with 24 μg Zn g^1 diet, while there was no
effect on Zn content of liver, kidneys,
spleen, brain, lungs, reproductive tissues,
muscle, skeleton or pelt (Windisch and
Kirchgessner, 1993). These and other
studies have indicated that rats are able to
maintain relatively constant tissue Zn
concentration up to 600 μg Zn g^1 diet;
however, higher intakes lead to substantial
increases in Zn content of most internal
organs and bone. Based on limited reports,
the ability to maintain tissue Zn homeo-
stasis with increasing dietary Zn concen-
tration varies considerably among species,
with quail and chickens having little
ability to maintain homeostasis, calves and
rats having moderate ability and mice
having the greatest dietary range of species
reported. Although the congruence is less
than desirable among studies, species and
physiological states with respect to tissues
affected by excess Zn and the dietary range
over which homeostasis can be main-
tained, it is possible to conclude that a
homeostatic plateau exists for some range
of Zn intakes above the nutritional require-
ment for most species.
Tissue Zn turnover varies among
tissues and is a factor in mobilization of Zn
and in its excretion from the body. Rapidly
turning over tissues can respond more
rapidly to a reduction in Zn intake.
Disappearance curve analysis of individual
tissues from sequentially sacrificed rams
following intravenous^65 Zn administration
showed that liver, heart, pancreas, salivary
glands, kidney cortex and spleen had rapid
Zn turnover ≥18% day^1 , while skeletal
muscle and bone had slow turnover, <2%
day^1 and other tissues had intermediate
values (McKenney et al., 1962). Comparison
of Zn kinetics in mice, rats, dogs and
humans shows that the biological half-life
of Zn increases as the size of the species
increases.
Simulation of zinc dynamics – model
developmentModel simulation provides a means of com-
piling existing knowledge of metabolism
from numerous studies to yield a more com-
prehensive view than can be provided by
individual investigations. Based on the
results of published studies, a model of Zn
metabolism (Fig. 8.1) was constructed in
order to simulate Zn dynamics. The simula-
tion was prepared with the aid of a compart-
mental modelling program, SAAM II (SAAM
Institute, 1994). The simulation has been
constructed for male, growing rats consum-
ing diets varying in Zn concentration from
deficient to adequate. Although constructed
for rats, similarities with other species are
apparent in the results of the simulation.
Williams and Mills (1970) thoroughly
documented feed intake, body weight
changes and whole-body Zn concentrations
over time with growing rats fed varying Zn
concentrations from deficient (3 μg g^1 ) to
adequate (12 μg g^1 ). Much of these data
have been incorporated into the model, pro-
viding basic Zn data for growing rats.
Dietary Zn intake varies with time (body
size) and dietary Zn concentration (Fig.
8.2). Several other studies have shown that
feed intake and growth rate no longer vary
with dietary Zn concentration for a
considerable range above the nutritional
requirement. Thus, feed intakes for 15 and
18 μg Zn g^1 diet were included in the
model equal to the intake at 12 μg Zn g^1
diet (Fig. 8.2). Whole-body Zn of young rats
increased with age and dietary Zn concen-
tration up to the nutritional requirement
(Williams and Mills, 1970) (Fig. 8.3). Since
Williams and Mills (1970) and others have
shown repeatedly that whole-body Zn or
tissue Zn does not increase with increases
in dietary Zn above the minimum require-
ment up to relatively high intakes, dietary
concentrations of 15 and 18 μg Zn g^1 diet
were included in the simulation with no
increase in whole-body Zn compared with
rats fed 12 μg Zn g^1 diet (Fig. 8.3).
Johnson et al. (1988) investigated the
effect of dietary Zn concentration on the
coefficient of absorption of Zn using grow-
ing male rats similar in age to those studied
by Williams and Mills (1970). Their
measurements of Zn absorption included
the dietary Zn range of 3.6–50 μg g^1. A
substantial reduction in coefficient of164 W.T. Buckley