Simulations of Cu metabolism in humans
also predict quantitatively greater changes
in endogenous excretion than absorption in
response to changes in dietary intake
(Buckley, 1996).
Manganese
Bone and muscle are the major body pools
of Mn. Manganese concentration in humans,
cattle and rats varies from 1.5 to 2.6 μg g^1
fresh weight in bone and from 0.06 to
0.80 μg g^1 fresh weight in muscle (Hurley
and Keen, 1987). Manganese concentration
in various body tissues is 0.1–0.005 times
that of Zn. The lower tissue concentrations
of Mn compared with Zn appear to be
associated with more rapid tissue turnover,
greater endogenous faecal excretion and
lower percentage absorption. Divalent Mn
entering the portal circulation system is
removed rapidly and efficiently by the liver.
The liver plays a major role in Mn homeo-
stasis, with biliary excretion being the main
route of endogenous loss. Non-biliary secre-
tion, including pancreatic secretion, into the
duodenum, jejunum and, to a lesser extent,
the ileum also occurs. Urinary excretion of
Mn is very small and plays a negligible role
in maintaining homeostasis.
In studies with growing rats, the
deposition of Mn in body tissues, including
liver and skeletal muscle, increased moder-
ately up to 35 μg Mn g^1 diet and thereafter
remained relatively constant up to the high-
est dietary concentration tested, 100 μg Mn
g^1 (Weigand et al., 1986). Exceptions were
bone Mn (femur), which appeared to reach
a plateau sooner at 4.5 or 11.2 μg Mn g^1
diet, and serum Mn, which reached a
plateau at 4.5 μg Mn g^1 and also increased
again at 100 μg Mn g^1 (Weigand et al.,
1986). Other studies at higher Mn intakes
show that rats are able to maintain a homeo-
static plateau with respect to liver and bone
Mn from the minimum dietary requirement
up to at least 600 μg Mn g^1 diet.
Early work in Mn dynamics was
interpreted mistakenly to indicate that
percentage absorption of dietary Mn
remained relatively constant and homeo-
static control was established predomin-
antly by variation in endogenous faecal
excretion. Later work, though, showed that
variation in percentage absorption with
changes in dietary Mn intake was quantita-
tively significant, especially at lower
dietary concentrations (Weigand et al.,
1986). True Mn absorption in the growing
rat decreased exponentially with increas-
ing dietary Mn concentration from 29% at
1.5 μg Mn g^1 diet to 2.0% at 100 μg Mn
g^1 (Weigand et al., 1986). At adequate
dietary intake, true absorption is greater
than apparent absorption, indicating that
endogenous excretion is also significant.
Turnover of Mn, estimated by loss of^54 Mn
from various tissues or the whole body,
increases with increasing dietary intake.
Serum and liver show the greatest change
in turnover with increasing intake, and
skeletal muscle and bone the least. The
increase in tissue Mn turnover is consistent
with a nearly threefold increase in excre-
tion of^54 Mn by rats consuming 100 μg Mn
g^1 diet compared with 1.5 μg Mn g^1
(Weigand et al., 1986). Studies of direct
collection of biliary Mn as well as tracer
investigations show that variations in
absorption as well as endogenous excretion
are important in Mn homeostasis in the rat.
Regulation of Mn homeostasis in calves
appears to be similar to that in rats. Calves
with higher Mn intake have greater tissue
and whole-body Mn turnover, reduced
percentage absorption and increased
biliary excretion.
Studies with the chick show a linear
relationship between dietary and tibia Mn
concentration with dietary levels up to
3000 μg Mn g^1 diet (Henry et al., 1989).
This relationship has been used in an assay
to determine availability of dietary Mn
supplements. Manganese can be stored in
avian bone and other tissues to be available
during a subsequent period of deficiency.
Linear accumulation of Mn in liver, kidney,
pancreas and muscle with increasing
dietary Mn in the chick has also been
observed. While these results might be
interpreted to indicate that there is no
homeostatic plateau in chicks with respect
to Mn metabolism, the Mn levels176 W.T. Buckley