Achoo! Time for Vitamin C?
Colds are common, with adults typically experiencing 2-4 colds per year. Although its perceived as a single illness, the common cold is caused by a wide range of viruses , most commonly rhinovirus which itself has over 100 subtypes, making a cure difficult.
Yet many people will reach for vitamin C supplements each year to stave off colds, to lessen duration or to alleviate symptom severity. Or take them when they have a cold to make their symptoms better.
The question is does it really work?
There is a popular acceptance that vitamin C has an impact on the common cold. Indeed, vitamin C has been proposed for treating respiratory infections since it was isolated in the 1930s. It became particularly popular in the 1970s when the respected scientist and Nobel laureate Linus Pauling concluded from earlier placebo-controlled trials that there was ‘strong’ evidence that vitamin C would prevent and alleviate the common cold. His book claimed that when given in doses of at least 1g daily, vitamin C significantly decreased the incidence and the severity of the common cold.
However, the scientific community generally does not accept Pauling’s analysis. Most human studies indicate that vitamin C supplementation with doses above the dietary reference values fail to reduce the incidence of colds in the normal population.
Recently a systematic Cochrane review restricted to placebo-controlled trials testing 0.2 g/day or more of vitamin C found that regular ingestion of vitamin C had no effect on common cold incidence in the ordinary population, based on 29 trial comparisons involving 11,306 participants.
The failure of vitamin C supplementation to reduce the incidence of colds in the general population indicates that routine vitamin C supplementation is not justified.
Can vitamin C reduce the incidence of colds in special circumstances?
Interestingly evidence suggests that persons exposed to brief periods of extreme physical exercise and/or cold environments may benefit from regular vitamin C intake above 200mg/day (0.2g) in terms of reducing the duration and severity of the common cold. In ﬁve trials with 598 participants (mainly marathon runners, skiers and military troops training in sub-arctic conditions), exposed to short periods of extreme physical stress, vitamin C halved the common cold risk. Vitamin C therefore may be useful in special circumstances for people exposed to brief periods of severe physical exercise or exposed to significant cold stress, but caution should be made in generalising this finding as these conditions do not apply to the general population.
Can vitamin C reduce the duration or severity of cold symptoms?
The Cochrane review also reported that based on 31 study comparisons with 9745 common cold episodes, regular supplementation had a modest but consistent effect in reducing the duration of common cold symptoms. Regular supplementation trials found that 0.2 g/day or more reduced common cold duration by 8% in adults and by 14% in children and 1 to 2 g/d of vitamin C in children reduced common cold duration by 18%. The practical relevance is not clear but this level of benefit does not seem to justify long term supplementation in its own right.
Are therapeutic doses of vitamin C after the onset of symptoms effective?
Since regular supplementation trials show a small effect on duration and severity but not incidence, it may seem rational to administer vitamin C therapeutically starting immediately after the first symptom. However therapeutic trails to test this theory have mostly been negative and pooled estimates do not show a difference between vitamin C and placebo, although there may be scope for more research to explore this further.
High doses of vitamin C administered therapeutically, starting after the onset of symptoms, thus show no consistent effect on the duration or severity of common cold symptoms. However, the authors cite a small number of trials showing benefit at high doses (4-8g) taken on the day of onset of symptoms. They report this suggestive finding as interesting but based on the majority of currently available research there is not a clear and significant benefit of therapeutic dosing, although more research may be warranted.
In summary, is there any benefit to taking daily vitamin C supplement or in taking a large dose of vitamin C when you feel a cold coming on? If you are planning to run a marathon or a winter adventure in the artic than perhaps, but for the rest of us though the science does not seem to merit the added expense.
Are there any adverse side –effects?
The published vitamin C common cold trials have not reported adverse effects of high doses of vitamin C, but there have been few controlled studies that specifically investigated adverse effects.
European reports on safe upper intake levels for vitamin C reported that high intakes of vitamin C supplements as currently consumed are unlikely to pose a risk of adverse health effects.
Overall, acute gastrointestinal intolerance (e.g., abdominal distension, flatulence, diarrhoea,) is the most clearly defined negative effect at high intakes, but there are limited data on the dose-response relationship in children, adults and older people. The available human data suggest that supplemental daily doses of vitamin C up to about 1 g, in addition to normal dietary intakes, are not associated with gastrointestinal effects, but that these may occur at higher intakes (3-4 g/day). There has though not been a systematic assessment of the safety of the long-term use of high dose vitamin C supplements.
What should we do for colds?
Antibiotics are ineffective, but adequate fluid, nutritious food and rest is advised, although this advice is based on medical opinion rather than scientific studies.
Vitamin C does have an impact on the immune system so we need to have an adequate intake. But the latest UK dietary surveys show that most of the population get enough vitamin C from their daily diets, on average were well above recommended levels ((40mg for adults) .
Good sources of vitamin C include the following foods, which in contrast to a single vitamin supplement will also provide other vitamins and minerals important to health.
- oranges and orange juice
- red and green peppers
- brussels sprouts
Whilst the pooled results of therapeutic studies do not justify routine vitamin C supplementation for the average person as a therapy for the common cold, including vitamin C rich foods as part of a healthy balanced diet is safe, nutritious and is important for immunity.
Ayela Spiro, British Nutrition Foundation – QCS Expert Nutrition Contributor
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