Oysters are a rich natural source of zinc.
Published in the Journal of Clinical Pathology, a publication from The BMJ, the findings come from an analysis of case notes from 70 patients prescribed zinc supplements at Glasgow hospitals in the decade from 2000-10.
The study found that 62% of patients were prescribed zinc at doses sufficient to cause copper deficiency.
"These findings underline the lack of awareness of zinc-induced copper deficiency," conclude the authors. They continue:
"Zinc is an essential trace element and so clinicians may consider it a safe nutrient rather than a drug carrying potential risk.
"This study offers persuasive evidence of a potential risk of iatrogenic copper deficiency being unwittingly caused by prescribing high doses of zinc."
The authors point out that the zinc requirement for adults is less than 10 mg a day. The dose most commonly prescribed, however, is 135 mg a day - yet "there is no evidence to support the prescription of zinc" at this level. The researchers found:
"In a significant number of patients, zinc was prescribed after zinc deficiency had been erroneously diagnosed following misinterpretation of a low plasma zinc concentration."
Too much zinc in the form of dietary supplements can disrupt the uptake of copper, the researchers explain, leading to neurological problems and anemia.
Zinc is an essential trace element. The daily need for men is between 5.5 and 9.5 mg, and 4-7 mg for women. Zinc supplements are usually available in formulations of 45 mg or 50 mg, though - and the US recommended tolerable limit is 40 mg a day.
Short-term zinc supplementation may not be harmful, consider the researchers - but longer-term use may be. The authors say:
"This study highlights the potential risk of developing zinc-induced copper deficiency as a result of such prescribing."
Nerve problems included pain and tingling
In the analysis of patient records, the researchers looked at:
- Reasons for advising zinc treatment
- Whether the family doctors had been warned about the potential impact of high doses of zinc
- Duration of treatment
- Development of anemia or neurological symptoms.
Lab test results were also collated where available, for levels of zinc, copper, albumin and C-reactive protein (CRP). (Low zinc levels are linked to low albumin levels and high CRP.)
Information on zinc supplements showed that 21 cases (43% of 52 patients yielding information) were for correcting zinc deficiency.
Zinc levels were measured before prescribing in over half of all cases studied - yet copper level was assessed only in two patients, and 60% of patients were prescribed zinc at daily doses of between 90 mg and 180 mg a day, say the researchers.
Only one set of case notes showed the GP had been warned about the potential effects of long-term zinc use.
The researchers found problems typically associated with zinc-induced copper deficiency: anemia, neutropenia (low white cell count) and/or neurological symptoms developed in 13 patients.
Example problems included neuropathy (peripheral nerve pain), paraesthesia (tingling in the fingers), ataxia (difficulties with balance and coordination) and leg nerve pain.