Zinc is clearly important for a healthy immune system: a deficiency impairs the functioning of virtually all immune cells, and supplementation restores this in at-risk groups. For people without a deficiency, zinc can shorten colds to some extent, but high doses are harmful and should be avoided.
Zinc plays an essential role in the functioning of virtually all types of immune cells. When zinc is deficient, monocytes, natural killer cells, neutrophils, T cells and B cells all perform worse. B cells even undergo increased cell death (apoptosis). This has been demonstrated in both laboratory research and human studies (PMID: 12730441, 23804522, 29186856, 12142956).
When a zinc deficiency has been confirmed, supplementation can genuinely restore immune function. This applies in particular to older adults, severely malnourished individuals and certain patient groups such as people living with HIV. For these at-risk groups, supplementation is well supported by evidence (PMID: 12730441, 16373990, 7478075).
For people without a deficiency the benefits are more modest, but several controlled studies show that zinc up to 30 mg per day can shorten the duration and severity of colds and other respiratory infections. The effect appears to be greatest in people with a mild deficiency (PMID: 16373990).
Important to know: more is not better. At very high zinc doses, some immune cells are actually suppressed (natural killer cells, T cells), while others become overactivated. Strikingly, this pattern resembles the damage that also occurs with a deficiency. Taking high doses of zinc without a clear reason is therefore not a good idea (PMID: 12730441).
During serious infections such as sepsis, zinc levels in the blood drop because zinc shifts to the liver. Low zinc levels are associated with a more severe disease course, but whether zinc supplementation improves outcomes in sepsis has not yet been sufficiently proven (PMID: 30060473).
The claims are based on multiple controlled studies and laboratory research (PMID: 12730441, 23804522, 29186856, 12142956, 16373990, 7478075, 30060473). The evidence is strong for zinc deficiency and recovery following supplementation; the evidence for the effect on colds in people without a deficiency is moderate. For sepsis, the evidence is limited and observational in nature.