Moon element
 When Johan
Jakob Berzelius discovered selenium in 1818 he explained that he had given
it that name because of properties similar to those of tellurium, named
from Tellus the Roman earth-goddess, though his reasoning seems a little
obscure. Selene, the Greek moon-goddess from whom he derived the name,
was daughter of the Titans.
In recent years the image of selenium, once regarded as a dangerous
and useless element, has undergone drastic revision, and in the October
issue of Chemistry in Britain Margaret Rayman of the University
of Surrey in Guildford has outlined its present significance as a dietary
essential factor.
Fifty years ago selenium was seen as a toxic factor in certain soils,
where plants reared there caused poisoning of cattle, notably in Dakota
and Wyoming in the United States and some regions of Venezuela and China.
Today it is hailed as a useful industrial product, for example in photocells,
and an essential element in the diet of humans as well as stock animals,
albeit hazardous in excess. It occurs in the amino-acid selenocysteine,
which features in the genetic code. It operates in biological redox reactions
and in electron transfer. So far, some 14 mammalian selenoproteins have
been identified, some of them able to remove the products of tissue damage
caused by free radicals. Some help to limit blood clotting and inflammatory
processes, in producing thyroid hormone, and in controlling cell division
and gene expression.
In Europe, sources of selenium in foodstuffs are deficient, but Brazil
nuts, kidney, liver, crab and other shellfish and fish are useful sources.
Recommended daily intakes in the United Kingdom are 75µg for men and 60µg
for women. Recent reductions in selenium levels in the UK have been attributed
to the reduced import of mainly Canadian wheat for bread-making, this
being selenium-rich compared with locally grown wheat.
Since selenium enters the food chain through plants that extract it
from the soil, its deficiency is seen in regions of volcanic, acid or
iron- or aluminium-rich soils. Animals reared in such areas show reproductive
impairment, growth depression and myopathy of heart and skeletal muscle.
Human diseases due to such deficiency include Keshan disease, a frequently
fatal cardiomyopathy encountered in north-east China, and Kashin-Beck
disease, a type of osteoarthritis in some regions of China, Tibet and
Siberia. Prophylactic treatment with selenium has reduced the incidence
of these.
Low selenium status has also been associated with an increased risk
of cardiovascular disease, asthma, rheumatoid arthritis, reproductive
problems, immune incompetence, cancer and adverse states of mood. Apparently
selenium is capable of reversing the onset of immune response decline
in the elderly.
Despite these beneficial effects, it has to be remembered that selenium
in excess is toxic and a high intake must be avoided. One practical method
of increasing its intake with minimal risk may be to spray crops with
a fertiliser containing sodium selenate, as has been tried in Finland.
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