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Getting ill often

Catching several colds a winter is normal, particularly for adults living with young children. The nutrients associated with immune function are worth knowing about, and the marketing around them is far ahead of the evidence.

General information only, not a diagnosis. A symptom is not proof of a deficiency — most symptoms here have many possible causes, and the common ones are rarely nutritional. Do not start a supplement to treat a symptom; the safe route is a blood test if one is warranted, and a doctor to interpret it. Use the red-flag list on each page, and see a clinician for anything that persists or worries you.

Start with the base rate, because it reframes the question. Adults typically get a handful of respiratory infections a year, and parents of preschool children get considerably more, because preschool children are efficient distributors of respiratory viruses. Exposure explains most of the variation between people. So does sleep: short sleep is one of the better-replicated risk factors for catching a cold after controlled exposure.

Vitamin C is the most famous immune nutrient and the clearest example of the gap between reputation and evidence. Regular supplementation does not prevent colds in the general population. It does appear to shorten them slightly — by a fraction of a day on average — and there is a specific exception for people undergoing extreme short-term physical stress, such as marathon runners and soldiers in subarctic conditions, in whom prevention trials have been positive. Starting it once you already have symptoms has not shown benefit.

Vitamin D has the more interesting recent story. Meta-analyses of randomised trials have suggested a small protective effect against acute respiratory infection, concentrated in people who were deficient to begin with and in those taking regular rather than large intermittent doses. That is a modest, conditional finding rather than a general recommendation, and it is also why the answer to "should I take vitamin D" in a northern winter is best answered by whoever knows your blood results.

Zinc has evidence for shortening the duration of a common cold when started very early, with meaningful caveats about formulation, dose and side effects — and a specific warning about intranasal zinc products, which have been associated with lasting loss of smell. This is a good example of a nutrient where the delivery route changes the risk entirely.

The unglamorous interventions remain the strongest: sleep, vaccination where it is offered, hand hygiene, and staying home when infectious. If you are getting unusually severe infections, infections that need antibiotics repeatedly, or infections in unusual places, that is a pattern worth investigating rather than supplementing.

Nutrients commonly discussed for frequent illness

Each nutrient below is rated for how strong the evidence is that it relates to this symptom. "Relevant only if you are deficient" means it can matter when your level is genuinely low, but topping it up when it is not low does nothing.

Vitamin D Mixed evidence

Pooled trial data suggest a small protective effect against acute respiratory infection, largest in people who were deficient. A conditional finding, not a general prescription.

Full ranking: foods highest in vitamin d.

Vitamin C Weak evidence

Does not prevent colds in the general population. Slightly shortens them when taken regularly beforehand; no benefit demonstrated when started after symptoms begin. Prevention trials in people under extreme short-term physical stress are the exception.

Full ranking: foods highest in vitamin c.

Zinc Mixed evidence

Some evidence for shortening a cold when started very early, with real caveats about dose and side effects. Avoid intranasal zinc products entirely — they have been associated with lasting loss of smell.

Full ranking: foods highest in zinc.

Protein Relevant only if you are deficient

Severe protein-energy undernutrition impairs immune function. This is a clinical state, not a description of eating slightly less protein than a target.

Selenium Relevant only if you are deficient

Involved in immune function, and deficiency is uncommon in mixed diets. Worth noting that the gap between an adequate and an excessive selenium intake is narrower than for most nutrients.

Full ranking: foods highest in selenium.

Source: Foods richest in each nutrient: USDA FoodData Central SR Legacy (public domain), per 100 g · checked 2026-09-02

Causes that are usually more likely

Before a nutrient, these are the explanations a clinician would normally consider first for frequent illness:

• Exposure — young children, public transport, healthcare and teaching work
• Short or poor sleep
• Smoking and vaping
• Chronic stress
• Asthma, allergy and other airway conditions
• Diabetes and other long-term conditions
• Medicines that suppress the immune system
• Not being up to date with offered vaccinations

When to see a doctor

Treat these as reasons to seek a professional opinion rather than to self-treat:

• Infections that repeatedly need antibiotics or hospital treatment
• Unusual infections, or infections in unusual sites
• Infections alongside unexplained weight loss or night sweats
• Persistent mouth ulcers, thrush or skin abscesses
• A wound or chest infection that will not clear
• Being told your white cell count is abnormal

This list is not exhaustive, and a symptom that is severe, persistent or getting worse always deserves a professional opinion. If you feel very unwell, seek urgent care rather than reading further.

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