Trace Elements: Zinc, Copper, Selenium and the Rest

What "trace" really means

Tiny quantity, non-negotiable role. A trace element is a mineral the body needs in vanishingly small amounts — milligrams or micrograms daily — yet cannot manufacture and cannot function without. Most work as enzyme cofactors: a single metal ion sitting at the active site of an enzyme, doing the chemistry the protein alone cannot. Because the requirement is so small, deficiency rarely comes from a normal diet. It comes from three recurring situations that every prescriber should learn to smell: malabsorption or high-output gastrointestinal losses (short bowel, fistulae, chronic diarrhoea), long-term parenteral nutrition without the right additives, and restrictive states such as after bariatric surgery. Hold onto that trio — it is the through-line of this whole chapter, and the reason trace elements belong in a pharmacology course at all.

Zinc: the cofactor of hundreds of enzymes

Zinc is the workhorse. It is a structural and catalytic cofactor for hundreds of enzymes and for the "zinc-finger" proteins that switch genes on and off, so it touches almost every fast-turnover tissue: skin, gut lining, immune cells, the growing child. Deficiency therefore hits all of them at once. The classic picture is impaired wound healing and a dermatitis that favours the mouth, the tips of the fingers and the perineum — the acral, perioral rash of acrodermatitis enteropathica. Add hair loss (alopecia), a blunted or distorted sense of taste and smell, and a weakened immune system with frequent infections. In children the signature is growth retardation and diarrhoea — and here zinc becomes a public-health drug: WHO recommends oral zinc alongside oral rehydration for childhood diarrhoea, because it shortens episodes and cuts diarrhoeal mortality. Causes follow the familiar trio: malabsorption, high-output GI losses, and parenteral nutrition given without zinc.

Zinc is also a deliberate therapy, not just a deficiency to correct. Two prescribing uses are worth knowing. In Wilson's disease, oral zinc is given precisely because it blocks copper absorption from the gut — it induces a protein called metallothionein in the intestinal cells that binds copper and carries it away in shed cells, so zinc is used as maintenance therapy to keep copper from re-accumulating (see the Toxicology / hepatology chapter). And in age-related eye disease, zinc is a component of the AREDS antioxidant formulation used to slow progression of age-related macular degeneration (a link across to the Ophthalmology section). The same property that makes zinc therapeutic in Wilson's — its power to shut down copper uptake — is exactly what makes casual high-dose zinc dangerous, as the next section shows.

Copper: iron's partner, and the Wilson's overload

Copper is the quieter partner, but it does two jobs students under-rate. First, it is essential for iron metabolism: the copper-containing protein ceruloplasmin (and its relatives) is needed to load iron for transport, so copper deficiency produces an iron-refractory anaemia — you can give all the iron you like and the anaemia will not budge. Second, copper enzymes cross-link connective tissue and build the covering of nerves. Deficiency is genuinely rare, but when it appears the causes are, again, the familiar trio plus one drug trap: malabsorption, bariatric surgery, parenteral nutrition without copper — and excess zinc ingestion, the metallothionein mechanism from the see-saw. The clinical picture is a beautiful mimic: anaemia and neutropenia that on a blood film can look exactly like myelodysplasia (a hard-won pearl for the Haematology section), together with a myeloneuropathy — a spinal-cord and nerve syndrome of numbness, tingling and unsteady gait. Recognize the pattern and you save the patient a marrow-failure work-up and simply replace the copper.

Copper's other face is not too little, but too much. Wilson's disease is the overload counterpart — an inherited failure to excrete copper into bile, so it accumulates in the liver, brain and eyes. The result is liver disease (from hepatitis to cirrhosis and acute liver failure), a movement and psychiatric disorder from copper in the basal ganglia, and the Kayser–Fleischer rings of copper in the cornea. The pharmacology is a neat mirror image of deficiency: you remove copper. Chelators — penicillamine or the better-tolerated trientine — bind copper and drag it out in the urine, and oral zinc is used, as above, to block further absorption for maintenance. Wilson's sits at the crossroads of this chapter, toxicology and hepatology, and is the reason copper appears twice in every exam: once as a deficiency mimicking marrow failure, once as an overload treated by chelation.

Selenium: the antioxidant metal

Selenium sits at the active site of glutathione peroxidase, one of the body's front-line antioxidant enzymes that mops up peroxides before they damage membranes — so selenium is fundamentally about protecting tissue from oxidative injury. Its deficiency syndrome is memorable because it is named: Keshan disease, a dilated cardiomyopathy first described in a selenium-poor region of China, alongside a skeletal myopathy. In modern practice selenium deficiency is a parenteral-nutrition problem — it is one of the trace elements that must be added to long-term TPN, exactly like zinc, copper, chromium and manganese. Miss it and a patient fed for weeks can develop a cardiomyopathy that no cardiac drug will fix, because the problem is a missing cofactor, not a failing pump.

The rest, in brief — and where iodine goes

The remaining essential trace elements deserve a line each. Chromium supports the action of insulin and normal glucose tolerance; deficiency (again, a long-TPN phenomenon) can produce a glucose intolerance that mimics diabetes. Manganese and molybdenum are cofactors for a handful of enzymes and are rarely deficient outside pure parenteral feeding. Fluoride is the dental element: too little and dental caries increase; too much and you get fluorosis — mottled, weakened enamel — a genuine excess syndrome, not just a deficiency one. Cobalt matters only as the cobalt atom sitting at the heart of vitamin B12, so its story is really the B12 story told in the Vitamins chapter. And iodine — the trace element with the biggest clinical footprint of all — gets its own dedicated chapter, because its single job (building thyroid hormone) and its deficiency disease (goitre and, in the fetus, congenital hypothyroidism) are large enough to stand alone; see the Iodine & thyroid chapter.