Is too much zinc harmful?

High zinc doses, well above the recommended daily intake of around 15 mg, are clearly harmful. With long-term intake of 100 to 300 mg per day, copper absorption becomes disrupted. This leads to anaemia and a deficiency of certain white blood cells (neutropenia), which weakens the immune system. This relationship is considered probably causal.

Anyone who suddenly takes an extremely high dose will quickly experience acute symptoms: nausea, vomiting, stomach pain and drowsiness. Zinc taken by mouth is relatively less toxic than when inhaled. People exposed to zinc fumes or intensive skin contact are also at risk, although the evidence for that is more limited.

Even at doses that are not dramatically high but still exceed the recommended amount, there are indications that the absorption of copper and iron decreases and that 'good' HDL cholesterol falls. This evidence is limited and based on associations, not hard cause-and-effect relationships.

The European food safety authority EFSA sets the safe upper limit at 25 mg per day; the American FDA uses 40 mg. Anyone who stays below that limit is unlikely to face acute danger under normal health conditions. The fact that the two authorities differ by 15 mg shows that the exact safe limit has not yet been established.

Finally, there is the broader picture of brain health. In Alzheimer's and Parkinson's disease, zinc levels in the blood are reduced, but accumulation of zinc in brain tissue appears to be harmful. Whether too much or too little zinc partly causes those diseases, or whether it is the other way around and a consequence of them, is something researchers do not yet know. This relationship is associative.