Page 118 - Small Animal Clinical Nutrition 5th Edition

Minerals and Vitamins 119

maintain zinc homeostasis. Thus, both absorption and excre- Mertz, 1987).
In most species, copper can be absorbed in all segments of
VetBooks.ir tion are important in regulating zinc balance. the GI tract; however, the small intestine is the major site of
Zinc deficiency is probably more of a practical concern with
pet foods than is toxicity, because: 1) zinc is relatively nontoxic
absorption (Davis and Mertz, 1987). Although the biochemi-
and 2) its availability is decreased by a number of factors (phy- cal mechanisms are not fully understood, there is good evidence
tate, high dietary levels of calcium, phosphate, copper, iron, that intestinal absorption of copper is regulated by the need of
cadmium and chromium). The antagonistic effects of calcium the animal and that metallothionein (a metal-binding protein)
are greatest when phytate is also present, resulting in the forma- plays a key role in regulation. Copper appears to be absorbed by
tion of a highly insoluble complex of calcium, phytate and zinc. two mechanisms, one saturable, suggesting active transport at
Signs of zinc deficiency have been reported to occur in dogs fed low dietary copper concentrations and the other unsaturable,
cereal-based dry foods (e.g., grains may contain significant con- suggesting simple diffusion at high dietary copper levels.
centrations of phytate), even when the zinc content of the food Most copper in plasma is bound to ceruloplasmin, a copper-
exceeded NRC minimum requirements (NRC, 2006; Morris binding protein. Newly absorbed copper, however, may be
and Rogers, 1994). transported from the intestine loosely bound to albumin or cer-
AAFCO recommends 120 mg/kg DM zinc for dogs and tain amino acids. In this form, the element is readily available
75 mg/kg DM zinc for cats (2007). For trace minerals, to the liver and other tissues, in contrast to the much more
AAFCO makes the same recommendations for adult mainte- tightly regulated distribution of ceruloplasmin-bound copper.
nance and growth/reproduction foods. In livestock, however, This difference in availability may explain the tissue damage
the requirement for zinc is greatly increased during growth caused by copper accumulation in hepatotoxicosis seen in
and reproduction. NRC (2006) recommends a minimum of Bedlington terriers and people with Wilson’s disease, in which
100 mg/kg DM zinc for growth vs. 60 mg/kg DM zinc for the ceruloplasmin transport protein is lacking.
adult dogs; NRC makes similar zinc recommendations for The liver is the central organ for copper metabolism. Hepatic
cats (i.e., 75 and 74 mg/kg DM zinc in foods for growth and concentrations reflect an animal’s intake and copper status.
adults, respectively). Copper is excreted primarily through the feces. Most fecal cop-
Signs of zinc deficiency include anorexia, decreased growth per is unabsorbed, but active excretion also occurs via the bile.
rate, alopecia, parakeratosis, impaired reproduction, depressed Copper homeostasis is maintained primarily through absorption.
immune function and growth disorders of the skeleton Dietary copper deficiency has been reported to occur in dogs
(Chapter 33). Naturally occurring zinc-responsive dermatoses and cats and thus is of practical concern (Morris and Rogers,
have been described (Chapter 32). 1994) (Case 6-3) Availability of copper from different foods
Although relatively nontoxic, excess dietary zinc can inter- and supplements can vary greatly, so the requirement for cop-
fere with other minerals (iron and copper), thus excesses per is difficult to define. The requirement can vary several-fold
should be avoided.The only reported cases of zinc toxicosis in depending on the source of copper in the food and the level of
dogs or cats have been due to dietary indiscretion (e.g., con- other ingredients/nutrients/non-nutrients (e.g., interactions
sumption of die-case nuts from animal carriers or pennies) with phytate, calcium, zinc and iron).The AAFCO (2007) rec-
(Case 6-2). Table 6-1 describes effects of zinc deficiency and ommendation for copper in dogs is 7.3 mg/kg DM.
excess. Ingredients naturally high in zinc include most meats, Separate copper requirements are recommended for extrud-
fiber sources and dicalcium phosphate. Zinc supplements ed cat foods (15 mg/kg DM) vs. moist cat foods (5 mg/kg
most commonly used in pet foods are zinc oxide, zinc sulfate, DM) during growth/reproduction. The recommended
zinc chloride and zinc carbonate. AAFCO (2007) copper level for maintenance of adult cats is
5 mg/kg DM, regardless of the food form. The rationale for
Copper separate copper requirements for cats (extruded vs. canned) is
Of the many copper-containing proteins, four enzyme systems unclear. Investigators demonstrated increased needs for cop-
may play key roles in the clinical signs associated with copper per during reproduction in queens fed extruded foods. a
deficiency: 1) the ferroxidase activity of ceruloplasmin explains Separate requirements for extruded and canned foods were
in part the disturbances of hematopoiesis in copper deficiency, recommended in the absence of reproduction data for cats fed
2) the monoamine oxidase enzymes may account for the role of moist foods.
copper in pigmentation and control of neurotransmitters and Researchers studied chicks to evaluate the availability of cop-
neuropeptides, 3) lysyl oxidase is essential for maintaining the per from feed ingredients typically used in pet foods (Aoyagi et
integrity of connective tissue, a function that explains distur- al, 1993). Results showed that copper availability was essential-
bances in lungs, bones and the cardiovascular system and 4) the ly zero from copper oxide and pork liver. Beef, sheep and turkey
copper enzymes cytochrome C oxidase and superoxide dismu- liver, however, were highly available sources of copper. AAFCO
tase (SOD) play a central role in the terminal steps of oxidative (2007) has recommended that pet food companies discontinue
metabolism and the defense against superoxide radicals, respec- the use of copper oxide as a copper source based on studies of
tively. These functions have been postulated to account for the swine, poultry, dogs and cats in which researchers have demon-
disturbances of the nervous system as seen in neonatal ataxia in strated the poor availability of copper from copper oxide
several animal species with copper deficiency (Davis and (Aoyagi and Baker, 1993; Cromwell et al, 1989; Czarnecki-