Nutrient deficiencies and immunity: what might your body be missing?

In autumn and winter, immunity comes up again and again. Vitamin C, vitamin D, zinc, selenium, multi-ingredient products and all kinds of "immunity shots" are especially popular at this time of year. Before reaching for yet another supplement, though, it's worth asking something far more basic: does your body actually have enough of the nutrients it needs for the immune system to work properly?

Deficiencies in certain vitamins and minerals can weaken the immune response and make you more prone to infections. That doesn't mean the more supplements you take, the better your immunity works. The benefit shows up mainly when there is a real deficiency or an increased risk of developing one.

Did you know deficiencies can affect your immunity?

The immune system isn't a single organ that you can simply "boost". It's a whole system that includes, among other things, the skin, mucous membranes, immune cells, antibodies and the substances that let cells communicate with each other.

For this system to work properly, it needs an adequate supply of energy, protein, vitamins and minerals. If one of the important nutrients is lacking for a longer time, the immune system can be affected too.

The following are particularly important:

This isn't a complete list. Vitamin B6, vitamin E, copper, magnesium and an adequate intake of protein and energy also matter.

Vitamin D and immunity

Vitamin D is involved in regulating both innate and adaptive immunity. Immune cells also carry vitamin D receptors, which is why an adequate level matters for how they work.

In Poland, vitamin D deficiency is common, especially in the months with little sunlight. Vitamin D status is assessed by measuring 25(OH)D in the blood. When to start supplementing and at what dose is covered in my article on vitamin D in autumn.

Keep in mind that an adequate vitamin D level doesn't guarantee you won't get an infection. Current research doesn't confirm that vitamin D supplementation clearly reduces the risk of respiratory infections in everyone. What matters most is preventing deficiency and correcting it if it is actually present.

Zinc and its role in the immune system

Zinc is needed for immune cells to develop, multiply and function properly. Its deficiency can impair both innate and adaptive immunity.

A more pronounced deficiency can lead to more frequent infections, slower wound healing, changes in taste and smell, loss of appetite or hair loss.

People on very restrictive diets, people with malabsorption and those with certain digestive diseases are particularly at risk of getting too little zinc.

Still, there's no need to take high doses of zinc preventively for the whole autumn and winter. Long-term excess can interfere with copper absorption and cause further problems. I explain when zinc supplementation makes sense in a separate article.

Iron: deficiency isn't just anaemia and fatigue

Iron is mostly associated with haemoglobin, but it also plays an important role in the immune system. Immune cells need it to multiply and function properly.

Importantly, iron deficiency can be present before anaemia develops. This means normal haemoglobin doesn't always rule out a problem.

Diagnosis relies, among other things, on ferritin, which reflects the body's iron stores. Interpreting it isn't always straightforward, though, because ferritin can rise during an infection or inflammation. Depending on the situation, a doctor may therefore also order a complete blood count, CRP, transferrin saturation or other parameters.

Iron shouldn't be supplemented without a reason. Both deficiency and excess can be harmful.

Vitamin C: does it really prevent colds?

Vitamin C is needed for immune cells to work properly and helps protect cells against oxidative stress. Its deficiency can impair the immune system, but that doesn't mean high doses of vitamin C will protect you from infection.

Studies show that regular vitamin C supplementation doesn't clearly reduce the risk of catching a cold for most people. It may, however, slightly shorten how long the symptoms last.

Good everyday sources of vitamin C include bell peppers, kiwi, berries, citrus fruit, broccoli and parsley. For most healthy people, an adequate diet covers their needs without supplements.

Vitamin A and the body's natural barriers

Vitamin A is important for healthy skin and for the mucous membranes of the respiratory and digestive tracts. These are among the first barriers that viruses and bacteria come into contact with.

A vitamin A deficiency can affect not only these tissues but also how some immune cells work and how antibodies are produced.

In people who eat a varied diet, severe vitamin A deficiency is relatively rare. So there's no reason for most people to take high-dose supplements preventively. Vitamin A can accumulate in the body, and too much of it is harmful.

Selenium: needed, but in the right amount

Selenium is part of enzymes involved in, among other things, protecting cells against oxidative stress and regulating the immune response.

A deficiency can harm immune function, but with selenium moderation is especially important. The gap between the amount the body needs and an excessive amount isn't large.

Taking high doses for a long time can cause side effects, so supplementation shouldn't be used without a specific reason.

How much of each nutrient do we need?

The values below come from the Polish Dietary Reference Values (Normy żywienia dla populacji Polski) 2024 and apply to healthy women and men aged 19–65. The table shows the RDA (Recommended Dietary Allowance) and, where no RDA has been set, the AI (Adequate Intake).

*Women's higher iron requirement applies mainly before menopause and is linked to regular blood loss during menstruation.

Keep in mind that these are reference values for the healthy population, not individual therapeutic recommendations. Requirements may differ during pregnancy and breastfeeding, in older adults, children and teenagers, and with certain diseases or deficiencies.

When should you think about deficiencies?

Frequent infections on their own don't mean you're deficient in vitamins or minerals. It is worth paying closer attention to your nutritional status, though, if your diet is very monotonous, you cut out whole food groups, eat too little, follow restrictive diets, or have digestive diseases or malabsorption.

In those situations, assessing your diet and, if needed, getting tested is a much better solution than randomly buying an "immunity" supplement.

Do you need to test all vitamins and minerals?

There's no need for broad test panels without a specific reason. The scope of testing should follow from symptoms, diet, medical conditions, medications and individual risk factors.

When justified, a doctor may order, among other things, a complete blood count, ferritin, transferrin saturation, vitamin B12, folate or 25(OH)D.

Deficiency vs. "immunity" supplements

These are two different situations. If your body is genuinely lacking a specific nutrient, replenishing it can help restore normal immune function.

If there's no deficiency, however, taking higher doses of vitamins and minerals usually doesn't make your immunity work any better.

That's why the foundation is still a well-balanced diet with vegetables and fruit, wholegrain products, protein sources, fish, dairy or suitable alternatives, plus nuts and seeds.

Supplementation may be needed, but it should follow from your specific situation, not from the simple fact that cold and flu season has started.

Frequently asked questions

Do frequent infections mean a vitamin deficiency?

Not always. How often you get infections also depends on sleep, stress, exposure to germs, age, physical activity, chronic diseases and medications. Deficiencies are only one possible factor.

Which deficiencies can affect immunity?

Vitamins A, C, D, E, B6, B12 and folate, as well as zinc, selenium, iron, copper and magnesium, all play a role. That doesn't mean you need to supplement all of them.

What does a suboptimal vitamin D level mean?

It's a result that isn't yet classified as a clear deficiency but sits below the range considered optimal. For vitamin D, this usually means a 25(OH)D concentration of 20–30 ng/ml.

Should I take zinc in autumn and winter?

There's no need to routinely take high doses of zinc throughout autumn and winter. Start by making sure you get enough from your diet. Long-term, high-dose supplementation can cause copper deficiency.

Does vitamin C protect against colds?

For most people, regular supplementation doesn't clearly reduce the risk of infection. It may, however, slightly shorten how long symptoms last.

Does vitamin D protect against infections?

Vitamin D is needed for the immune system to work properly, but it doesn't act like a drug that prevents infections. What matters most is preventing deficiency and correcting it.

Does a normal blood count rule out iron deficiency?

No. Iron deficiency can appear before anaemia does. That's why, when justified, ferritin and other iron parameters are assessed as well.

Is an “immunity” multivitamin worth taking?

If your diet covers your needs and you have no deficiencies, extra high doses of vitamins and minerals usually bring no additional benefit.

Can you have several deficiencies at once?

Yes. This can happen especially with a very monotonous diet, restrictive diets, malabsorption or malnutrition.

References

1. Rychlik, E., Stoś, K., Woźniak, A., Mojska, H. (eds.) (2024). Normy żywienia dla populacji Polski 2024 [Dietary Reference Values for the Polish population 2024]. Warsaw: National Institute of Public Health NIH – National Research Institute. https://www.pzh.gov.pl/normy-zywienia-2024/
2. National Institutes of Health, Office of Dietary Supplements (2025). Dietary Supplements for Immune Function and Infectious Diseases: Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/ImmuneFunction-HealthProfessional/
3. Calder, P.C., Carr, A.C., Gombart, A.F., Eggersdorfer, M. (2020). Optimal Nutritional Status for a Well-Functioning Immune System Is an Important Factor to Protect against Viral Infections. Nutrients, 12(4), 1181. doi:10.3390/nu12041181
4. Gombart, A.F., Pierre, A., Maggini, S. (2020). A Review of Micronutrients and the Immune System – Working in Harmony to Reduce the Risk of Infection. Nutrients, 12(1), 236. doi:10.3390/nu12010236
5. World Health Organization (2020). WHO guideline on use of ferritin concentrations to assess iron status in individuals and populations. Geneva: WHO. https://www.who.int/publications/i/item/9789240000124
6. Frost, J.N., Drakesmith, H. (2025). Iron and the immune system. Nature Reviews Immunology, 25(12), 885–899. doi:10.1038/s41577-025-01193-y
7. Płudowski, P., Kos-Kudła, B., Walczak, M. et al. (2023). Guidelines for Preventing and Treating Vitamin D Deficiency: A 2023 Update in Poland. Nutrients, 15(3), 695. doi:10.3390/nu15030695
8. Jolliffe, D.A., Camargo, C.A. Jr, Sluyter, J.D. et al. (2025). Vitamin D supplementation to prevent acute respiratory infections: systematic review and meta-analysis of stratified aggregate data. The Lancet Diabetes & Endocrinology, 13(4), 307–320. doi:10.1016/S2213-8587(24)00348-6
9. European Food Safety Authority (2017). Dietary Reference Values for nutrients: Summary report. EFSA Supporting Publications, 14(12), e15121. doi:10.2903/sp.efsa.2017.e15121
10. National Institutes of Health, Office of Dietary Supplements (2026). Zinc: Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/Zinc-HealthProfessional/
11. National Institutes of Health, Office of Dietary Supplements (2025). Vitamin C: Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/VitaminC-HealthProfessional/
12. National Institutes of Health, Office of Dietary Supplements (2025). Vitamin A and Carotenoids: Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/
13. National Institutes of Health, Office of Dietary Supplements (2025). Selenium: Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/Selenium-HealthProfessional/