Page 108 - Small Animal Clinical Nutrition 5th Edition

Minerals and Vitamins 109

sium in bones and teeth, 2) constituents of body fluids and tis- prevent diarrhea). However, high doses may result in detrimen-
VetBooks.ir sues such as electrolytes concerned with the maintenance of tal side effects after prolonged use. The pharmacologic actions
of nutrients differ in several ways from their physiologic func-
osmotic pressure, acid-base balance, muscle contraction,
tions: 1) doses greatly exceeding the amount of a nutrient pres-
membrane permeability and tissue irritability (e.g., sodium,
potassium, chloride, calcium and magnesium in blood, cere- ent in foods are usually needed to obtain a therapeutic response,
brospinal fluid and gastric juice) and 3) catalysts/cofactors in 2) the specificity of the pharmacologic action is often different
enzyme and hormone systems, as integral and specific com- from the physiologic function and 3) chemical analogues of the
ponents of the structure of metalloenzymes, or as less specific nutrient that are often most effective pharmacologically may
activators within those systems. Table 6-1 lists specific func- have little or no nutritional activity (RDA, 1989) (Box 5-7).
tions of each mineral. Claims of nutritional adequacy of pet foods are based on the
current AAFCO nutrient allowances (“profiles”). These levels
Homeostasis are neither minimal requirements nor necessarily optimal
Specific concentrations and functional forms of minerals must intake levels. It isn’t possible to establish optimal levels without
be maintained within certain limits for optimal growth, health additional information about nutrient requirements for all
and fertility. Higher organisms possess homeostatic mecha- lifestages and information concerning the availability of nutri-
nisms that attempt to maintain concentrations of minerals at ents from pet food ingredients and complete diets. In some
their active sites within narrow physiologic limits despite over- cases, insufficient margins of safety have been given to account
or under-ingestion. Such mechanisms include control of intes- for population variation, product diversity, processing effects
tinal absorption or excretion, the availability of specific stores and potentially low nutrient availabilities of certain pet food
for individual elements and the use of “chemical sinks” such as ingredients. In the case of trace minerals, the ratio between
metallothionein that can bind potentially toxic amounts of ele- dietary allowance and absolute requirement can be as large as
ments in an innocuous form (Underwood and Mertz, 1987). 100:1 (e.g., chromium) because of incomplete absorption, or
The degree of homeostatic control varies from one element can approach unity (e.g., iodine) when absorption is high
to another. Continued ingestion of diets or exposure to envi- (Underwood and Mertz,1987).The nature of typical diets con-
ronments that are severely deficient, imbalanced or excessively sumed strongly influences dietary allowances because numer-
high in a particular trace element, or in an interfering substance ous interactions among dietary components and different
such as phytate or certain fibers, can induce changes in func-
tioning forms, activities or concentrations of that element in
body tissues and fluids so that they fall below or rise above the
desired limits. Altered metabolism develops in these circum-
stances, which may affect physiologic function. Structural dis-
orders may also arise in ways that differ with various elements,
with the degree and duration of the dietary deficiency or toxic-
ity and with the age, gender and species of the animal involved.

Deficiency/Adequacy/Toxicity
Traditionally, minerals were classified as “essential” or “toxic,”
but as more information was gathered, elements shifted from
the latter to the former category (e.g., selenium). However, tox-
icity may occur with all elements. A “biologic dose-response
curve” exists for each element (Underwood and Mertz, 1987).
This curve (Figure 5-2) identifies a range of concentrations
that spans three primary areas: 1) at low concentrations, physi-
ologic function is consistently and reproducibly impaired
(defined as deficiency), 2) at optimal concentrations, the nutri-
ent is provided at levels necessary to meet the requirements of
the animal and 3) at excessive concentrations, pharmaco-toxi-
cologic effects occur. The intakes or dose levels at which these
phases become evident and the width of the optimal plateau
vary widely among minerals and can be markedly affected by
Figure 6-1. Mineral interrelationships. Minerals connected by a line
the extent to which various other elements and compounds are clinically or experimentally interact with the other mineral. This inter-
present in the animal’s body and in the food consumed (Box 6- action may be bidirectional (each mineral affects the use of the other)
1).Table 6-1 lists specific signs of mineral deficiencies and tox- or unidirectional (one mineral affects the use of the second mineral
icities. but not vice versa). (Adapted from Puls R. Mineral Levels in Animal
Several nutrients have specific therapeutic uses at high Health. Clearbrook, British Columbia: Diagnostic Data Sherpa
dosages (e.g., zinc is fed at growth-promoting levels in swine to International, 1990; 19.)