Page 109 - Small Animal Clinical Nutrition 5th Edition

110 Small Animal Clinical Nutrition

Box 6-1. Mineral Balance Studies.
VetBooks.ir The requirements for most nutrients are derived from experimen- good as 5%, especially in complex matrices such as food, urine

tal and clinical evidence of deficiency and the amount of nutrient
needed to prevent signs of deficiency. When balance studies are and feces.Thus, balance studies will not detect nutritionally impor-
tant mineral differences when absorption efficiencies are low and
used to estimate requirements, the requirement is defined as the radioisotopes or stable isotopes are not used.
intake at which zero balance is attained, or when intake is equal to Probably the biggest criticism of balance studies is that balance
excretion in urine and feces. However, the zero balance point will studies better reflect habitual intake than a requirement or zero
underestimate the requirement, if the measurement does not balance. For example, an intake of 1 µg of selenium/kg body
account for endogenous losses and losses in sweat. weight maintains a zero balance in Americans. Approximately one-
Mineral balance studies have been criticized as inadequate and tenth of that intake maintains a balance in people living in China;
erroneous measures of body requirements. Small percentage China is an area in which the risk of selenium deficiency is high
errors in determining intakes and excretion can result in significant and prevalence of Keshan disease is significant. Thus, zero bal-
differences in balance calculations. One of the biggest problems in ance does not necessarily indicate absence of disease. In New
conducting balance studies is separating feces into time intervals Zealand, selenium balance is maintained on an intake of one-third
that can be related to intake. Fecal markers aid in separation, but the amount required by Americans.
peristaltic reflux may still confound results. In addition, one ani- Analogous situations also exist worldwide for calcium balance.
mal’s rate of passage can vary markedly. Adaptation to a different Widely different intakes of calcium result in zero balances in differ-
intake level may occur in a few days or weeks. Some adaptations, ent countries. Thus, the previous dietary intake exerts a significant
however, take several months or even years to occur. Thus, the effect on the nutrient level that results in a zero balance and is
adaptation period and collection period need to be sufficiently long more a reflection of the intake required to maintain an existing
to take into account animal adjustments to new foods, rates of mineral pool size.
passage and homeostatic adaptation. Balance studies should be evaluated with caution when
Balance studies are probably more reliable when the mineral is attempting to determine requirements. A summary of their limita-
excreted in the urine, rather than in the feces. This finding is true tions follows:
for sodium, potassium and selenium. When absorption of a nutri- • Prior long-term habitual intake influences whether positive,
ent is low, as is the case for a number of minerals, the amount of negative or equilibrium balance occurs at a particular intake.
fecal mineral is large compared with the amount absorbed (e.g., • The duration of the study may not be long enough to allow for
the mineral concentration in feces is attributed to unabsorbed min- homeostatic adaptation.
eral and endogenous secretion). Failure to measure endogenous • Cumulative errors occur from environmental contamination,
secretion may markedly underestimate the true amount of individual variability and analytical methods. Thus, balance
absorbed mineral. However, if radioisotopes or stable isotopes are studies that demonstrate no treatment differences, may in
used in conjunction with a balance study, the endogenous secre- fact be a result of the insensitivity and imprecision of the bal-
tions can be distinguished from unabsorbed mineral and a meas- ance method.
ure of true absorption attained (as opposed to “apparent” absorp-
tion, which does not account for endogenous losses). Balance The Bibliography for Box 6-1 can be found at
studies without the use of isotopes are fraught with inaccuracy and www.markmorris.org
variability. Few analytical methods give a coefficient of variation as

chemical forms in individual foods determine biologic avail- The rigors of processing can affect the availability of min-
ability. Thus, it is important to understand the limitations of erals either positively or negatively via changes in solubility,
AAFCO nutrient profiles. pH, reduction potential and charge density and creation of
complexes (Clydesdale, 1988). Charge density refers to the
Mineral Interactions valence state and size of a metal. For example, the cations in
A tremendous number of mineral-mineral interactions exist the periodic table in groups 6B to 2B, with a relatively high
+
+
(Figure 6-1). In general, these interactions can be antagonistic ionic charge ( 2, 3) and small size, form a large number of
(the presence of one mineral reduces the transport or biologic stable complex ions, whereas the large alkali metal cations,
+
+
efficacy of the other) or synergistic (the two minerals act in a such as Na and K , with a small charge are much less likely
complementary fashion either by sparing or substituting for the to form complexes with proteins or carbohydrate moieties via
other mineral or the two together enhance a biologic function). ionic, coordinate or covalent bonds. Furthermore, among the
Most mineral interactions are antagonistic and can occur via a transition metals, which may form more than one cation,
+
+3
+3
number of different mechanisms that include interactions complexes formed by the 3 valence state (e.g., Cr , Co )
(Solomons, 1988): 1) in the food during processing, before con- are more numerous and more stable than complexes formed
+
sumption, 2) in the digestive tract, where there is competition by their respective 2 ions. Charge is also involved with cell
for uptake sites or intracellular-level mechanisms, 3) at the tis- permeability and ion solubility before ions enter cells.
sue level, either at storage sites or inhibition of enzyme activity, Solubility varies tremendously depending on ion size and
4) at the time of transport and 5) in the excretory pathway. degree of polarity or charge.