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Supplement

Zinc

Mineral · Zinc bisglycinate / zinc picolinate

As a component of more than 300 enzymes, zinc is involved in immune function, and in a meta-analysis with 5,446 adults, there were 5 fewer respiratory infections per 100 person-months under zinc – a number needed to treat of 20, with moderate certainty of evidence. The large 2024 Cochrane review, by contrast, missed its primary endpoint for cold prevention, and the often-cited 2.37-day reduction in cold duration is not a robust figure because of extreme heterogeneity.

What zinc is

Zinc is an essential mineral that is involved in more than 300 enzymes and in immune function, DNA synthesis and wound healing. Dietary sources include oysters, beef, pumpkin seeds, lentils and cashews. In the BK-Score it gets 7 out of 10 points for human evidence and 8 out of 10 for mechanism.

The cold studies mostly used lozenges, usually as gluconate, at 45 to 276 mg per day over 4.5 to 21 days.

Why 97 percent heterogeneity devalues the figure

Heterogeneity, given as I², measures how much the individual results of a meta-analysis differ from one another. At I² = 97 %, practically all of the variation is due to real differences between the studies, not to chance. The studies then do not measure the same quantity.

In this case, a common mean describes no reality but merely adds up contradictory results into one number. That is exactly the case with the 2.37-day reduction in cold duration – accordingly, the authors rate the certainty of evidence as low.

What is well supported

  • In the second large analysis, prevention reached significance. Hunter 2021 pooled 28 RCTs with 5,446 adults and reports 5 fewer respiratory infections per 100 person-months (1 to 8) with moderate certainty of evidence, corresponding to a number needed to treat of 20. It is the finding with the highest certainty of evidence in this body of data – the 2024 Cochrane review came to a different result on the same question.
  • On cold duration, both analyses point in the same direction. Nault 2024 gives 2.37 days shorter for Cochrane (−4.21 to −0.53), Hunter 2021 around two days (0.61 to 3.50). Both figures are supported with low and very low certainty of evidence, respectively, and in Cochrane additionally with a heterogeneity of I² = 97 %. The direction is robust, not the size.
  • The role in the immune system is understood. Zinc is involved in more than 300 enzymes and in immune function, DNA synthesis and wound healing. In the BK-Score, mechanism gets 8 out of 10 points and track record of use 10 out of 10.
  • An officially derived maximum level. The BfR proposes 6.5 mg per daily dose in food supplements, derived from the tolerable upper intake level of 25 mg per day set by the EU Scientific Committee on Food in 2003 and the small margin between this limit and the 95th percentile of intake from a normal diet. The frequency of side effects is also quantified: RR 1.34 (1.15 to 1.55) with moderate certainty of evidence.

What the studies show

Cochrane 2024: prevention missed, treatment uncertain

Nault 2024 analyzed 34 RCTs with 8,526 adults and children, 15 of them on prevention and 19 on treatment. The primary endpoint of the prevention studies, the risk of a cold, was not met: RR 0.93 (0.85 to 1.01), low certainty of evidence. For treatment, cold duration was shortened by 2.37 days (−4.21 to −0.53) at I² = 97 % and low certainty. Symptom severity remained unchanged (SMD −0.03; −0.56 to +0.50, very low certainty).

A second analysis with a different scope

Hunter 2021 pooled 28 RCTs with 5,446 adults, with oral, sublingual and intranasal use. For prevention, it reports 5 fewer respiratory infections per 100 person-months (1 to 8) with moderate certainty, corresponding to a number needed to treat of 20 – an example of how relative and absolute effects differ. Symptoms resolved on average around two days earlier (0.61 to 3.50), but with very low certainty of evidence.

The clearest negative finding

In the same analysis, sublingual zinc after controlled rhinovirus inoculation prevented not a single cold: RR 0.96 (0.77 to 1.21) with moderate certainty of evidence. This is the methodologically cleanest approach to the prevention question, because the time of infection is known. Two letters of criticism were also published on the 2024 Cochrane review; the analysis is disputed among experts.

Where the data stop

  • Preventing colds. The primary endpoint of the Cochrane review was missed; the confidence interval reached up to 1.01. Sublingual zinc did not prevent a cold after targeted rhinovirus exposure.
  • The figure of 2.37 days. At I² = 97 %, the included studies contradict each other so strongly that the pooled mean is not a robust point estimate. Nevertheless, the figure regularly appears in advertising without this caveat.
  • Which zinc form and which dose works. The Cochrane review cannot make a statement on this. Marketing individual compounds as superior has no basis in these data.
  • Lower symptom severity. The effect was SMD −0.03, with an interval of −0.56 to +0.50 and very low certainty of evidence.

Status, approval and legal

For food supplements, the German Federal Institute for Risk Assessment proposes a maximum level of 6.5 mg zinc per daily dose. The value is derived from the tolerable upper intake level of 25 mg per day set by the EU Scientific Committee on Food in 2003 and the small margin between this limit and the 95th percentile of intake from a normal diet.

Safety

Non-serious side effects occurred more often under zinc: RR 1.34 (1.15 to 1.55) with moderate certainty in the Cochrane review, RR 1.41 (1.17 to 1.69) in Hunter 2021, with a number needed to harm of 7. With long-term intake above 40 mg per day, there is a risk of copper deficiency, because zinc competes with copper for absorption. This text is for information and does not replace medical advice.

BK-Score Supported, with caveats

Human evidence7
Mechanism8
Safety data8
Hype gap6
Track record of use10

For deficiency and for cold duration there are controlled studies and meta-analyses, and the immune function is mechanistically clear. Overdosing disturbs the copper balance – this rarely appears in advertising.

The score rates the state of knowledge, not the substance. “Safety data 9” means well studied – not harmless. “Track record of use 9” means used long and widely – that is not proof of efficacy.
Subjective assessment by Biohacking Kompakt based on published scoring rules – not a scientific rating and not a medical recommendation. Rules and all ratings (German)

Frequently asked questions about zinc

How does zinc work?

Zinc is a component of more than 300 enzymes and is involved in immune function, DNA synthesis and wound healing. This role is mechanistically well understood. The cold studies mostly used lozenges, usually as gluconate. Which zinc form and which dose is effective, however, cannot be determined from the available data.

Does zinc prevent colds?

According to the best available analysis, no. The 2024 Cochrane review, with 34 studies and 8,526 participants, missed the primary endpoint of cold risk, with a relative risk of 0.93 and an interval up to 1.01. Sublingual zinc also did not prevent a cold after targeted rhinovirus exposure.

Does zinc shorten a cold?

The often-cited reduction of 2.37 days is not a robust figure. It comes from a meta-analysis with a heterogeneity of 97 percent, in which the individual studies contradict each other massively. The authors rate the certainty of evidence as low. Symptom severity did not change in the same analysis.

What does a heterogeneity of 97 percent mean?

It means that the pooled studies practically do not measure the same thing. The I-squared measure indicates how much of the variation between the individual results is due to real differences rather than to chance. At 97 percent, the common mean describes no shared reality but combines contradictory results into one number.

How much zinc is allowed in food supplements?

The German Federal Institute for Risk Assessment proposes a maximum level of 6.5 milligrams per daily dose. The basis is the tolerable upper intake level of 25 milligrams daily set by the EU Scientific Committee on Food in 2003 and the small margin between this limit and the 95th percentile of intake from a normal diet.

What side effects does zinc have?

Non-serious side effects occurred more often in both large analyses, with relative risks of 1.34 and 1.41 and a number needed to harm of seven. With long-term intake above 40 milligrams per day, there is a risk of copper deficiency, because zinc competes with copper for absorption. Not medical advice.

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Information only, not medical advice and not a usage or dosage recommendation. Prescription-only and unapproved substances belong in the hands of a physician. Last updated: 2026-10-04.