Farm Animal Metabolism and Nutrition

absorption occurred with increasing dietary
Zn concentration. Their data (Johnson et al.,
1988) were subjected to non-linear regres-
sion, to yield the following relationship
which was incorporated into the model:

Abs = 0.986 
0.0233 Zn p.p.m.

+ 0.0002 Zn p.p.m.^2 (8.1)

where Abs = coefficient of absorption and
Zn p.p.m. = dietary Zn concentration, μg
g
^1 (Fig. 8.4).
Plasma Zn was held in steady-state in
the simulation because it is only about
0.5% of whole-body Zn in the rat, and
variations in the plasma Zn pool size,
which are observed during deficiency, are
negligible with respect to changes in the
major tissue Zn pools. It was assumed that

the relative distribution of plasma Zn to

tissues in the body of the growing rat

would be similar to that found for mature

rats in a recent compartmental modelling

study (House and Wastney, 1997). Thus,

values for the fractional transfer coefficients

to tissues from plasma (determined by

House and Wastney, 1997) are incorporated

into the current simulation. The values

utilized were 7.1 day
^1 for transfer of

plasma Zn to muscle (k(5,2) in Fig. 8.1),

5.62 day
^1 for transfer to skin and hair

(k(6,2)), 35.7 day
^1 for transfer to internal

organs (k(3,2)) and 7.22 day
^1 for transfer

to bone (k(4,2)). Thus, Zn transfer from

plasma to internal organs, mainly liver, in

the simulation is relatively rapid, which is

consistent with other observations on the

Trace Element Dynamics 165

5

Muscle

6

Pelt

4

Bone

7

Urine

3

Internal

organs

9

Gut^1 Faeces

2

Plasma

Dietary Zn

k(5,2)

k(2,5)

k(2,1)

k(6,2)

k(2,6)

k(2,4) k(4,2)

k(2,3)

k(3,2)

k(7,2)

k(1,3)

k(9,1)

Fig. 8.1.Kinetic model of Zn metabolism in rats. Circles represent body Zn compartments, which are
numbered arbitrarily. Arrows are fractional transfer coefficients (day
^1 ), k(i,j), indicating transfer to
compartment i from compartment j; for example, k(2,1) is the fraction of compartment 1 (gut Zn) which
is transferred to compartment 2 (plasma Zn) per day.