The abuse and toxicity of zinc

Zinc is a nutritionally essential element. It is necessary for growth and is involved in several physiological functions. Its deficiency results in severe health problems. More than 70 human enzyme systems depend on zinc for their functions. It helps in the normal tissue function as well as the digestion and metabolism of phosphorus.

The range of normal plasma zinc level is from 85 to 110 microgram per deciliter (ug/dL). Severe deficiency may decrease plasma zinc to 40 to 60 ug/dL. Zinc concentration in tissues varies widely. Liver receives up to about 40% of a dose. The greatest concentration of zinc in the body is in the prostate.

The recommended dietary allowance for zinc is 15mg to 18mg per day for men and 12mg per day for women. Children need approximately 10mg and infants 5mg every day. Young men who do not get enough zinc may have poorly developed ex organs.

A zinc deficiency in a pregnant woman may lead to growth retardation in her offspring. In the newborn, zinc insufficiency may be manifested by dermatitis, hair loss, impaired healing and susceptibility to infections.

Zinc enters the environment in a number of ways. Some is released through natural processes but most comes from human activities such as mining, coal burning, steel production and the burning of waste.

Zinc attaches to soil, sediment and dust particles in the air. Its compounds can move into the ground water as well as lakes, streams and rivers. It is also present in certain foods. These include oysters, meats, whole grains, dairy products, nuts and legumes.

Zinc also has many commercial uses. It is used as coating to prevent rusting and mixed with other metals to produce alloys such as brass and bronze. Its compounds are also widely used in the manufacturing of paint, rubber, dye, wood preservatives and ointments. Some of the compounds and their uses are listed in Table 1.

However, some of these can be hazardous. These include zinc chloride, zinc sulfate, zinc sulphide, zinc chromates and zinc oxide. Zinc oxide has been identified as the main cause of metal fume fever. Among its symptoms are headache, fever, chills, muscle aches and vomiting. Metal fume fever occurs in workplace environments involving welding, melting and smelting processes. The fume is irritating to the upper respiratory tract. An outdoor exposure to zinc chloride fume may also be encountered from smoke generators and smoke bombs.

Zinc salts are widely used in the cosmetics industry. If ingested, they can produce irritation or corrosion of the gastrointestinal system. A mixture of the zinc salts of stearic and palmitic acids are used to prepare zinc soaps. It is also used in bath preparations, hand creams, lotions and ointments. The inhalation of its powder may cause lung problems, leading to death in infants.

Zinc phosphide, which emits an unpleasant smell, is always subjected to scrutiny as well. It is often used as an active ingredient in rodenticide but some of its formulations have been classified as highly toxic. Zinc phosphide reacts with water and acid in the stomach to release phosphine gas which in turn causes cell toxicity with necrosis of the gastrointestinal tract. It also injures other organs such as the liver and kidneys. Symptoms of acute zinc phosphide poisoning include nausea, shock, weak heart beat and low blood pressure.

The ingestion of elemental zinc has been shown to cause lethargy. Eating large amount of zinc can cause stomach cramps, nausea and vomiting. Most reports of acute toxicity are related to food poisoning incidents. Several resulted from storage of food or drinks in galvanised containers. In a chronic exposure, it can cause anaemia and pancreas damage. It also tends to lower the concentration of high density lipoprotein cholesterol.

The intake of zinc, even at a low supplementary concentration, has been suggested to interfere with the utilisation of copper and iron and to adversely affect cholesterol concentrations.

Severe copper deficiency can result from prolonged therapy with zinc supplement. The physiological requirement for zinc is 10mg to 15mg per day. However, the suggested pharmacological dose of zinc is 10 times higher, a dose at which toxicity has been reported. Even at a certain therapeutic dose, toxicity to the bone marrow has been implicated.

In retropect, there are a few cases of massive zinc ingestion which are worth mentioning. In the first case, a man was diagnosed with having a peculiar form of anaemia. Armed with some medical knowledge, this man had ingested excessive zinc supplement in his attempt to relieve his symptoms of rage. He associated his condition with an excess of body copper and hence ingested zinc to overcome it.

The patient knew that zinc interfered with the action of copper which caused it to be depleted from the body. However, he was unaware of the consequences. It seemed he had taken the message "prevention is better than cure" a bit too far.

In another incidenct, a man sought medical help after suffering for a few months from a feeling of illness, discomfort and tiredness. A medical examination showed that he had developed profound anaemia. It was later discovered that his previous medications included a zinc supplement of 450mg per day for two years.

The history of this massive zinc ingestion suggested a connection of his anaemia with zinc as it intefered with copper absorption and copper deficiency. Oral zinc increases faecal excretion of copper and blocks the absorption of ingested minerals. In this case, a series of complex zinc-copper relationship and metabolism resulted in the inhibition of copper absorption and increased faecal loss of copper through saliva, gastric juices and biliary secretions.

Two other cases reported also involved patients taking self-prescribed zinc to treat "prostatitis". They both had taken the medication for at least two years at a daily dose of between 200mg and 400mg. In both cases, the patients responded to the withdrawing of zinc or to giving copper.

These cases sufficiently delineate that although zinc is essential for life, excessive exposure may be harmful.

The writer is a pharmacist at the National Poison Centre, Universiti Sains Malaysia, Penang.