Vitamin C, zinc and vitamin D for immunity: what supplementation does and what it does not

Vitamin C, zinc and vitamin D are needed for the immune system to work normally, and a deficiency weakens the body's defences and increases susceptibility to infection. In people without a deficiency, supplementation has modest effects: vitamin C taken regularly did not reduce the number of colds in the general population, zinc did not prevent them, and vitamin D reduced the risk of respiratory infections only slightly.

The article goes through each nutrient, with the figures from the large analyses, then explains when a blood test makes sense and what an infusion can — and cannot — do.

Why these nutrients matter

The US National Institutes of Health (NIH), through its Office of Dietary Supplements, sums up the consensus as follows: an adequate intake of vitamins A, C, D and E, selenium and zinc is important for immune function, and clinical deficiencies of these nutrients weaken immunity and can increase susceptibility to infection.

Vitamin C, for example, helps maintain the integrity of epithelial barriers and contributes to the proliferation of B and T lymphocytes and to phagocytosis. A deficiency impairs immune function.

The key point comes straight after: the fact that a deficiency does harm does not mean that a surplus helps. That has been tested separately, and the results are more modest.

Vitamin C and the common cold: what Cochrane found

The most cited analysis is the 2013 Cochrane review by Hemilä and Chalker, which included only placebo-controlled trials with doses of at least 0.2 g a day.

• Prevention in the general population: in trials with 10,708 participants, vitamin C taken daily did not significantly reduce the risk of catching a cold (relative risk 0.97).
• Intense physical exertion: in marathon runners, skiers and soldiers on sub-arctic exercises — 598 participants, in five trials — the risk of a cold was roughly halved (relative risk 0.48).
• Duration of the cold: in those who took it regularly, colds lasted 8% less in adults and 14% less in children.
• Taken only after symptoms started: no consistent effect on duration or severity was seen.

The authors' conclusion: routine supplementation in the general population is not justified, although it may be useful for people exposed to short periods of severe physical exertion.

The NIH notes another relevant point: the effects of supplementation may vary with each person's vitamin C level, and some data suggest a greater benefit in those with a low level.

What the intravenous route changes and what it does not

An NIH pharmacokinetic study published in 2004 showed that vitamin C taken by mouth produces blood concentrations that the body controls tightly, while intravenous administration reaches much higher concentrations, rising further as the dose increases. The mechanism is explained in detail in the article about vitamin C ampoules and infusion.

What does not follow from this: that a higher concentration brings a greater clinical benefit. The largest randomised trial of intravenous vitamin C, LOVIT, published in 2022, included 872 adults with sepsis in intensive care. The group that received vitamin C had a higher, not lower, risk of death or persistent organ dysfunction at 28 days compared with placebo.

This is a very different context from a wellness drip and says nothing about the risk in a healthy person. It does show, however, that “more, straight into the vein” does not automatically translate into “better”.

Zinc: not for prevention, possibly for duration

The 2024 Cochrane review analysed 34 trials, with 8,526 participants, that tested zinc alone against placebo.

• Prevention: zinc may have little or no effect on the risk of catching a cold (low-certainty evidence).
• Treatment: taken during a cold, it may shorten its duration, but the certainty of the evidence is low and the results of the trials varied widely.
• Adverse effects: used as a treatment, zinc probably increased the risk of mild adverse effects; among those tracked in the trials were an unpleasant taste and digestive upset.

Half of the trials used lozenges, and the rest tablets, syrups or intranasal forms. The results cannot be transferred to zinc given by infusion.

Zinc also has a clear safety limit. Doses of 50 mg or more, taken for weeks on end, can inhibit copper absorption and can even reduce immune function. The tolerable upper intake level set for adults is 40 mg a day.

Vitamin D: a small effect, linked to daily doses

A meta-analysis published in 2021 in The Lancet Diabetes & Endocrinology pooled 43 randomised trials, with data for 48,488 participants. Vitamin D supplementation reduced the risk of having at least one acute respiratory infection, but the reduction was small: 61.3% compared with 62.3% in the placebo group.

Protection was associated with daily doses of 400–1,000 IU, with durations of up to 12 months, and with an age of 1–16 years at enrolment. Supplementation was safe.

The practical consequence: high-dose injectable vitamin D is not a way to prevent colds. It is a way to correct a confirmed deficiency, covered in detail in the article about injectable vitamin D3. Unlike vitamin C, excess vitamin D can cause hypercalcaemia.

When a deficiency should be confirmed

Targeted supplementation makes sense when there is a deficiency. For each of the three nutrients, the blood test has its own particularities:

Tests are useful mainly in at-risk groups. For zinc: digestive conditions, bariatric surgery, a vegetarian or vegan diet, chronic alcohol use. For vitamin C: smokers, people with a very limited diet and those with malabsorption.

Results are interpreted at a consultation, not on your own and not from a message.

What an infusion does not do

• It does not prevent infections and does not replace vaccination.
• It does not treat a cold, flu or any other infection. If you are ill right now, with a fever, the right place to go is your GP (family doctor).
• It does not make up for lack of sleep or a long-term unbalanced diet.

What we offer at Synerva

The protocols in this category are on the immunity IV drips page:

• Immunity Boost — vitamin C, B complex, minerals and trace elements, 60 minutes, 600 RON;
• High-dose vitamin C 5000 mg — 30 minutes, 300 RON, details on the injectable vitamin C page;
• Glutathione + vitamin C — 45 minutes, 450 RON;
• Minerals & trace elements — includes zinc, 30 minutes, 250 RON;
• Immunity course — 4 Immunity Boost sessions, 1990 RON, compared with 2400 RON separately.

Injectable vitamin D — 100,000 IU at 200 RON, 300,000 IU at 250 RON — is given only after a medical assessment, based on the measured level. Details on the vitamin D page.

The medical assessment and sterile materials are included in the price. For a home visit a travel fee is added: 100 RON in Bucharest, free for orders of 600 RON or more, and 150 RON in Ilfov, free for orders of 900 RON or more. If at the assessment we do not see a reason for an infusion, we tell you.

Sources

NIH Office of Dietary Supplements — Dietary Supplements for Immune Function and Infectious Diseases.

Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database of Systematic Reviews, 2013, CD000980.

Padayatty SJ et al. Vitamin C pharmacokinetics: implications for oral and intravenous use. Annals of Internal Medicine, 2004;140(7):533-7.

Lamontagne F et al. Intravenous Vitamin C in Adults with Sepsis in the Intensive Care Unit (the LOVIT trial). New England Journal of Medicine, 2022;386(25):2387-98.

Nault D et al. Zinc for prevention and treatment of the common cold. Cochrane Database of Systematic Reviews, 2024, CD014914.

Jolliffe DA et al. Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis. Lancet Diabetes & Endocrinology, 2021;9(5):276-92.

NIH Office of Dietary Supplements — fact sheets for health professionals: Vitamin C, Zinc, Vitamin D.

For information only. It does not replace a medical consultation and does not interpret test results.