Zinc

Zinc is a metallic chemical element (symbol Zn) that, in trace amounts, is essential to human, animal, plant, and microbial life ^[1]. In people it is the most abundant trace metal after iron, and it is distinctive among metals for appearing as a cofactor in every class of enzyme; well over a thousand human proteins are thought to depend on it ^[1][2]. Beyond catalysis, zinc has a structural role in zinc-finger proteins that regulate gene expression, and it is required for normal immune function, cell division, protein synthesis, growth, reproduction, and the senses of taste and smell ^[1][2].

Dietary zinc is supplied chiefly by animal foods such as oysters and other shellfish, red meat, and poultry, along with legumes, nuts, seeds, and whole grains; plant sources are less bioavailable because compounds called phytates bind zinc in the gut ^[1][2]. Zinc deficiency is common worldwide, particularly where diets are low in animal protein, and it can cause impaired growth and delayed sexual maturation in children, weakened immunity, skin lesions, poor wound healing, hair loss, and loss of taste ^[1][2]. Supplemental zinc is used to correct deficiency, and zinc treatment is part of the standard management of acute diarrhea in children in developing countries ^[1].

One of the most studied supplemental uses is for the common cold. Meta-analyses of randomized trials indicate that zinc lozenges, when taken soon after symptoms begin, can shorten the duration of colds; pooled estimates found cold length cut by roughly a third, with both zinc acetate and zinc gluconate showing benefit ^[3][4]. Common salts used in supplements and foods include zinc gluconate, zinc sulfate, zinc acetate, and zinc picolinate ^[1].

Although zinc is generally safe at intakes near dietary requirements, excessive intake causes problems. Acute overdose can produce nausea and vomiting, while chronic high intake interferes with copper absorption and can lead to copper deficiency, anemia, and neurological signs; nutrition authorities therefore set a tolerable upper intake level ^[1][2]. Zinc is regulated as a nutrient and dietary supplement rather than as a drug for most uses ^[1].

At sensible doses zinc is safe. High single doses can cause nausea, especially on an empty stomach. The bigger issue is chronic high intake, which causes copper deficiency, anemia, and neurological problems; long-term use above roughly 40 mg daily should include copper or medical oversight.

Zinc is a divalent cation, and most of its interactions follow from that. It chelates tetracyclines and fluoroquinolones in the gut lumen, cutting absorption of both the antibiotic and the zinc; the same applies to penicillamine, trientine and oral bisphosphonates, where enough zinc can render a dose meaningfully less effective. This is a spacing problem rather than a contraindication.

The interaction with copper is the one that causes real harm. Zinc induces metallothionein in the enterocyte, which traps copper and prevents it entering circulation, so sustained high dose zinc produces copper deficiency. The consequence is sideroblastic anaemia, neutropenia and a myeloneuropathy resembling subacute combined degeneration that does not always reverse once copper is restored. Cases have followed months of supplementation well above nutritional intakes, and also chronic use of zinc containing denture adhesives.

Thiazide diuretics increase urinary zinc losses over time, and high dose iron or calcium taken at the same moment competes for the same intestinal uptake pathway.

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