane review which analyzed 29 clinical trials reported that: “In the general community trials involving 10,708 participants, the pooled risk ratio (RR) was 0.97 (95% confidence interval (CI) 0.94 to 1.00).” Since the relative risk in the group treated with regular vitamin C supplementation is close to 1, regular vitamin C supplementation does not actually reduce the incidence of the common cold in the general population. [Learn more about the concept of risk ratio.]
The general consensus among scientific institutes is in agreement with these findings. The U.S. National Institutes of Health states: “Overall, the evidence to date suggests that regular intakes of vitamin C at doses of at least 200 mg/day do not reduce the incidence of the common cold in the general population.” The Linus Pauling Institute of Oregon State University likewise states that “routine supplementation with vitamin C (0.25 to 2 grams/day) does not reduce the occurrence of the common cold in the general population.”
However, there appear to be exceptions to who benefits from vitamin C supplementation, such as individuals who undergo high-intensity physical training like marathon runners. The 2013 Cochrane review reported that “five trials involving a total of 598 marathon runners, skiers, and soldiers on subarctic exercises yielded a pooled RR of 0.48 (95% CI 0.35 to 0.64)”, indicating that the incidence of a common cold was halved with vitamin C supplementation.
Another study demonstrated that high-dose vitamin C supplementation of approximately 6 g/day improved the survival rate among severely ill patients in intensive care units, although this has not been confirmed in randomized, controlled clinical trials. For reference, the daily recommended vitamin C intake for healthy individuals is 75 mg for women and 90 mg for men.
This is because vitamin C undergoes a quicker turnover when we are ill, particularly during respiratory illnesses and sepsis, explained Vissers. “The sicker you are, the more vitamin C is required to maintain your body’s supply.”
However, she also cautioned that “prevention of [coronavirus] infection is quite a different matter than treatment. We really have no idea whether having more vitamin C will prevent anyone contracting the virus.”
In summary, maintaining an adequate level of vitamin C intake is indeed important for health, but there is no evidence showing that regular vitamin C supplementation in otherwise healthy people prevents COVID-19 infection or the common cold in general. Excessive intake (more than 1 g/day) can even be harmful, resulting in diarrhea and stomach pain.
Professor, Centre for Free Radical Research, University of Otago
The story of vitamin C and infections, particularly viral infections, is a very hotly contested area, and unfortunately for many years lots of claims have been made both by proponents and skeptical medical doctors that are not founded in fact. Fortunately in the past few years much better scientific data is available and we should concentrate on this and review it carefully. The papers cited in the article are mostly quite old and are criticised for this reason (not always fair). Here’s what I think is pertinent to the coronavirus discussion:
- It is now well established that vitamin C is rapidly turned over in our bodies when we are sick, and that the normal daily intake of around 200 mg per day is nowhere near enough to keep up with demand. This is particularly the case for respiratory illness and sepsis. Also, the sicker you are, the more vitamin C is required to maintain your body’s supply. This amount was shown to be around 6 g per day for the severely ill.
- Some recent studies have indicated a remarkable survival advantage when critically ill patients are given vitamin C. These studies have generated a lot of interest and there are a number of clinical trials underway at the moment.
- We now know that vitamin C is necessary for dozens of critical body functions that require the body stores to be adequately maintained, and that we should maintain levels above 50 μM in plasma to achieve this.
Interestingly, a clinical trial for administration of vitamin C to patients with coronavirus has been registered in China. This will take the hype out of the discussions, hopefully.
However, prevention is quite a different matter than treatment. We really have no idea whether having more vitamin C will prevent anyone contracting the coronavirus. However, as it is a viral pneumonia, there is a distinct possibility that additional vitamin C may help with treatment. There are no studies with this virus that show this, but some benefit has been suggested with other severe respiratory illnesses. The clinical trial in China should address this issue.
Associate Professor, Department of Pathology and Biomedical Science, University of Otago
The claim that “the coronavirus pandemic can be dramatically slowed, or stopped, with the immediate widespread use of high doses of vitamin C” is not accurate.
Vitamin C has well-known antioxidant and immune-enhancing functions, and its levels in the body decrease significantly during severe respiratory infections such as pneumonia, which is a major complication of COVID-19. Low vitamin C status may also increase susceptibility to severe respiratory infections, as indicated by pneumonia being a common complication of the vitamin C deficiency disease scurvy. These observations indicate a strong link between vitamin C and immune defense against respiratory infections.
Meta-analysis of several studies investigating prophylactic vitamin C (at doses of 50 – 2,000 mg/day) for the prevention of pneumonia has indicated that the vitamin may decrease the incidence of pneumonia. These studies were carried out in soldiers, marine recruits and institutionalized schoolboys. However, there is as yet no direct evidence that high oral doses (i.e. 3,000 mg/day) can specifically slow or stop infection by the latest coronavirus outbreak in the general population, particularly if already well-nourished. Specific subgroups, such as people with low vitamin C status, may however benefit from supplementation.
Health Feedback has produced a number of other claim reviews on COVID-19. You can view them here.
Prof. Carr’s comment was received and included after the initial publication. This comment further supports the original verdict and did not modify it.
- 1 – Pauling (1971) The Significance of the Evidence about Ascorbic Acid and the Common Cold. PNAS.
- 2 – Hemilä and Chalker. (2013) Vitamin C for preventing and treating the common cold. Cochrane Database of Systematic Reviews.
- 3 – Carr et al. (2017) Hypovitaminosis C and vitamin C deficiency in critically ill patients despite recommended enteral and parenteral intakes. Critical Care.
- 4 – Marik et al. (2017) Hydrocortisone, Vitamin C, and Thiamine for the Treatment of Severe Sepsis and Septic Shock: A Retrospective Before-After Study. Chest.
- 5 – Carr and Maggini. (2017) Vitamin C and immune function. Nutrients.
- 6 – Carr AC. Vitamin C in pneumonia and sepsis. In: Chen Q, Vissers M, editors. Vitamin C: New Biochemical and Functional Insights. Oxidative Stress and Disease. Boca Raton, FL CRC Press/Taylor & Francis; 2020. p. 115-35.
- 7 – Hemilä H. (2017) Vitamin C and infections. Nutrients.
- 8 – Hemilä and Louhiala. (2013) Vitamin C for preventing and treating pneumonia. Cochrane Database of Systematic Reviews.