Page 111 - Small Animal Clinical Nutrition 5th Edition

112 Small Animal Clinical Nutrition

but not all of the minerals evaluated. Additional mineral sup-
Box 6-2. Organic vs. Inorganic Minerals. plementation is warranted for foods known to have reduced
VetBooks.ir There is some debate about whether organic forms of trace min- mineral availability.
erals are more available than inorganic forms. The answer
depends on the specific mineral, the dietary conditions and the Macrominerals
physiologic state of the animal. Clearly, the organic forms of cer- Calcium
tain minerals (e.g., selenium, chromium, iron) are better used Calcium serves two important functions: 1) as a structural com-
than inorganic forms. (See the selenium, chromium and iron
sections of this chapter.) Which form is better used is less clear ponent in bones and teeth and 2) as an intracellular second
for other minerals (e.g., zinc, copper). For example, there are as messenger that enables cells to respond to stimuli such as hor-
many studies that have failed to show increased availability with mones and neurotransmitters. Calcium’s two major physiologic
zinc/copper organic forms as there are studies demonstrating functions in bone are to serve as a structural material and as an
improved availability. ion reservoir. When calcium in bone acts as an ion reservoir, it
A number of factors influence the outcome of availability stud- is in equilibrium with serum ionized calcium and under tight
ies, including: 1) the presence of non-nutritional factors (e.g., homeostatic control.
phytate, fiber, goitrogens), 2) nutrient interactions (e.g., excess- The mechanism of calcium homeostasis in blood is complex
es of other minerals) and 3) physiologic state (e.g., demand for and involves several organs. Blood concentrations of ionized (or
certain minerals increases with reproduction and growth com- free) calcium are the major initiator of calcium regulatory
pared to that of maintenance, thus in these situations, the differ- mechanisms in the body. Calcium in blood is in equilibrium
ences in availability are magnified between organic and inorgan-
ic sources). between a free or ionized state (~50%), a protein-bound state
Results of studies in puppies showed that as calcium levels (~40 to 45%) and a complexed or chelated state (~5 to 10%).
increased from 1.0 to 1.5%, zinc usage (as measured by The effects of changing ionized calcium concentration in blood
changes in plasma zinc concentrations) decreased, irrespective are highlighted below (Figure 6-2) (Nap and Hazewinkel,
of whether the source was organic (zinc propionate) or inorgan- 1994). Low concentrations of ionized calcium:
ic (zinc oxide). Zinc from zinc propionate was approximately 1.8 • Stimulate PTH secretion, which stimulates conversion of
to 2 times more available than from zinc oxide. 25-hydroxycholecalciferol (25-OH-D ) to the biologically
3
Other investigators likewise noted increased zinc retention (as more potent 1,25-dihydroxycholecalciferol (1,25-(OH) -
2
measured by zinc deposition in hair and fecal zinc excretion) for D ) in the kidneys
3
adult dogs fed a zinc-amino acid chelate compared with the • 1,25-(OH) -D stimulates calcium uptake in the gut via
2
3
same dogs fed zinc polysaccharide or zinc oxide. Increasing cal- receptor-mediated mechanisms
cium from 1.2 to 3.2% reduced zinc retention when dogs were
3
2
fed zinc polysaccharide or zinc oxide, but not the zinc-amino • 1,25-(OH) -D , in conjunction with PTH, stimulates
acid chelate. bone resorption
Similarly, researchers have demonstrated in livestock and fish • PTH induces phosphaturia.
that growth rate and calcium and phytate levels are factors that High or normal concentrations of ionized calcium:
significantly affect zinc use. Thus, these factors determine • Stimulate calcitonin secretion, which does not stimulate
whether the use of organic zinc sources is beneficial. Together, 1,25-(OH) -D production
3
2
these data suggest little or no benefit to using organic zinc in • 24,25-dihydroxycholecalciferol (24,25-(OH) -D ) is now
3
2
foods low in phytate and calcium (e.g., low calcium is defined as produced in the kidneys, which is considered biologically
calcium levels approximating NRC recommendations for respec- less active
tive species). However, as phytate and/or calcium levels • No stimulation of gut absorption or bone resorption occurs
increase, or demand for zinc increases (e.g., rapid growth rate), • Increased renal calcium excretion results
there is greater zinc use from organic zinc sources (parameters
used to assess zinc availability included bone zinc, immune • Calcitonin decreases osteoclastic activity.
2
3
response and/or growth rate). Furthermore, the more rapidly the PTH, calcitonin and 1,25-(OH) -D act together to main-
animal grows, the greater the benefit demonstrated for organic tain calcium homeostasis in the face of variable dietary intakes
zinc (e.g., fish >chicks [broiler breeds >leghorn-type] >puppies and changing calcium requirements during growth, pregnancy
>pigs). The efficacy decreased as the animal matured, suggest- and lactation.
ing organic zinc sources in foods may be less beneficial for adult The amount of true calcium absorption may range from 25
animals. to 90%, depending upon calcium status, calcium form or
In summary, organic forms of minerals may be beneficial intake (Nap and Hazewinkel, 1994). This exchangeable pool
when dietary or physiologic conditions limit mineral availability. consists of the small amount of calcium in blood, lymph and
These conditions include: 1) mineral antagonisms caused by other body fluids, and accounts for 1% of the total body cal-
phytate, fiber and imbalances/excesses of other minerals and 2) cium. The remaining 99% is located in bones and teeth.
increased metabolic demand such as rapid growth rate, repro-
duction and immune challenge. There are three routes of calcium absorption in the intestine.
One is an active, saturable, transcellular process that occurs
primarily in the duodenum and proximal jejunum. The
The Bibliography for Box 6-2 can be found at process is regulated by vitamin D and involves a vitamin D-
www.markmorris.org. dependent, calcium-binding protein (CaBP or cal-bindin).