Immune-boosting supplements: an evidence-based overview

Supplements for immunity are one of the most popular categories on the health product market, but not every product is backed by equally strong evidence. The scientific literature clearly supports a few active ingredients — above all vitamin C, vitamin D, zinc and probiotics — while the effects of others on the human body remain unclear. In this article, we look at what the studies actually say, which doses are justified and which misconceptions are worth avoiding.

Which immune system supplements are scientifically proven?

EFSA (European Food Safety Authority) has approved health claims for several micronutrients that contribute to the normal function of the immune system: vitamin C, vitamin D, zinc, folic acid, vitamins B6 and B12, vitamin A, selenium and iron. This does not automatically mean that everyone needs supplements — but it does mean that a shortfall in these active ingredients can clearly affect immune defense.

If you want a broader understanding of when and why supplements are used at all, the article Questions and answers: Dietary supplements gives a good overview.

What does the science say about vitamin C?

Vitamin C is an antioxidant that supports both the innate and the adaptive immune response, stimulates the production of neutrophils and lymphocytes, and reduces oxidative stress (Carr & Maggini, 2017). According to a Cochrane systematic review, regular vitamin C intake (≥ 200 mg per day) shortens the duration of the common cold by an average of 8% in adults and up to 14% in children during intense training or chronic stress (Hemilä & Chalker, 2013).

That said, under normal circumstances vitamin C probably does not prevent infections in the general population — it is more useful during periods of heavy training load and stress. Read more about how training affects micronutrient needs: Supplements for athletes.

Does vitamin D help prevent viral illnesses?

Vitamin D is involved in the maturation of immune cells and the regulation of cytokines. An individual participant data meta-analysis published by Martineau et al. (2017; 25 RCTs, over 11,000 participants) showed that vitamin D supplementation reduced the risk of acute respiratory tract infections — especially in people whose baseline vitamin D level was low, below 25 nmol/l (adjusted OR 0.58; 95% CI 0.40–0.82).

In Estonia, vitamin D deficiency is especially common in autumn and winter, when skin synthesis from sunlight is almost absent. The NIH Office of Dietary Supplements recommends 600–800 IU per day for adults as baseline protection, but a doctor may recommend a higher dose for people with a deficiency — you can find more detailed dosing information in the NIH vitamin D fact sheet.

What is the role of zinc and probiotics?

Zinc is essential for the normal function of more than 300 enzymes and for the development of T cells and NK cells. Even a mild zinc deficiency impairs lymphocyte proliferation and antibody production (Rink & Gabriel, 2000). Using zinc lozenges within the first 24 hours of a cold has been associated with a shorter illness (Hemilä, 2011).

Probiotics — live microorganisms that, when consumed in adequate amounts, have a positive effect on the host — have also been studied in the context of immune function. A Cochrane review (Hao et al., 2015; 12 RCTs, over 3700 participants) found that probiotics reduced the incidence and duration of upper respiratory tract infections compared with placebo; the best-studied were Lactobacillus and Bifidobacterium strains.

Combining supplements is something to approach with caution — synergies and interactions are less well studied than individual ingredients. Read more about how to combine different products safely: SUPPLEMENTS: whether, when, how much.

What should you keep in mind with immune-supporting supplements?

Supplements do not replace the foundation of nutrition, sleep and physical activity. Supplements: what you should know explains the general principles that apply to all supplements.

Some practical recommendations:

• Vitamin C: 200–1000 mg per day is the range used in studies; under normal circumstances, a varied diet often covers it.
• Vitamin D: 1000–2000 IU per day for adults in autumn and winter is a common preventive dose; higher doses only based on blood test results.
• Zinc: 8–11 mg per day usually covers the need; doses above 40 mg per day may interfere with copper absorption.
• Probiotics: the effect is strain- and dose-specific; choose products with clinical evidence for the specific strain and amount.

Consult your doctor if you have a chronic condition, take medication or are considering doses that exceed the recommended daily amount.

References

• Carr AC, Maggini S. (2017). Vitamin C and Immune Function. Nutrients. PMID: 29099763
• Hemilä H, Chalker E. (2013). Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev. PMID: 23440782
• Martineau AR, et al. (2017). Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. PMID: 28202713
• Rink L, Gabriel P. (2000). Zinc and the immune system. Proc Nutr Soc. PMID: 10946996
• Hemilä H. (2011). Zinc lozenges may shorten the duration of colds: a systematic review. Open Respir Med J. PMID: 21769305
• Hao Q, et al. (2015). Probiotics for preventing acute upper respiratory tract infections. Cochrane Database Syst Rev. PMID: 26695080

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