Minerals and Trace Elements 221(which corresponds to lower status and intake than
that needed to saturate platelet glutathione peroxi-
dase activity) in the vast majority (97.5%) of all indi-
viduals in a population. The RDAs for both men and
women is 55 μg/day in the USA. In the UK, the refer-
ence nutrient intake (RNI) has thus been set at 75 and
60 μg/day selenium for men and women, respectively.
Blood selenium concentrations in the UK population
have declined by approximately 50% over the past 30
years and current UK intakes are only about 50% of
the RNI. As explained previously, however, there is
uncertainty as to what constitutes optimum selenium
status and the intakes of selenium in various dietary
regimens needed to achieve optimum status. Optimum
status may not necessarily be refl ected in saturated
glutathione peroxidase activity. The UL for adults is
set at 400 μg/day in the USA and at 300 μg/day in
the EU.
Selenium enters the food chain through plants that,
in general, largely refl ect concentrations of the element
in the soil on which the plants were grown. The
absorption of selenium by plants, however, is depen-
dent not only on soil selenium content but also on
pH, microbial activity, rainfall, and the chemical form
of selenium. Higher plants absorb selenium preferen-
tially as selenate and can synthesize organic selenium
compounds, e.g., selenomethionine, and to a lesser
extent selenocysteine. Brazil nuts contain high con-
centrations of selenium because of the seleniferous
soils in the Andes mountains but also the effi ciency
of accumulation of selenium by the plants species.
Selenium concentrations of cereals and staples are
much lower, but the content and bioavailability of
selenium in wheat usually make this a major con-
tributor to overall selenium intakes because of the
high quantities of wheat consumed as bread and
other baked products. Wheat is the most effi cient
accumulator of selenium within the common cereal
crops (wheat > rice > maize > barley > oats). There
are major varietal differences in selenium uptake and
for wheat, tomatoes, soybean, and onions, there are
up to fourfold differences in uptake of selenium from
soils amongst cultivars. The ability of plants to accu-
mulate selenium has been useful for agronomic bio-
fortifi cation, which differs from food fortifi cation
where the nutrient is added during food processing.
The Finnish Policy (1984) has led to a 10-fold increase
in cereal grain selenium concentration as well as
marked increases in fruit and vegetables and meat
concentrations as a result of adding selenium to fertil-
izers used for grain production and horticulture
and fodder crop and hay production. The resulting
increase in the selenium status of the population is
largely owing to wheat (bread) consumption but the
biofortifi cation of vegetables may also have an impact
on public health as, in contrast to wheat, where the
major selenocompound is selenomethionine, seleno-
methylselenocysteine is the predominant form in
vegetables; the last compound may have important
cancer chemoprotective effects (see also Figure 9.8)
Fish, shellfi sh, and offal (liver, kidney) are rich sources
of selenium, followed by meat and eggs. Animal
sources, however, have lower bioavailability of sele-
nium than do plant sources.Micronutrient interactions
Selenium is an antioxidant nutrient and has impor-
tant interactions with other antioxidant micronutri-
ents, especially vitamin E (Figure 9.8). Vitamin E, as
an antioxidant, can ameloriate some of the symptoms
of selenium defi ciencies in animals. Copper defi ciency
also increases oxidative stress, and the expression of
glutathione peroxidase genes is decreased in the
copper-defi cient animal.
The metabolic interactions between selenium and
other micronutrients, however, extend beyond those
between selenium, vitamin E, and other antioxidants.
Peripheral deiodination of thyroxine (T 4 ), the pre-
dominant hormone secreted by the thyroid, to the
more biologically active triiodothyronine (T 3 ) in
extrathyroidal tissues is accomplished through the
selenium-dependent deiodinase enzymes. Selenium
defi ciency, therefore, can contribute to iodine defi -
ciency disorders, and goiter complications have been
noted in up to 80% of Keshan’s disease casualties after
autopsy. Moreover, higher serum T 4 concentrations
were found in patients with subacute Keshan’s disease
and in children with latent Keshan’s disease compared
with the respective controls. All thyroid hormone
concentrations in these studies were within normal
ranges, suggesting that selenium defi ciency, or even
suboptimal selenium status, was blocking optimum
thyroid and iodine metabolism.
Excess selenium intake interferes with zinc bio-
availability, decreases tissue iron stores, and increases
copper concentrations in the heart, liver, and kidney.
Vitamins C and E, sulfur amino acids and sulfate,
arsenic, and heavy metals can decrease the toxicity of