Farm Animal Metabolism and Nutrition

rate of transfer of plasma^65 Zn to liver in
humans, rats and cattle.
Tracer studies with rats, cows and
calves show enhanced retention and/or
increased specific activity of tracer in
tissues of Zn-deficient animals compared
with Zn-adequate animals, which indicates
that Zn transfer from tissue to plasma is
decreased in Zn deficiency (e.g. Windisch
and Kirchgessner, 1994). On the other
hand, excessive Zn intake leads to a reduc-
tion in tissue tracer specific activity or
retention. These results indicate that tissue
Zn turnover is decreased in Zn deficiency
and increased when Zn supply is exces-
sive. Thus, variations in tissue Zn mass in
the simulation were accomplished through
changes to tissue Zn egress transfer coeffi-
cients. The value of each coefficient was
controlled by a function designed to adjust
the compartment size, expressed as a
percentage of whole-body Zn, to match
tissue Zn contents obtained from several
studies (Giugliano and Millward, 1984;
Windisch and Kirchgessner, 1994; House
and Wastney, 1997). The tissue egress

coefficients decreased with decreasing Zn

intake in the simulation, as would be

expected from published observations of

changes in tissue Zn turnover. Percentages

of whole-body Zn in muscle and bone of Zn-

adequate growing rats used in the simula-

tion were 26.7 and 27.9%, while

percentages for Zn-deficient growing rats

were 40.0 and 13.4%, respectively (from

Giugliano and Millward, 1984, after

correcting for pelt Zn). The percentage of

whole-body Zn in the internal organs was

fixed at 15.1 (from Giugliano and Millward,

1984, after correcting for pelt Zn). The

percentage of Zn in the pelt was fixed at 30.5

(House and Wastney, 1997). Percentages of

whole-body Zn in internal organs and in the

pelt were fixed in the simulation because

they were unaffected by variations in dietary

Zn (Giugliano and Millward, 1984;

Windisch and Kirchgessner, 1994).

Measurements of urinary Zn excretion

for growing rats at various dietary Zn

intakes from the study of Johnson et al.

(1988) were subjected to non-linear regres-

sion to yield the following relationship:

Trace Element Dynamics 167

Fig. 8.4.Absorption of Zn by young growing male rats fed a semi-synthetic diet with varying Zn
concentrations. Adapted from Johnson et al.(1988).