Chemistry Reference
In-Depth Information
CHAPTER
9
Essential Metals: Assessing Risks
from Defi ciency and Toxicity
GEORGE C. BECKING, MONICA NORDBERG, AND GUNNAR F. NORDBERG
ABSTRACT
defi cient and excess intakes or exposures of EMs. Risk
assessment of EMs needs a multidisciplinary scien-
tifi c approach. Only a combination of data on required
nutritive intakes, defi ciency, and excess exposure can
give appropriate intake recommendations from both
the nutritional and toxicological point of view. Prin-
ciples and methods for developing such recommen-
dations will be described in this chapter. Assessing
risks includes evaluation of the essentiality of metals,
nutritional requirements and related adverse effects of
defi ciencies, homeostasis, maximum allowable con-
centration, and the critical adverse effects of intakes
above this concentration.
The traditional criteria for essentiality for human
health is that the absence or defi ciency of the element
from the diet produces either functional or structural
abnormalities and that the abnormalities are related to,
or a consequence of, specifi c biochemical changes that
can be reversed by the presence of the essential metal
(WHO, 1996). At present, the following trace elements
are regarded as essential for human health: iron, zinc,
copper, chromium, cobalt, molybdenum, and selenium
(WHO, 1996). Details of their essentiality and toxicity
are reviewed in the respective chapters in this Hand-
book. As explained in Chapter 32, a basic intake of this
metal through the diet is required by humans to main-
tain health, and based on this information manganese
should be added to the list of essential metals.
In this chapter the principles for risk assessment of
EMs are outlined and discussed. For the assessment
of risks from toxic effects, methods are similar to
those used for metals in general (cf Chapter 14) with
the addition that the essential nature of the metal has
Recommendations aimed at protecting the public
from toxicity of essential elements have usually been
developed separately from those recommendations
aimed at protection from defi ciency. Because of the
uncertainties involved in the evaluations, these recom-
mendations have sometimes been in confl ict, emphasiz-
ing the need for a new approach, including a balanced
consideration of nutritional and toxicological data.
In developing these new principles of evaluation,
some basic concepts based on interindividual variabil-
ity in sensitivity to defi ciency and toxicity must be con-
sidered. Such variation translates into one interval of
(low) daily intakes, at which there is risk of developing
defi ciency, and another interval of (high) dietary intakes
at which toxicity may occur. In most instances, there is
a third set of intakes in between, which represents the
acceptable range of oral intakes (AROI) in which no
adverse effects occur. It must be noted, however, that
such a range cannot be found that protects all persons
from adverse effects. Those persons with genetically
determined sensitivity may require higher intakes to
avoid defi ciency or lower intakes to avoid toxicity than
those defi ned by the AROI. AROI is defi ned as protect-
ing 95% of an unselected human population from mini-
mal adverse effects of defi ciency or toxicity.
1 INTRODUCTION
Risk assessment of essential metals (EMs) in human
health involves methods for analyzing the limits of
