How do you support your immune system? What the evidence supports

This entry is part of the Nutri Tailor Health Reference Library — cited research on supplements, nutrients and adjacent areas of health.

Summary

A healthy immune system is not improved by boosting, and the useful work is removing what suppresses it. Sleep comes first, because adults sleeping under six hours were four times more likely to catch a cold after deliberate virus exposure. Vitamin D through the UK autumn and winter comes second, where trials pooling over 75,000 people found a small reduction in respiratory infections. Daily moderate movement follows, then stress, smoking and heavy alcohol brought down, iron and zinc corrected where a blood test or diet says they are low, and blood sugar kept in range. Vitamin C does not reduce how often most people catch colds, though it shortens them modestly. Repeated or unusual infections need a GP.

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How it works

Immunity is a system with a budget, and the things that drain the budget are what make people catch every passing infection. Sleep is when the adaptive response consolidates, which is why short sleepers make weaker antibody responses to vaccines and catch more colds on controlled exposure. Vitamin D is a signalling molecule for innate defence in the airways, and most UK adults run low between October and March because the sun is too weak to make it. Iron and zinc are required for the rapid cell division that fighting an infection demands, so deficiency slows the response. Cortisol and the stress response suppress immune signalling directly, which is the mechanism behind the link between stress and illness, and alcohol and smoking damage the airway lining that forms the first barrier. None of this is exotic. The interventions that work are the ones that remove a drain on the system.

Timing

Sleep changes show in how you feel within a fortnight and in infection frequency over a season. Vitamin D takes around three months to reach steady state at a given dose, so an October start covers the winter. Iron correction runs months, zinc weeks. The fair read on any immune programme is a winter, not a week, counted in infections caught and days lost rather than in how you feel on a given morning.

Safety profile

Some patterns are not a lifestyle question. Infections that are unusually frequent, unusually severe, keep needing antibiotics, or sit in unusual places, alongside unexplained weight loss, night sweats, persistent swollen glands or bruising, need a GP and blood tests, because they can signal diabetes, low white cells or other conditions that no supplement addresses. Vitamin D above 4,000 IU a day long term needs a blood level to justify it; zinc above 40 mg a day long term depletes copper; vitamin C at gram doses can cause loose stools and raises kidney stone risk in those prone to them. Anyone on immune-suppressing prescriptions, anyone with an autoimmune condition, and anyone pregnant should check before adding immune-active herbs. Nutri365 checks for interactions and contraindications before suggesting any nutritional step.

Special populations

Older adults have a weaker vaccine response and benefit most from sleep and vitamin D being right before the winter jabs. Vegans and vegetarians are the group most likely to be low in zinc, iron and B12 together, a combination that shows up as repeated infections and fatigue. Heavy trainers and marathon runners are the one group in whom vitamin C trials halved cold incidence. People with diabetes should take a run of infections as a prompt to check HbA1c. Children under five have separate NHS vitamin D advice.

Guideline positions

NHS guidance advises everyone in the UK to consider a daily 10 microgram vitamin D supplement from October to March, because sunlight is insufficient to make it in those months. The trial evidence behind that for infections: a 2017 pooled analysis of individual data from 25 trials (10,933 people) found vitamin D reduced acute respiratory infections overall, with the largest effect in people starting very deficient; the 2021 update across 46 trials and 75,541 people found a smaller but still significant protective effect; the 2025 update sits at the edge of significance. On sleep, 164 adults under quarantine exposure to a cold virus showed those sleeping under six hours were over four times more likely to develop a cold than those sleeping seven or more. On vitamin C, the Cochrane review found no effect on how often the general population catches colds, an 8 percent shorter duration in adults, and halved incidence only in people under heavy physical stress such as marathon runners.

Practical framework

The causes are worked through in order. Sleep comes first, at seven hours or more held for a month, with the method on [how to fall asleep faster](/apps/learn/how-to-fall-asleep-faster-what-the-evidence-supports). Vitamin D comes second, at 10 micrograms a day from October to March for everyone, with a blood test for anyone who is always ill, because a level under 25 nmol/L needs more than that, read on the [vitamin D results hub](/apps/learn/vitamin-d-blood-test-results-explained-uk-what-your-number-means). Movement comes third, daily and moderate, because very heavy training on poor sleep drains rather than builds. Stress, alcohol and smoking come fourth, with the stress side on [how to lower cortisol](/apps/learn/how-to-lower-cortisol-what-the-evidence-supports). Iron and zinc come fifth, where a blood test or a low-meat, low-variety diet points that way, with the [ferritin hub](/apps/learn/ferritin-blood-test-results-explained-uk-what-your-number-means) and [zinc for immunity](/apps/learn/best-zinc-supplement-for-immunity-form-dose-timeline) covering the detail. Blood sugar comes sixth, because a raised HbA1c makes infections more frequent and slower to clear. The supplement layer is ranked on [immune supplements, what the evidence supports](/apps/learn/immune-supplements-what-the-evidence-supports). Nutri365 reads sleep, blood results and reported symptoms together over weeks to find which of the six applies, checks for interactions and contraindications, and refers questions outside scope to a health specialist.

Common misconceptions

There is no evidence that immune-boosting products raise immunity in people who are not deficient in something, and the marketing phrase has no clinical meaning. Vitamin C does not stop most people catching colds. Echinacea has not been shown to help much in the Cochrane review, with at most a weak possible effect from some products. Feeling run down is a reason to check sleep, vitamin D, iron and stress, not a reason to buy a multi-ingredient blend at random.

Sources

Martineau AR, Jolliffe DA, Hooper RL, et al 2017. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. PMID: 28202713
Jolliffe DA, Camargo CA Jr, Sluyter JD, et al 2021. Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials. Lancet Diabetes and Endocrinology. PMID: 33798465
Prather AA, Janicki-Deverts D, Hall MH, Cohen S 2015. Behaviorally Assessed Sleep and Susceptibility to the Common Cold. Sleep. PMID: 26118561
Hemila H, Chalker E 2013. Vitamin C for preventing and treating the common cold. Cochrane Database of Systematic Reviews. PMID: 23440782 · DOI: 10.1002/14651858.CD000980.pub4
Karsch-Volk M, Barrett B, Kiefer D, Bauer R, Ardjomand-Woelkart K, Linde K 2014. Echinacea for preventing and treating the common cold. Cochrane Database of Systematic Reviews. PMID: 24554461 · DOI: 10.1002/14651858.CD000530.pub3
2024. Vitamin D. NHS.

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