day^1 ) and endogenous faecal excretion
continued to rise, which is the pattern
observed by Weigand and Kirchgessner
(1978). As dietary Zn intake was increased
to 141 μg Zn g^1 diet in the study of
Weigand and Kirchgessner (1978), the
quantity of endogenous faecal Zn excretion
also increased until it was 90% of the
value of apparent Zn absorption. Thus,
regulation of endogenous excretion plays a
critical role in conservation of body Zn
during Zn deficiency and, also, in the
elimination of Zn when excess Zn is
absorbed.
Although regulation of absorption
alone was not sufficient to maintain
homeostasis in response to elevated dietary
intake (Fig. 8.11), the threefold reduction
in coefficient of absorption over the range
of dietary intakes investigated is substan-
tial (Fig. 8.4). A reduction in the coefficient
of absorption with increasing dietary
consumption of Zn has been observed
repeatedly in rats, and also in calves,
sheep, human infants and human adults.
Changes in absorption may be of greater
significance at higher dietary Zn concentra-
tions in ruminants than in monogastrics.
Studies with ewes and calves indicate that
homeostasis was maintained predomin-
antly by changes in absorption, with very
little if any change in endogenous faecal
excretion when dietary Zn ranged from
adequate to excessive.
Redistribution of Zn within the major
Zn pools of the body has been observed
several times in response to Zn deficiency
in growing rats (e.g. Giugliano and
Millward, 1984) as well as in mature rats
and chicks. Redistribution involves the
relatively greater loss of Zn from bone com-
pared with other tissues during deficiency.
Loss of bone Zn has also been demon-
strated in Zn-deficient young pigs,
although changes in other tissues have not
been measured. Redistribution appears to
be a mechanism of maintaining Zn concen-
tration in preferred tissues at the expense
of others and, as such, may be an important
metabolic strategy influencing the health of
deficient animals. Changes in skin and hair
Zn as well as bone, muscle and internal172 W.T. Buckley
Fig. 8.11.Consumption, absorption and excretion of Zn in young growing male rats. The figure is a
simulation as described in Fig. 8.6.