g^1 diet was unable to support as rapid
growth as 12–18 μg Zn g^1 diet during the
full 28-day period. During the last days of
the simulation, whole-body accumulation
of Zn at 9 μg Zn g^1 diet was nearly
parallel to that at 12 μg Zn g^1 diet (Fig.
8.3), leading to nearly equivalent balance
values. Thus, due to observed differences
in feed intake, percentage absorption and
percentage excretion, the simulation
reveals that 9 μg Zn g^1 diet should be able
to meet the nutritional requirement for Zn
at the end of the 28-day period but not at
the beginning.
Control of endogenous faecal Zn
excretion is recognized as an important
mechanism of Zn homeostasis. In the
current simulation, endogenous faecal
excretion was almost negligible at 3, 6 and
9 μg Zn g^1 diet in the growing rat (Fig.
8.10). The whole-body retention of Zn
found by Williams and Mills (1970) was
achieved by the nearly complete inhibition
of endogenous faecal excretion. It shows
that the rat has considerable capacity for
regulating loss of Zn from the body, since
there was very little loss at intakes less
than the nutritional requirement. The
ability of organisms to minimize endo-
genous faecal excretion of Zn in response
to dietary deficiency has been reported in
studies with rats, calves, goats and human
infants.
Although the coefficient of absorption
decreases as dietary Zn concentration
increases (Fig. 8.4), quantitative true
absorption (μg day^1 ) increases because the
elevated Zn intake is not fully matched by
the reduction in the coefficient of absorp-
tion. This relationship was observed by
Weigand and Kirchgessner (1978) and is
shown by solving the simulation for
dietary concentrations up to 48 μg g^1 (Fig.
8.11). Since there was no further increase
in whole-body Zn concentration above the
nutritional requirement, the additional
absorbed Zn was excreted in the faeces.
Apparent absorption remained relatively
constant above the nutritional requirement
(Fig. 8.11), while both true absorption (μgTrace Element Dynamics 171Fig. 8.10.Endogenous faecal excretion of Zn by young growing male rats. The figure is a simulation as
described in Fig. 8.6.