Farm Animal Metabolism and Nutrition

Chapter 8

Trace Element Dynamics

W.T. Buckley

Agriculture and Agri-Food Canada, Brandon Research Centre,

Brandon, Canada

Introduction

Trace element dynamics in animals refers
to the quantitative metabolism and kinetics
of trace element absorption, distribution,
storage and excretion. Control of these
processes normally yields a reasonably
constant and optimum internal environ-
ment with respect to trace element func-
tions in metabolism. Within a range of
conditions, trace element homeostasis may
be achieved and maintained. Trace element
dynamics may be viewed as the shifting or
maintenance of trace element status, which
depends on numerous factors including the
element and species in question, and the
influence of homeostatic mechanisms. It is
the objective of this chapter to summarize
current knowledge of trace element
dynamics with respect to the responses of
whole-body trace element metabolism to
changes in dietary intake. The reader is
also referred to other reviews on the topic
(Miller, 1973; Kirchgessner, 1993).
Trace elements can be divided into two
groups with respect to their route of endo-
genous excretion: those for which homeo-
stasis is partially dependent on endogenous
faecal excretion, controlled by the intestinal

tract, liver and pancreas, and those for

which homeostasis is dependent upon renal

excretion, controlled by resorption in the

proximal renal tubule. Cationic elements

including Cu, Mn and Zn fall into the first

category while elements present in the body

as anions including Cr, F, Mo and Se fall

into the second category. In the cation

group, control of homeostasis through varia-

tion in absorption usually is the most signifi-

cant factor, although variation in excretion is

also important. Iron, though, is unique

because its homeostasis is dependent essen-

tially, if not entirely, on control of absorp-

tion. In the anionic group, control of

excretion predominates and absorption

plays a minor role. In this chapter, we will

discuss Zn, Cu, Mn, Fe and Se.

The trace element content of tissues

may respond in basically two ways to

changes in a dietary trace element intake:

(A) no change over a range of intakes,

beyond which a decrease or increase of

tissue content occurs; or (B) a continuous

change in tissue content over a range of

intakes. In case A, a homeostatic plateau

occurs, which is not apparent in case B.

There are various combinations of tissue

responses within individuals because some

© CAB International2000. Farm Animal Metabolism and Nutrition

(ed. J.P.F. D’Mello) 161