From an old-fashioned faith in the healing powers of chicken soup to more modern obsessions with so-called super foods, we like to think some things we eat can help ward off infections. The vast majority of these beliefs have little evidence to back them up, but there are dietary interventions that appear to work.
Numerous supplements are sold on the basis of supposed immune-boosting powers, but their health claims usually stem from tests done on cells in the lab. That is just the first stage of gathering evidence, though; the only way to know for sure if something will work is a randomised, controlled trial done on people, preferably several trials.
By that measure, zinc supplements probably come out best, with evidence they can both prevent colds and shorten their duration if started within 24 hours of the symptoms first appearing (Cochrane Database of Systematic Reviews, DOI: 10.1002/14651858.CD001364.pub3).
Zinc may work by stopping the cold virus from replicating or preventing it from gaining entry to cells lining the airways.
That old favourite vitamin C doesn’t seem to prevent colds, although as a treatment it might reduce symptoms slightly. The only other supplement with any credibility is echinacea, an extract of the purple coneflower – although again only as treatment, not prevention, and even then the evidence is mixed.
Vitamin C boosts immune cell activity in theory so why does it perform so poorly in practice? It seems that while vitamin supplements help people who are malnourished avoid diseases caused by vitamin deficiency, such as scurvy, there is no extra benefit to exceeding the recommended levels, which most people in the west hit anyway. In fact, popping vitamin pills – including vitamin C – may even be harmful overall (New Scientist, 5 August 2006, p 40).
If you really want to support your immune system the best approach is simply to eat a plentiful supply of fruit and vegetables. They contain not just vitamins but thousands of other compounds called phyto-chemicals, which have numerous beneficial effects we are only just starting to understand.
It is also important to focus on the quantity of food, not just its quality. People who are obese are more likely to get a range of infections, including respiratory, skin and urinary ones (The Lancet Infectious Diseases, vol 6, p 438). Piling on the pounds not only makes it harder to breathe, which predisposes people to colds and flu, but the excess fat releases chemical signals that interfere with immune functioning.
Think carefully about how you shed the pounds, though, because yo-yo dieting is also harmful. Frequent cycles of weight loss and regain seem to reduce the performance of natural killer cells, an important branch of the immune system that targets cancerous cells and those infected with viruses (Journal of the American Dietetic Association, vol 104, p 903).