Urinary Zn = 7.4
(1 e0.0074 Zn intake) (8.2)where units of urinary Zn and Zn intake
are μg day^1 (Fig. 8.5). This expression was
utilized in the simulation to describe the
loss of Zn via urine.
All aspects of the model were derived
from published results except for the rate
of endogenous faecal excretion, which was
determined for various nutritional states by
solving the model. The model was solved
for endogenous faecal excretion rather than
using available data from the literature
because endogenous faecal excretion
cannot be varied independently from
changes in whole-body Zn mass, and
changes in whole-body mass were entered
into the model from the data of Williams
and Mills (1970) as discussed above.
Simulation of zinc dynamics – model outputThe model was solved for dietary Zn intake
by young growing male rats consuming
diets varying from 3 μg g^1 (deficient) to
18 μg g^1 under conditions in which 12 μg
g^1 provided adequate Zn. Urinary and
faecal Zn excretion are shown in Figs 8.6
and 8.7. As has been well documented in
animals, the simulation shows that faeces
are the main route of excretion of dietary
Zn. However, at lower dietary Zn intakes,
urinary Zn excretion became a significant
percentage of total Zn excreted (Fig. 8.8).
Similar results have been found in humans
and calves.
As might be expected, Zn balance
varied considerably in response to dietary
intake according to the simulation (Fig.
8.9). Except for the lowest level of dietary
Zn (3 μg g^1 ), Zn balance, expressed as μg
day^1 , varied with time in the growing rats,
and was highest for 9–18 μg Zn g^1 diet at
28 days. The simulation shows that
measurement of balance alone may not be
sufficient to distinguish an adequate from a
deficient diet. At 28 days, balance for rats
fed 9 μg Zn g^1 diet was as great as for
12–18 μg Zn g^1 diet, even though 9 μg Zn168 W.T. Buckley
Fig. 8.5.Urinary excretion of Zn by young growing male rats fed a semi-synthetic diet with varying Zn
concentrations. Adapted from Johnson et al. (1988).