Published: July 2000
Prescriber Update 20: 39-42
Medsafe Editorial Team
Selenium is an essential trace element. Although concentrations in New Zealand soils are low, there is no
indication that this has resulted in any detrimental effects on the health of New Zealanders. With current
levels of animal and poultry supplementation of selenium and consumption of imported plant foods, especially
wheat and legumes, it appears that intake of selenium by most New Zealanders is at or around recommended
levels, as indicated by the 1997 National Nutrition Survey and the 1997/98 New Zealand Total Diet Survey.
The current recommended daily intake in the US for adults is 55µg.
Some New Zealanders take selenium supplements with the intention of reducing the oxidative damage of free radicals. The daily dose recommended on the label of these supplements is usually 50-200µg. The Dietary Supplements Regulations 1985 require supplements to have a maximum adult dose of 150µg/day. The maximum safe daily intake is 400µg.
Symptoms of selenium toxicity include a garlicky odour in the breath, fatigue, gastrointestinal symptoms, transverse lines on the nails, alopecia, and peripheral neuropathy. Treatment is by supportive care. There is no known effective antidote. Symptomatic recovery may be quite rapid, occurring within 2 weeks in one case.
Selenium is an essential trace element, used in particular in the glutathione peroxidase enzyme system which protects intracellular structures against oxidative damage. In foods it is present largely as the amino acids selenomethionine and selenocysteine, in which it replaces the usual sulphur atom.
A deficiency of selenium in an area of China has resulted in an endemic form of cardiomyopathy, called Keshan disease. Selenium has low concentrations in most New Zealand soils, but there has been little indication that the low intake has resulted in any detrimental effects on the health of New Zealanders. The disease patterns for coronary artery disease, hypertension and cancer are similar to those in Western countries with far higher selenium intakes.1
An evaluation of selenium requirements completed in 2000 by the US Institute of Medicine revised the American recommended adult intake to 55µg/day, the level at which the enzymes with antioxidant functions are at maximum activity.2 A recent study3 conducted in New Zealand estimated that a suitable minimum intake for New Zealanders, achievable without use of supplements, is 39µg/day. At this level plasma glutathione peroxidase is at two-thirds of maximal activity which was thought to be sufficient by the WHO/IAEA/FAO Expert Committee.4
The 1997 New Zealand National Nutrition Survey,5 based on recall of food consumed during the previous day, calculated a mean daily selenium for men aged ≥ 15 years of 60µg/day and for women aged ≥ 15 years of 44µg/day. The 1997/98 Total Diet Survey6 examined selenium intake in two groups of men (young male 19-24 years and adult male > 25 years) and two groups of women (adult female > 25 years and lacto-ovo vegetarian female 19-40 years) using simulated diets. Estimated intakes for both groups of men were in excess of the US recommendation, while the estimated intakes for the women coincided with the US recommended level, 55µg/day. The intake calculated in the National Nutrition Survey is considered to be a more representative indication of dietary intake of selenium by New Zealanders.
The intake of selenium by New Zealanders has increased since the earlier Total Diet Surveys in 1982 and 1987/88.6 To prevent animal diseases, farm animals are drenched with selenium-enriched products and the meal fed to poultry has selenium added. Generally bread made in the South Island is lower in selenium than bread made in the North. Since deregulation of the grain industry much North Island bread has a significant proportion of imported, largely Australian wheat which is selenium-rich. But South Island bread is made predominantly with wheat grown locally in low-selenium soils. Current practices need to continue for the selenium intake of New Zealanders to remain around recommended levels.
Meats, eggs, dairy products and bread are the main sources of selenium in New Zealand diets.6 Kidney, liver and seafood, and for the vegetarian, imported legumes are rich in selenium.
Some people use selenium supplements as a prophylactic against cancer and cardiovascular disease, but its value for either purpose is not well established.4,7 One placebo-controlled study of patients with a history of basal cell or squamous cell carcinomas of the skin found a significantly lower rate of total cancer incidence among the group taking selenium.8 These results need to be confirmed by further large scale long term studies.8.9
The dose recommended on the label of selenium supplements is usually 50-200µg daily. The Dietary Supplements Regulations 1985 require selenium supplements to be manufactured and labelled so that the recommended daily dose is no more than 150µg.
A maximum safe daily dietary intake has been estimated at 400µg.2,4 At an intake of 750-850µg functional signs of toxicity can be expected.4 In an American publication, the normal range in serum is said to be 0.84-1.3 µmol/L,10 but what is regarded as ‘normal’ will vary from country to country and region to region.
Selenium, like arsenic, inactivates the sulphhydral groups of amino acids. Toxicity has been associated with a garlicky odour in the breath (caused by methylated selenium), fatigue, gastrointestinal disturbances, transverse lines on the nails, alopecia and peripheral neuropathy. Treatment involves discontinuation of the source of excessive intake and supportive care. There is no known antidote or suitable chelator.
In a published11 case of selenium poisoning, the patient took 10 tablets a day for 2 weeks following a loading dose of a supplement containing an unknown amount of selenium. During this time he developed diarrhoea, worsening fatigue, a tingling sensation in the extremities and became completely bald. Two weeks after discontinuing the supplement he had a serum selenium level of 8.26 µmol/L and appeared healthy with regrowth of hair and normal neurological examination.