Zinc
What it is
Zinc is an essential trace mineral and the second most abundant trace element in the human body after iron. It is found in highest concentrations in muscle and bone, with significant amounts also present in the liver, kidney, pancreas, and prostate. Unlike some other minerals, the body has no dedicated zinc storage system -- status is maintained through dietary intake and tight homeostatic regulation of absorption and excretion.
Zinc functions as a catalytic, structural, and regulatory component in over 300 enzymatic reactions. Its roles span DNA synthesis and cell division, protein synthesis, immune cell development and signalling, wound healing, taste and smell perception, insulin storage and secretion, and reproductive function. These broad physiological roles explain why zinc deficiency produces a diverse and sometimes subtle clinical picture. These roles are biochemical and do not imply prevention of infection or general immune enhancement in individuals with adequate zinc status.
Dietary sources include red meat, shellfish (particularly oysters, which have exceptionally high zinc content), poultry, legumes, nuts, seeds, and wholegrains. Bioavailability from plant sources is substantially lower than from animal sources due to the binding of zinc by phytates in plant cell walls.
Supplemental forms vary considerably in bioavailability and tolerability. Zinc gluconate, zinc citrate, and zinc bisglycinate are well-absorbed and better tolerated than zinc sulfate. Zinc picolinate is marketed as superior but comparative trial data are limited. Zinc oxide has poor bioavailability and is not appropriate for supplementation purposes.
What the evidence shows
The zinc evidence base reflects the same pattern seen with several other micronutrients: robust and clinically meaningful effects in low-status or deficient individuals, and a much weaker case for routine supplementation in replete healthy adults.
Deficiency correction and repletion are well-established. Zinc deficiency produces a recognisable clinical syndrome including impaired immune function, poor wound healing, taste and smell disturbances, skin changes, and reproductive dysfunction. Correction of deficiency reliably resolves these manifestations. This is a nutritional correction effect and is not contested.
Common cold -- zinc lozenges. The evidence for zinc lozenges initiated within 24 hours of cold symptom onset is more robust than for most other cold remedies. A Cochrane review (Hemila) found approximately 33% reduction in cold duration with zinc acetate or zinc gluconate lozenges. The effect is formulation-dependent -- ionic zinc release appears to be the active mechanism, and formulations that chelate zinc ions (such as those containing citric acid) show weaker effects. This is a therapeutic effect at symptom onset, distinct from the prevention effect discussed for vitamin C.
Testosterone and male reproductive function. Zinc is required for testosterone synthesis and spermatogenesis. In men with low zinc status, supplementation raises testosterone and may improve sperm parameters. In replete men, the effect is considerably smaller and inconsistent. The widespread marketing of zinc as a testosterone booster for healthy men is not well-grounded in the available trial evidence.
Acne vulgaris. Oral zinc has a reasonable evidence base for acne, with multiple trials showing reductions in lesion counts. The effect is smaller than for oral antibiotics but zinc has the advantage of not contributing to antibiotic resistance and being suitable for longer-term use. This is a genuine clinical application with an appropriate evidence base.
Age-related macular degeneration. The AREDS trials established high-dose zinc as part of a combination formulation that reduces progression to advanced AMD in specific patient groups. This is a well-evidenced clinical indication but involves doses (80mg) well above routine supplementation levels and is specific to individuals with intermediate or advanced AMD.
Immune function in replete adults. Zinc modulates a range of immune biomarkers. As with vitamin C, these changes do not translate into consistent reduction in infection incidence in well-nourished populations. Immune biomarker improvements should not be interpreted as evidence of clinical infection prevention in replete adults.
The five questions
Does low status cause harm that supplementation corrects?
Yes, clearly. Zinc deficiency produces a well-characterised clinical syndrome. Low zinc status -- even when short of frank deficiency -- is associated with impaired immune biomarkers, slower wound healing, and reduced reproductive function, though clinical thresholds for mild insufficiency are not precisely defined and plasma zinc is an insensitive marker. Repletion in individuals with clearly low status reliably corrects these effects. Low zinc status is more common than is generally appreciated, particularly in older adults, vegetarians, and those with malabsorption.
Does supplementation prevent disease in at-risk populations?
Yes, in specific contexts. The strongest evidence is for diarrhoea prevention and treatment in zinc-deficient children in low-income settings, where zinc is a WHO-recommended intervention. For cold prevention specifically, the evidence supports therapeutic use at symptom onset rather than prophylactic supplementation. For AMD progression, the AREDS formulation is indicated in a defined patient group.
Does zinc produce meaningful biomarker effects?
Yes, across multiple systems -- testosterone, immune markers, inflammatory markers, and wound healing parameters all respond to zinc supplementation in low-status individuals. The important interpretive boundary is that biomarker improvements in deficient individuals do not predict equivalent benefits in replete populations.
Does zinc improve outcomes in clinical populations?
Yes, in well-defined contexts: acne, AMD progression, male infertility associated with low zinc status, and wound healing in depleted patients. For most other clinical applications the evidence is insufficient or inconsistent.
Does zinc benefit healthy, replete adults?
The evidence for routine zinc supplementation in replete healthy adults is not compelling beyond the lozenge application for cold treatment. Immune and testosterone benefits in this population are not well-supported. The case for supplementation is strongest in individuals at risk of low status.
Individual variation
Vegetarians and vegans have substantially higher zinc requirements -- estimated at approximately 50% above omnivore recommendations -- due to phytate inhibition of absorption from plant foods. Soaking and sprouting legumes and grains reduces phytate content and improves zinc bioavailability.
Older adults are at elevated risk of low zinc status through a combination of reduced dietary intake and age-related decline in absorption efficiency. This group represents one of the clearer candidates for routine supplementation consideration.
Athletes, particularly endurance athletes, lose zinc through sweat and may have increased requirements. High-dose iron supplementation can impair zinc absorption via shared intestinal transporters and is worth considering in individuals taking both.
Testing and status assessment
Zinc status is difficult to assess accurately. Plasma or serum zinc is the most commonly used measure but is insensitive -- it is buffered by homeostatic mechanisms and may remain within the normal range even when tissue zinc is depleted. Plasma zinc below 10 micromol/L indicates deficiency; values between 10 and 12 micromol/L suggest borderline status. Importantly, acute phase responses during infection or inflammation lower plasma zinc independently of true zinc status, which can confound interpretation in unwell individuals.
No single biomarker reliably identifies low zinc status short of frank deficiency, and clinical thresholds for mild insufficiency are not well defined. Clinical assessment including dietary history, risk factors, and symptom review is often more informative than a single plasma zinc measurement. Testing is most useful in individuals with clear risk factors or clinical features consistent with deficiency.
Safety
Zinc is well-tolerated at standard supplementation doses. GI side effects are the main dose-limiting factor and are more pronounced with zinc sulfate than with better-tolerated forms. The tolerable upper intake level of 40mg per day reflects the primary long-term risk of chronic high-dose supplementation, which is copper deficiency via competitive absorption inhibition. This is a clinically important interaction -- copper deficiency from zinc overuse can cause anaemia and neurological effects and is a documented consequence of unsupervised high-dose zinc use. Individuals taking more than 25mg per day on an ongoing basis should ensure adequate copper intake. Nasal zinc sprays should be avoided entirely given the documented risk of permanent anosmia.
What can reasonably be concluded
Zinc has important and well-established roles across multiple physiological systems. The evidence for benefit in deficient and low-status individuals is clear and clinically relevant. The case for routine supplementation in replete healthy adults is more limited than commercial messaging typically suggests.
The therapeutic lozenge effect for cold treatment is the most clearly evidence-based application in otherwise healthy individuals. The testosterone and immune benefits that drive much of the consumer market for zinc are principally repletion effects in low-status individuals and do not reliably generalise to replete populations.
Individuals most likely to benefit from supplementation are older adults, vegetarians and vegans, those with malabsorption conditions, and men with reproductive concerns in the context of low or borderline zinc status. Where evidence is limited or outcomes are uncertain, conclusions should be treated as provisional and subject to revision as the evidence base develops.