liver Cu occurs. Laying hens maintain a
constant liver Cu concentration up to a
relatively high dietary concentration of
600 μg Cu g^1 diet, above which homeo-
static control breaks down. Ponies also
have an ability to maintain Cu homeostasis
over a range of dietary intake.
Absorption of Cu decreases with
increasing dietary Cu concentration. The
results of a number of studies of dietary Cu
absorption by humans were compiled by
Turnlund (1989) and show an exponential
reduction in absorption from about 55% in
humans consuming 0.8 mg Cu day^1 to
about 15% in humans consuming about
7.5 mg day^1. Apparent and true absorp-
tion of Cu by rats also decreases with
increasing dietary intake (e.g. Johnson and
Lee, 1988). While the coefficient of true
absorption decreased about twofold in rats
fed 21 compared with 0.4 μg Cu g^1 diet,
endogenous Cu excretion increased from 1
to 46 μg g^1 (Johnson and Lee, 1988).
These results and others show that rats are
efficient at conserving Cu when fed a defi-
cient diet. The response is similar to that
found with Zn and Mn. The biological half-
life of^67 Cu decreased from 2.36 to 1.96
days with increasing dietary Cu concentra-
tion in the study of Johnson and Lee
(1988), indicating that turnover of tissue
Cu accelerated with increasing absorbed
Cu. It appears that only intestinal endoge-
nous losses are important in homeostatic
regulation of Cu, since urinary and salivary
Cu were unaffected by changes in dietary
Cu in humans. Biliary Cu secretion
increases with increasing dietary Cu in
rats, cattle and chickens, but not in sheep.
At adequate Cu intakes, changes in absorp-
tion may play less of a role than endoge-
nous faecal excretion in maintaining Cu
homeostasis. Endogenous excretion of Cu
by humans was reduced from 1.0 to 0.5 mg
day^1 when dietary intake was reduced
from 1.4 to 0.9 mg day^1 , while there was
no measurable change in percentage
absorption of Cu (Milne et al., 1990).Trace Element Dynamics 175Fig. 8.14.Affect of dietary Cu concentration on liver Cu accumulation by animals. Data were taken from
the published reports indicated. Mean data are plotted as symbols. Data extracted from graphs are plotted
without symbols. Dietary Cu for cattle (Stoszek et al., 1986) is supplemental Cu without the basal diet
contribution.