absorption in humans vary between 53 and
98%, and absorption by ruminants is some-
what lower, between 11 and 46%.
Homeostatic regulation of Se retention
has been investigated in rats (Kirchgessner
et al., 1997). Growing rats fed 40–3000 ng
Se g^1 diet absorbed 93–97% of Se, which
showed that there was no homeostatic
regulation of Se metabolism at the site of
gastrointestinal absorption. Endogenous
faecal excretion played a relatively minor
role and was responsible for elimination
of only 6.4–9.0% of Se consumed.
Homeostatic control of Se retention was
achieved mainly by regulation of urinary
excretion. Urinary excretion was small
(75–140 ng day^1 ) up to 100 ng Se g^1 diet
and then jumped to 671 ng day^1 for rats
fed 150 ng Se g^1 diet, which was inter-
preted as a transition from deficient to
sufficient Se intake. Thereafter, urinary Se
excretion increased from 32 to 63% of
dietary intake, or 671–26,700 ng day^1.
However, urinary elimination was not
100% efficient and dietary retention varied
from 55% of intake at 150 ng Se g^1 diet to
26% at 3000 ng Se g^1 diet, leading to
excessive whole-body retention especially
at the highest dietary concentrations.
Nevertheless, whole-body Se increased
only 20% over the range of 150–600 ng Se
g^1 diet, indicating reasonably effective
homeostatic control. The selenium concen-
tration of most tissues, especially the liver,
increased with increasing dietary intake.
As has been observed with other elements
(Zn, Mn), whole-body turnover of the
element increased, which indicated that
excessive Se intake exchanged with tissue
Se prior to elimination from the body.
Selenium dynamics in ruminants is
poorly understood and research reports to
date have been somewhat conflicting.
Ruminants excrete a higher proportion of
dietary Se in the faeces than the urine,
possibly due to the formation of unavail-
able forms of Se in the gastrointestinal
tract, which may explain the lower absorp-
tion of Se by ruminants compared with
monogastrics. Sheep responded to changes
in dietary Se concentration with changes in
urinary excretion, while faecal excretion
remained relatively constant (Langands et
al., 1986), which is consistent with mono-
gastric metabolism. Examination of gastro-
intestinal contents of sheep showed that
two to three times the quantity of Se
ingested entered the anterior portion of the
small intestine each day, indicating that
endogenous secretions of Se were sub-
stantial, although the secretions were
reabsorbed in lower sections of the small
intestine (Langands et al., 1986). On the
other hand, biliary secretion of^75 Se
measured directly in cattle following intra-
venous injection of labelled selenite was
<10% of that recovered in urine, suggesting
that endogenous secretions are small
(Symonds et al., 1981). Recent studies
revealed a significantly higher percentage
absorption of dietary Se by ewes fed Se-
deficient hay (46%) compared with ewes
fed Se-adequate hay (18%) (Krishnamurti
et al., 1997), which does not agree with the
constant percentage absorption observed in
rats. Further work will be required to
clarify the roles of absorption and excre-
tion in the homeostasis of Se.Concluding Remarks
The study of trace element dynamics
provides insight into the significance of
various body pools of elements as well as
the regulation of their routes of absorption
and excretion. It is apparent that the large
pools of an element, often in muscle, bone
or liver, play an important role in homeo-
stasis and whole-body metabolism.
However, except for Zn and Fe, the meta-
bolism of the major body pools of trace
elements has received little attention. Large
pools may serve as a source or sink during
periods of dietary insufficiency or excess.
On the other hand, as in the case of muscle
Zn in rats, the body attempts to maintain a
constant concentration of the element at
the expense of other pools during periods
of deficiency. Compartmental modelling is
an effective tool for compiling information
on trace element dynamics. It also serves as
a planning assistant in the design of studies
and greatly facilitates understanding theTrace Element Dynamics 179