Page 119 - Small Animal Clinical Nutrition 5th Edition

120 Small Animal Clinical Nutrition

DM for cats, which is similar to NRC (2006) recommenda-
VetBooks.ir tions. The manganese requirement for birds is 10 to 12 times
higher than that for people, pigs, dogs and cats (McDowell,
1992).Manganese functions as an enzyme activator or as a con-
stituent of metalloenzymes. Although there are only a few
manganese-containing metalloenzymes (e.g., arginase, pyru-
vate carboxylase and manganese-superoxide dismutase), many
enzymes are activated by manganese, including hydrolases,
kinases, decarboxylases and transferases. Other cations (espe-
cially magnesium), however, can partially substitute for man-
ganese with little or no loss in enzymatic activity, thus man-
ganese deficiency may not adversely affect physiologic or meta-
bolic function (McDowell, 1992).
Manganese is also essential in bone and cartilage develop-
ment because it activates glycosyltransferases (i.e., enzymes
important for polysaccharide and glycoprotein synthesis). In
addition, manganese is involved in reproduction and lipid
metabolism (e.g., manganese is involved in the biosynthesis of
choline and cholesterol).
Manganese homeostasis is maintained through regulation of
absorption and excretion. Manganese is absorbed throughout
Figure 6-3. Diagram showing pathways of thyroid-hormone synthe-
the small intestine in a rapidly saturable process. Low molecu-
sis from iodine within the thyroid gland. (Adapted with permission
lar weight ligands, such as L-histidine and citrate, enhance
from Hetzel BS, Maberly GF. Iodine. In: Mertz W, ed. Trace
absorption, whereas excessive concentrations of phosphorus,
Elements in Human and Animal Nutrition, 5th ed. San Diego, CA:
Academic Press Inc, 1986; 147.) See text for details. iron and cobalt can reduce absorption. Manganese is excreted
via several routes that combine to provide an efficient homeo-
static mechanism to regulate manganese levels in tissues. Bile
Mauldin et al, 1993; Fascetti et al, 1998, 2000). flow is the primary route of excretion, but manganese is also
Signs of copper deficiency in cats include poor reproductive excreted in pancreatic juice and in the small intestine.
performance, early fetal loss, fetal deformities, cannibalism, coat Table 6-1 lists signs of manganese deficiency and excess.
a
hypopigmentation, kinked tails and inverted carpi. Clinical Ingredients rich in manganese include fiber sources, menhaden
signs in dogs include hair depigmentation and hyperextension fish meal and dicalcium phosphate. Manganese supplements
of the distal forelimbs (Zentek and Meyer, 1991). Table 6-1 include manganese oxide, manganese sulfate, manganous chlo-
lists signs of deficiency and excess. ride and manganese carbonate.
Copper excess in dogs and cats with normal metabolism is
of much less practical concern than copper deficiency, but can Iodine
interfere with iron and zinc use. Bedlington, West Highland Iodine is a constituent of the thyroid hormones 3,5,3’,5’-
white and Skye terriers, however, are predisposed to heredi- tetraiodothyronine (thyroxine, T ) and 3,5,3’-triiodothyronine
4
tary autosomal recessive disease resulting in copper hepatoxi- (T ). Thyroid hormones have an active role in thermoregula-
3
cosis (Chapter 68). Anti-copper therapies such as zinc supple- tion, intermediary metabolism, reproduction, growth and
mentation and orally administered tetrathiomolybdate have development, circulation and muscle function. Thyroid hor-
been used to treat dogs with this genetic disorder (NRC, mones also: 1) influence physical and mental growth and dif-
2006). AAFCO (2007) has set a safe upper limit of 250 ferentiation and maturation of tissues, 2) affect other endocrine
mg/kg DM copper for dogs, but no safe upper limit for cats. glands, especially the hypophysis and the gonads, 3) influence
NRC (2006) lists no safe upper limit of copper for either dogs neuromuscular functioning and 4) have an effect on the integu-
or cats. ment, hair and fur (McDowell, 1992).
Most meat ingredients, especially organ meats, are rich in The thyroid glands actively trap iodine daily to ensure an
copper. Ruminant livers are extremely high in copper; concen- adequate supply of thyroid hormone.This trapping mechanism
trations are five to 10 times higher than in monogastric livers. b regulates a more or less constant iodine supply to the thyroid
Typical copper supplements include cupric sulfate, cupric car- glands over a wide range of plasma iodide levels. Figure 6-3
bonate and cupric chloride. outlines the steps of thyroid-hormone biosynthesis (Hetzel and
Maberly, 1986). Iodine trapping is an active transport mecha-
+
+
Manganese nism linked to Na /K -ATPase activity. Thyroid-stimulating
Manganese deficiency is of little practical relevance in dogs and hormone, which is released from the pituitary gland to regulate
cats, but is of practical relevance in birds. AAFCO (2007) rec- thyroid activity, also regulates this mechanism. A thyroid-per-
ommends 5 mg/kg DM manganese for dogs and 7.5 mg/kg oxidase enzyme oxidizes iodide, which is released from thyroid