selenium poisoning / selenosis

Introduction

selenium is an essential trace element however, it is toxic if taken in excess at levels > 400-800mcg/d and can lead to selenosis
Recommended Dietary Allowances (RDAs) for selenium is 55 μg/day
major food sources of selenium in the American diet are breads, grains, meat, poultry, fish, and eggs.
toxicity risk is mainly in China where people who ingest corn grown in extremely selenium-rich stony coal (carbonaceous shale which has selenium content of 9.1%)
elsewhere, the most frequent ingestion resulting in acute selenium toxicity is that of gun bluing - children who ingest this suffer cardiovascular collapse within minutes of ingestion
large amounts of brazil nuts has caused selenosis (each nut contains 68-91mcg)
chronic toxicity may occur in taking selenium supplements
selenium poisoning of water systems may result whenever new agricultural runoff courses through normally dry, undeveloped lands
elemental selenium and most metallic selenides have relatively low toxicities because of their low bioavailability
selenates and selenites are very toxic, having an oxidant mode of action similar to that of arsenic trioxide
- chronic toxic dose of selenite for humans is about 2400-3000 micrograms of selenium per day
selenium also occurs in organic compounds, such as dimethyl selenide, selenomethionine, selenocysteine and methylselenocysteine, all of which have high bioavailability and are toxic in large doses

garlic odor on the breath
metallic taste
gastrointestinal disorders
alopecia
sloughing of nails
fatigue
irritability
neurological damage - hyperreflexia, peripheral paresthesia, anesthesia and hemiplegia
extreme cases may develop:
- cirrhosis
- acute pulmonary oedema (APO)
- rapid cardiovascular collapse and death may occur in large acute poisonings
- and may be fatal

may occur in those with severe GIT malabsorption issues, on total parenteral nutrition, those aged over 90yrs and those dependent on foods grown in selenium poor soils
it generally becomes symptomatic only when other stresses are concurrent such as mercury exposures or vitamin E deficiency
it rarely occurs in absence of other issues hence effect of selenium deficiency on health remains uncertain