Essential trace metals occupy a narrow physiological window between deficiency and toxicity. At intakes below threshold, biochemical functions that depend on metalloenzymes, structural metalloproteins, and redox-active cofactors become impaired. At intakes above the tolerable upper level, the same metals that sustain enzyme function generate oxidative stress, displace competing divalent cations, and produce organ damage. The literature reviewed below covers iron, zinc, copper, selenium, manganese, cobalt, molybdenum, and chromium, synthesizing human epidemiological, clinical trial, and mechanistic evidence from eight peer-reviewed sources deposited with this topic.
Global deficiency burdens are substantial. Approximately 372 million preschool-aged children and 1.2 billion women of childbearing age are deficient in at least one key micronutrient, contributing to roughly 7% of the global disease burden annually [7]. Iron, zinc, and iodine deficiencies are the most prevalent, together with folate deficiency [7]. More than 68% of the global population (over 5 billion people) do not consume adequate iodine; deficiencies in iron, zinc, and folate affect more than 50% of the global population [7]. Subclinical deficiencies are estimated to be twice as prevalent as clinically manifest conditions [7]. Toxic element co-exposure compounds the problem in low-income settings, as lead, cadmium, and arsenic compete with or displace essential divalent metals, exacerbating functional deficiency at the same dietary intake level [7].
Iron
Body distribution and reference ranges
The human body contains approximately 4 to 5 g of iron, distributed primarily in hemoglobin, myoglobin, cytochromes, and storage forms (ferritin and hemosiderin) [4]. Normal serum iron levels are 55 to 176 µg/dL in men and 40 to 170 µg/dL in women [4]. Transferrin saturation is the principal indicator of iron transport capacity; saturation above 40% supports a diagnosis of iron overload [4]. When transferrin saturation rises to 60 to 70%, the quantity of non-transferrin-bound (free) iron increases substantially, raising the probability of oxidative tissue damage, hepatic accumulation, and cardiac muscle injury [4].
Deficiency
Iron deficiency is the most prevalent nutritional deficiency worldwide. According to the World Health Organization, anemia affects over 30% of women of reproductive age and is primarily attributed to iron deficiency in most regions [7]. Approximately 30% of women and 40% of pregnant women and children under 5 years of age have iron deficiency globally [4]. Clinical manifestations include hypochromic microcytic anemia, fatigue, impaired immune function, and neurodevelopmental impairment in children [7]. In aging populations, iron deficiency anemia responds to intravenous iron supplementation; multiple randomized controlled trials in heart failure patients have demonstrated reductions in cardiovascular events by correcting iron deficiency, although this evidence was not a primary focus of the sources reviewed here [2].
Iron deficiency is particularly difficult to assess in clinical settings because ferritin, the most widely used storage marker, is an acute-phase reactant. In inflammatory states, malnutrition, liver disease, and malignancy, ferritin may be elevated regardless of iron stores; concurrent measurement of acute-phase proteins such as C-reactive protein and alpha-1-acid glycoprotein is recommended to adjust ferritin interpretation [7]. The reticulocyte hemoglobin equivalent, available in most modern hematology analyzers, can detect functional iron deficiency without the acute-phase confounding that affects ferritin [7].
Overload
Excess iron causes tissue damage through non-transferrin-bound iron-mediated generation of hydroxyl radicals (Fenton chemistry). When transferrin saturation exceeds 60 to 70%, the amount of free iron increases substantially and is more likely to damage tissues, accumulate in the liver, and cause oxidative stress damage to cardiac muscle cells [4]. Chronic iron overload (hereditary hemochromatosis or transfusional overload) produces hepatic fibrosis, cirrhosis, cardiomyopathy, arrhythmias, and endocrine dysfunction.
An extreme case of disturbed iron regulation is documented in anorexia nervosa. A 318-woman outpatient cohort found low serum iron in only 6.2%, whereas ferritin was often elevated and transferrin sometimes reduced [8]. One severely malnourished woman with anorexia nervosa had extreme hyperferritinemia, with serum ferritin reaching 9,096 µg/L [8]. This pattern illustrates how nutritional depletion can dissociate storage iron markers from functional iron status, complicating clinical assessment.
Drug interactions affecting iron status
Several commonly prescribed medications impair iron status. Antacids, bile acid sequestrants, lipid-lowering drugs, non-steroidal anti-inflammatory agents, opioids, and proton pump inhibitors have been identified as clinically significant drugs compromising iron bioavailability or absorption [2].
Zinc
Body distribution and reference ranges
The adult human body contains 1.4 to 2.3 g of zinc, distributed across skeletal muscle, bone, the liver, kidney, skin, and prostate [5, 4]. Serum zinc is the most widely used clinical indicator, though it has low specificity; levels plateau once physiological requirements are met and are sensitive to fasting, diurnal variation, and systemic inflammation [7]. The recommended dietary allowance is 11 mg/day for adult men and 8 mg/day for adult women; the tolerable upper intake level is 40 mg/day for adults [3, 1].
Deficiency
Zinc deficiency affects approximately one-third of the global population [7]. Clinical features include growth retardation, impaired immune function, delayed wound healing, dermatitis, hypogonadism in males, and cognitive impairment [7]. In children and adolescents, zinc deficiency increases infectious morbidity and reduces linear growth [7]. In anorexia nervosa, zinc deficiency was present in 64.3% of patients in an inpatient cohort of 374 participants (mean serum zinc 64.5 to 65.7 µg/dL versus a clinical reference of 80 to 120 µg/dL) [8].
Globally, vegetarian and vegan dietary patterns confer risk of zinc deficiency because plant-based zinc is less bioavailable than zinc in animal-source foods; phytates in whole grains and legumes inhibit zinc absorption [7]. Aging populations are additionally vulnerable because reduced food intake, impaired absorption, and polypharmacy all depress zinc status [2].
Zinc deficiency in pregnancy has been examined in a meta-analysis of 16 randomized controlled trials, which reported a 14% reduction in preterm births following zinc supplementation, with no other statistically significant maternal or neonatal benefits observed [1]. The majority of trials in that meta-analysis administered 20 to 30 mg of zinc predominantly as zinc sulfate [1].
Excess
Excess zinc above the tolerable upper level of 40 mg/day produces toxicity through several converging mechanisms. At the enzymatic level, excess zinc dysregulates copper-zinc superoxide dismutase (Cu/Zn SOD), reduces glutathione peroxidase (GPX) and catalase (CAT) activity, and impairs antioxidant defense [3]. Dyslipidemia, insulin resistance, leptin resistance, and hypertension have been reported with excess zinc [3].
Epidemiological data from a cross-sectional analysis found a 3.24-fold higher risk of prediabetes and a 51% higher risk of type 2 diabetes mellitus in participants with higher serum zinc concentrations, with proposed mechanisms including impaired glucose transport and altered insulin signaling [3]. Elevated serum zinc was associated with higher systolic and diastolic blood pressure, and with increased odds of hypertension (odds ratio approximately 1.68, though the precise figure in that study requires verification against p. 4 of the source) [3].
Zinc-copper antagonism is the best-characterized nutrient-nutrient interaction in zinc toxicity. At supplemental doses of 150 mg/day of elemental zinc for six weeks, significant declines in serum copper and ceruloplasmin activity were observed in clinical studies [1]. The mechanism is induction of intestinal metallothionein by excess zinc; metallothionein has a higher affinity for copper than zinc and sequesters copper in enterocytes, reducing its absorption and producing secondary copper deficiency [1, 4].
Supplementation in aging
In an RCT of adults over 65 years (n = 725), zinc supplementation together with selenium, vitamins C, E, and beta-carotene over two years significantly reduced the risk of upper respiratory tract infections and shortened infection duration compared with placebo (p less than 0.01 for both outcomes) [2]. In zinc-deficient elderly populations specifically, supplementation significantly reduces the number of episodes and duration of acute upper airway infections [2].
Copper
Dietary requirements and sources
The recommended dietary allowance for copper in adults is 900 µg/day; requirements increase to 1,000 µg/day during pregnancy and 1,300 µg/day during lactation [1]. Copper is concentrated in foods of both animal and plant origin. Oysters contain 44,996 µg per 100 g; beef liver contains 6,434 µg per 100 g; and cocoa contains approximately 5,000 µg per 100 g [1]. Nuts, legumes, whole grains, and dark leafy vegetables are significant plant-based sources [4].
Deficiency
Copper deficiency is clinically manifest as anemia (not responsive to iron supplementation), neutropenia, and neurological complications including myelopathy and peripheral neuropathy [7]. Bone abnormalities including osteoporosis occur with chronic copper deficiency [7]. Secondary copper deficiency arising from excess zinc is mechanistically established: zinc induces intestinal metallothionein, which preferentially sequesters copper and prevents its absorption [1, 4].
Drugs that compromise copper status include antacids and anticonvulsants, primarily by reducing absorption [2].
Toxicity
Acute copper excess causes nausea, vomiting, diarrhea, and hepatic necrosis. Chronic copper accumulation occurs in Wilson’s disease, an autosomal recessive disorder of copper transporter ATP7B, producing hepatic cirrhosis, neurological deterioration, and psychiatric disturbance [4]. The clinical assessment of copper status uses serum copper and ceruloplasmin; ceruloplasmin reflects the functional copper pool but is an acute-phase reactant and is elevated during pregnancy and in inflammatory states [7].
Copper-zinc superoxide dismutase
The Cu/Zn SOD (superoxide dismutase 1, SOD1) requires both copper and zinc for its antioxidant function. Copper at the active site performs the redox cycle that converts superoxide to hydrogen peroxide; zinc stabilizes the protein structure. Dysregulation of this enzyme by either copper deficiency or excess zinc is therefore a convergent mechanism linking both metals to impaired antioxidant defense [3, 1].
Selenium
Status assessment and deficiency threshold
Selenium status is most accurately assessed using plasma selenoprotein P (SePP), which reflects selenium supply to tissues, though SEPP1 assays require specialized equipment not widely available in clinical settings [7]. Serum selenium reflects recent dietary intake but is not specific to the functional selenium pool and is affected by dietary selenium forms [7]. Glutathione peroxidase (GPx) activity is a functional marker sensitive to deficiency states, particularly effective for detecting low selenium, but less useful for detecting excess because GPx activity plateaus once selenium is sufficient [7].
A deficiency threshold of plasma selenium below 80 µg/L was used in clinical studies in anorexia nervosa, approximating the concentration required for optimal selenoprotein activity [8]. Applying this threshold, selenium deficiency was present in 40.6% of patients in a 153-patient cohort; a separate cohort of 153 patients found 53.6% (82 of 153) with selenium below 80 µg/L [8]. Selenium serum reference values across European and North American populations (Table 7 of Lopez-Alonso et al. 2025) range from a median of 65.7 µg/L (Serbia) to 130 µg/L (Australia), illustrating substantial geographic variation driven by differences in soil selenium content and dietary patterns [7].
Deficiency
Classical selenium deficiency diseases are geographically circumscribed. Keshan disease, an endemic cardiomyopathy, is associated with very low selenium in soils and food of certain regions of China. Kashin-Beck disease, an osteochondropathy, occurs in selenium-deficient populations in China and Central Asia. Beyond these endemic presentations, low selenium is associated with impaired thyroid function, compromised immunity, and reduced muscle strength in aging populations [2]. Mercury exposure further compounds selenium deficiency because inorganic mercury binds selenium, reducing its bioavailability for selenoprotein synthesis [7].
In aging populations with low selenium, supplementation with selenium (200 µg/day) combined with coenzyme Q10 (200 mg/day) for 48 months reduced cardiovascular event rates and improved age-related biomarkers including insulin-like growth factor 1, adiponectin, and osteoprotegerin in a Swedish RCT of 441 adults (p less than or equal to 0.02 for the biomarker comparisons) [2].
Toxicity
Selenium has the narrowest margin between deficiency and toxicity of the essential trace metals. Excess selenium causes selenosis, characterized by garlic-breath odor (from dimethyl selenide exhalation), hair loss, nail brittleness, neurological symptoms, and in severe cases, liver failure. The safe dose range for population supplementation is narrow; clinical trials have used selenium at doses of 100 to 200 µg/day, which are generally well tolerated when baseline selenium status is confirmed as deficient or low-normal.
Drugs that interfere with selenium status include diuretics (which increase urinary selenium loss) and statins (which interfere with selenoprotein synthesis) [2].
Manganese
Essential functions
Manganese is required for carbohydrate, protein, and lipid metabolism, and is a structural cofactor in manganese superoxide dismutase (MnSOD), the primary mitochondrial antioxidant enzyme [4]. Manganese-dependent enzymes include arginase, glutamine synthetase, phosphoenolpyruvate decarboxylase, pyruvate carboxylase, and manganese superoxide dismutase [4]. Daily manganese requirements differ across health institutions; the US Food and Drug Administration estimates of adequate intake are 2.3 mg/day for adult men, 1.8 mg/day for adult women, 1.5 mg/day for children 4 to 8 years, and 600 µg/day for children 7 to 12 months [4].
Deficiency
Clinical manganese deficiency is uncommon because manganese is present in numerous dietary sources. Experimentally induced manganese deficiency in humans produced transient skin eruptions, decreased serum cholesterol, and effects on bone remodeling, as well as impaired reproductive development, altered mood, increased premenstrual pain, and lower scores on tests of cognitive flexibility and processing speed in children [4].
Toxicity
Manganese intoxication is of increasing importance as a public health concern, particularly in susceptible populations including children [4]. Environmental manganese accumulates in the brain, particularly in the cerebral basal ganglia, and produces manganism, a neurological disorder clinically resembling Parkinson’s disease [4]. Manganism was initially described in occupationally exposed workers in mining, metallurgy, battery production, and ferromanganese alloy plants. High manganese exposure in children is associated with reduced IQ, increased oxidative stress, mitochondrial dysfunction, endoplasmic reticulum stress, increased cell death, neuroinflammation, and interference with neurotransmitter metabolism [4]. Epigenetic alterations affecting neurological development and vulnerability to other nervous system disorders have also been reported with excess manganese [4].
A comprehensive survey of 14,863 soil samples across India found that 5% of soils were potentially deficient in manganese, alongside 49% deficient in zinc, 33% in boron, 12% in iron, 11% in molybdenum, and 3% in copper [6]. Because human manganese exposure is primarily dietary and because plant manganese content depends on soil availability, regional soil deficiency directly affects human dietary adequacy.
Cobalt
Essential function and requirements
Cobalt’s essential role in human metabolism is confined to its function as the redox-active core of vitamin B12 (cobalamin). Cobalt is required for the activity of methionine synthase and methylmalonyl-CoA mutase, enzymes essential for DNA synthesis, erythropoiesis, and neurological integrity [4]. The general population total dietary intake of cobalt (including inorganic cobalt from food and water) ranges from 5 to 50 µg/day; established safety limits (AFSSA acceptable daily intake) range from 1.6 to 8 µg/kg per day [4]. Serum cobalt concentrations of 300 to 700 µg/L are associated with reversible hematological and endocrine effects; levels at or above 700 to 800 µg/L can cause more severe neurological, reproductive, and cardiac effects [4].
Deficiency
Cobalt deficiency occurs exclusively as vitamin B12 deficiency. Clinical manifestations include megaloblastic anemia, macrocytosis, and ineffective erythropoiesis [4]. Neurological manifestations include neuropathy from cobalamin deficiency, cognitive deterioration, motor alterations, and myelin damage. Biochemically, hyperhomocysteinemia and methylmalonic aciduria increase the risk of cardiovascular disease and neurodegenerative disease [4]. Vegetarian and vegan populations are especially vulnerable to vitamin B12 deficiency [4].
Toxicity
Excess cobalt, particularly as free Co2+ ions, produces cobaltism, a multisystemic syndrome with neurological manifestations (hearing loss, tinnitus, vertigo, and peripheral neuropathy with paresthesias and weakness), cardiomyopathy, tachycardia, hypothyroidism, and goiter through iodine uptake inhibition [4]. Cobaltism is primarily a concern in patients with metal-on-metal hip prostheses, which release Co2+ ions into circulation; it is also seen with high-dose cobalt supplementation formerly used as an anti-anemia agent in patients with renal failure [4].
Molybdenum
Essential function and dietary intake
Molybdenum’s essential biological function derives from its incorporation into the molybdenum cofactor (Moco), a pterin-based structure (molybdopterin) that coordinates the molybdenum ion and is a component of four mammalian enzymes: sulfite oxidase (oxidizes sulfur-containing amino acids), xanthine oxidoreductase (metabolizes purines), aldehyde oxidase (metabolizes aldehydes and exogenous compounds), and mitochondrial amidoxime reductase (mARC1/2, involved in lipogenesis and nitrite conversion) [4]. Human dietary intake represents the main source of molybdenum, with an estimated daily intake of 0.1 to 0.5 mg; food rich in molybdenum includes legumes, cereal grains, liver, kidneys, milk, and certain leafy vegetables [4].
Deficiency and toxicity
Nutritional molybdenum deficiency in humans is an infrequent phenomenon because molybdenum is present in a wide variety of foods. The most clinically documented form is molybdenum cofactor deficiency (MoCD), an autosomal recessive genetic disorder affecting 1 in 200,000 newborns [4]. MoCD presents at birth with seizures and rapid neurological deterioration caused by accumulation of sulfite and S-sulfocysteine due to loss of sulfite oxidase activity; it results in severe neurodegeneration and lens dislocation [4]. Excess molybdenum (molybdenosis) is rare in humans; in ruminants, it leads to secondary copper deficiency by forming thiomolybdates in the rumen that bind copper [4].
Chromium
Status and essentiality
Chromium occupies a contested position in human nutrition. The historical concept of a “glucose tolerance factor” containing chromium that facilitates insulin action has not been structurally characterized, and a specific glucose tolerance factor is no longer accepted by the scientific community [4]. Recent evidence suggests that the association between chromium status and type 2 diabetes mellitus may be confounded by other variables and does not persist after rigorous statistical correction [4]. The recommended maximum daily intake of chromium is 10 to 40 µg for children up to six months, 35 µg in men, and 25 µg in women [4].
Clinical studies have reported lower serum chromium levels in patients with type 2 diabetes mellitus compared with healthy controls, but large-scale intervention trials and meta-analyses have shown inconsistent and at best modest effects of chromium supplementation (such as chromium picolinate) on glycemic control markers including glucose and HbA1c [4]. While chromium is an essential trace element for some animal species, its essentiality in humans has not been definitively proven [4].
Toxicity by oxidation state
Chromium(III) (the trivalent form found in foods) is considered to have low toxicity; it is poorly absorbed, rapidly eliminated in urine, and not a strong oxidant [4]. Chromium(VI) (hexavalent chromium) is highly toxic and carcinogenic; it is a strong oxidant with high oral and inhalation absorption, is classified as a Group 1 carcinogen for lung cancer by IARC, and accumulates in organs before being reduced to Cr(III) and eliminated in urine [4]. Acute Cr(VI) exposure by inhalation or oral route causes respiratory distress, vomiting, hemorrhage, dizziness, and can produce multiorgan failure, coma, and death; chronic exposure causes asthma, lung cancer, allergic dermatitis, and kidney and liver failure [4].
The deficiency-to-toxicity continuum: cross-metal considerations
Several principles cut across individual metals.
The U-shaped dose-response is universal for essential metals: deficiency at low intakes and toxicity at high intakes bracket a range of adequacy, with the optimal intake window varying substantially by metal [4, 7]. For selenium, the adequacy window is narrow (approximately 55 to 400 µg/day for adults, with considerable individual variation); for manganese, the adequacy window is wide because manganese is efficiently regulated by biliary excretion [4].
Metal-metal competition governs absorption and function at multiple loci. Iron, zinc, copper, and manganese compete for divalent metal transporter 1 (DMT1) in the intestinal apical membrane; excess supplementation with one metal can produce secondary deficiency of another [4, 7]. Zinc excess induces metallothionein, which sequesters copper and reduces copper absorption, producing secondary copper deficiency as a recognized clinical complication of zinc supplementation [1]. Mercury antagonizes selenium by forming an irreversible mercury-selenium complex (tiemannite), sequestering selenium and impairing selenoprotein synthesis [7].
Aging amplifies both the risk of deficiency and the complexity of assessment. Aging is associated with reduced food intake, impaired absorption (particularly of iron from plant sources, zinc from phytate-rich diets, vitamin B12-bound cobalt from protein-bound food sources), and increased prevalence of medications that compromise micronutrient status [2]. In a physiology-based analysis of supplementation strategies for healthy aging, the critical micronutrients to consider for combined deficiency screening in adults aged 50 years and older include vitamins A, B-complex, C, D, and E, together with iron, selenium, and zinc [2]. An RCT evidence table from Berger et al. 2026 [2] identified the AREDS (antioxidants and zinc), COSMOS (multivitamin), DO-HEALTH (vitamin D plus EPA plus DHA), VITACOG (B vitamins), and Swedish selenium-coenzyme Q10 trials as producing positive outcomes across immunity, AMD progression, cognitive decline, infection incidence, and cardiovascular biomarkers respectively, illustrating that targeted supplementation in populations with demonstrated deficiency or suboptimal status can produce clinically meaningful benefits.
Biomarker limitations constrain clinical and epidemiological assessment across all essential metals. Serum or plasma concentrations are the most widely available indicator, but they reflect recent intake, are affected by inflammation (acute-phase response), and plateau once physiological requirements are met, limiting their utility for detecting excess [7]. Functional biomarkers (erythrocyte SOD activity for zinc, GPx activity and SEPP1 for selenium, ceruloplasmin for copper, ferritin and transferrin saturation for iron) provide information on the adequacy window and are more sensitive to deficiency [7]. Harmonized international reference intervals for serum trace minerals do not exist; Table 7 of Lopez-Alonso et al. 2025 [7] documents substantial between-country variation in serum copper (median 729 µg/L in Serbia to 1,226 µg/L in Denmark), serum zinc (median 660 µg/L in Denmark to 1,150 µg/L in Australia), and serum selenium (median 65.7 µg/L in Serbia to 130 µg/L in Australia), driven by soil mineral content, dietary patterns, and analytical methodology differences.
What remains uncertain
The essentiality of chromium in human metabolism remains scientifically unresolved. Existing evidence for chromium’s role in glucose tolerance is inconsistent and may reflect confounding rather than a causal relationship between chromium status and glycemic control [4].
Dose-response relationships for manganese toxicity at environmental (non-occupational) exposure levels in adults are not established from the reviewed sources. The reviewed studies characterize high-exposure occupational or mining contexts rather than dietary manganese toxicity.
The optimal intake window for selenium supplementation in aging populations is not defined from the reviewed sources, which describe selenium deficiency thresholds and RCT outcomes but do not establish dose-response relationships for the transition from benefit to harm in older adults.
The biomarker evidence base for cobalt status (outside of industrial exposure) is limited. Established serum thresholds for cobalt toxicity in the general population are derived primarily from metal-on-metal prosthesis surveillance studies and may not translate to dietary or supplemental cobalt exposure contexts.
Soil mineral content is the primary upstream driver of human selenium, zinc, and manganese exposure through the food supply. India-wide survey data from Mandal et al. 2026 [6] found 49% of soils potentially deficient in zinc and 11% potentially deficient in molybdenum, but comprehensive global mappings of soil-to-food-to-human transfer efficiency for these metals across regions are not available from the reviewed sources.
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