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OCR for page 30
Formulated Diets
for Cats
Cats require specific nutrients, not specific feedstuffs.
Nevertheless, cats are quite individualistic in their feed-
ing behavior and frequently exhibit food preferences
that have been conditioned by previous dietary experi-
ence. Conditioned diet preference should not be con-
fused with nutrient requirements. While cats are car-
nivorous in the wild, satisfactory diets containing
significant amounts of vegetable matter have been de-
veloped. However, in commercial cat diets a proportion
of animal tissue is retained to satisfy specific nutrient
requirements, e.g., arachidonic acid and taurine, and
to improve the acceptability of the diet. Thus, strict veg-
etarian diets fed alone are not nutritionally adequate for
cats, even if such diets are sufficiently palatable to be
readily eaten.
The nutritional needs of domestic cats are increas-
ingly being met by commercial cat foods. While there
are a large number of products available, these fall into
the three basic types described below.
DRY-TYPE CAT FOODS
Low in moisture content (usually about 7 to 12 per-
cent), these foods commonly contain ground, whole or
dehulled cereal grains (e.g., wheat, corn, oats, barley),
cereal by-products (e.g., corn gluten meal, mill run,
wheat germ meal), soybean products (e.g., soybean
meal, soy flour, soy protein concentrate), animal prod-
ucts (e.g., poultry by-product meal, meat meal, and
meat and bone meal, meat by-products), marine prod-
fied fat on an extruded product or incorporating fat into
the core material. Crude protein content varies from 28
to 36 percent on a dry basis. Processing methods (fre-
quently expansion, extrusion, or baking) serve to par-
tially gelatinize starch for improved digestibility. The
extruded product is often sprayed with various protein
digests to increase product acceptability.
SEMIMOIST CAT FOODS
Intermediate in moisture content (usually 25 to 35
percent), these foods incorporate a level of water-solu-
ble solids such as sugar, sodium chloride, sorbates, and
low-molecular-weight alcohols (e.g., propylene glycol)
sufficient to stabilize the product through available wa-
ter control and to prevent spoilage without refrigera-
tion. Semimoist cat foods commonly contain fresh or
frozen meats (e.g., liver, kidney, tripe), animal by-
product meals (e.g., meat, poultry, liver), whole or de-
hulled cereal grains (e.g., corn, wheat, barley, oats),
cereal by-products (e. g., corn gluten meal, wheat
flour), marine products (e.g., fish meal, condensed fish
solubles), soybean products (e.g., soybean meal, soy
flour, soy protein concentrate), fats and oils (e.g., ani-
mal fat), and mineral and vitamin supplements. Crude
protein content usually ranges from 28 to 40 percent and
crude fat from 10 to 15 percent on a dry basis. These
foods are commonly marketed in a sealed pouch of a size
convenient for feeding single meals.
ucts (e.g., fish meal, condensed fish solubles), milk
products (e.g., dried whey, dried skimmed milk, so- CANNED CAT FO ODS
drum case~nate), fats and oils (e. g., cod liver oil, animal
fat), and mineral and vitamin supplements. Crude fat
content usually ranges from 8 to 12 percent on a dry
basis. These fat levels are achieved by spraying a liqui-
30
Relatively high in moisture content (usually 72 to 78
percent in the United States), these foods are usually for-
mulated to be nutritionally complete. They may also
OCR for page 31
Nutnent Requirements of Cats 31
serve as a highly palatable, specialty food that adds vari-
ety to the cat's diet but which may not be nutritionally
complete. The label should be examined for guarantees
of nutritional adequacy. Complete foods usually have
crude protein levels ranging from 28 to 50 percent and
crude fat levels from 10 to 40 percent on dry basis. They
are commonly formulated from fresh or frozen meats
(e.g., lung, liver, kidney, spleens, tripe, uclders, bone,
blood, poultry by-products), animal by-product meals
(meat, poultry), marine products (e.g., fish, fish meals),
and mineral and vitamin supplements. Some formula-
tions also contain whole or dehullec] cereal grains (e.g.,
corn, wheat, barley), soybean products (e.g., textured
soy protein derived from soy flour or soy protein con-
centrate3, fats and oils (e.g., vegetable oil), and cereal
by-products (e.g., wheat bran). Specialty canned for-
mulations are generally high-protein (40 to 65 percent)
meat- or fish-based products with mineral or vitamin
supplements added. In all canned cat foods particular
attention is needed to ensure that adequate thiamin lev-
els are present post-processing, because thiamin is espe-
cially heat labile and up to 80 percent may be lost during
canning. Cannel] cat foods should be supplemented to
several times the thiamin requirement to ensure ade-
quacy after processing.
NUTRIENT CONTENT
OF COMMERCIAL DIETS
Metabolism trials have been conducted on commer-
cial cat diets (M. A. Norvell, Quaker Oats Company,
personal communications, 1976; Kendall et al.,
1982a,b, 1985~. The proximate composition, apparent
digestibility, and metabolizability of the various types
of commercial cat diets are shown in Table 3. The aver-
age apparent digestibilities of crude protein, crude fat,
and nitrogen-free extract were appreciably less (78, 75,
and 72 percent, respectively) than the digestibility fig-
ures (91, 96, and 96 percent, respectively) used in devel-
oping Atwater's (Harris, 1966) estimated ME values of
4, 9, and 4 kcal/g. Table 3 also outlines calculated me-
tabolizable energy (CME) values by diet type according
to four different methods (OMEN to CME4~. Method 1
(CME~) calculates ME on the basis of Atwater factors;
method 2 (CME2) uses measured apparent digestibility
coefficients to adjust E values to an estimated ME;
method 3 (CME3) uses the regression equations derived
by Kendall et al. (1982a,b) to calculate ME of dry and
canned diets; method 4 (CME4) adjusts measured in
viva DE values to ME by subtracting 0.9 kcal/g digest-
ible crude protein in the diet, which is the E value of
urinary urea. The latter value does not include the en-
ergy from components in urine that are not associated
with the complete catabolism of protein. However, the
0.9 kcal/g digestible crude protein has been verified ex-
perimentally in cat metabolism trials (P. T. Kendall,
Pedigree Petfoods, personal communication, 1986~.
The footnotes to Table 3 explain the derivation of CME~
to CME4 values in detail.
Kendall et al. (1985) compared these four methods of
estimating ME content of cat diets with in viva deter-
mined ME values. The exception was that an adjust-
ment factor of 1.25 kcal/g digestible crude protein was
used for CME4 rather than 0.9 kcal/g digestible crude
protein reported in Table 3:
Prediction of ME Linear Regression
from Equation
Coeffi-
cient
of Deter-
mination
(R2)
0.96
Residual
Standard
Deviation
(RSD)
Atwater factors(CMEI)~ ME = 0.09 + 0.76CMEIb
Measured apparent
digestibility
coefficients and
E values (CME2)
Regression
equations (CME3)
(a) dry diets
(b) canned
DE by subtracting 1.25
keel digestible crude
protein (CME4)
0.214
ME = 0.09 ~ 0.93 CME2 0.96 0.228
ME = 0.47 + 0.84 CME3 0.78 0.149
ME = 0.02 + 0.96 CME3 0.95 0.049
ME = 0.01 + 1.02 CME4 1.00 0.04
.
aSubscripts refer to the venous methods.
bIn viva determined ME (keal/g, as fed).
All methods, except CME3 for dry diets, reliably esti-
mated relative ME values of cat diets, but absolute ME
was overestimated by methods 1 and 3 (dry diets only)
and to a lesser extent by method 2. Method 3 gives close
agreement with in viva ME of canned diets, while
method 4 closely agrees with in viva ME for all diets and
would be further improved by the use of 0.9 kcal/g di-
gestible crude protein rather than 1.25 kcal/g. Thus, an
accurate estimate of in viva ME content of commercial
cat diets can be obtained by adjustment of DE values by
subtracting an assumed energy value for urinary urea
(CME4), or by applying apparent digestibility coeffi-
cients and E values for major organic nutrients (CME2~.
In addition, the ME of certain canned diets can be pre-
dicted from proximate composition according to the
multiple regression equation of Kendall et al. (1982~.
However, no single method appears to be applicable for
cat diets of all types unless preceded by in viva digestibil-
ity studies. The use of Atwater factors is not recom-
mended unless apparent digestibility values above 90
percent are measured for crude protein, fat, and nitro-
gen-free extract.
Estimates of daily food requirements for cats are pre-
sented in Table 4. These estimates take into consider-
OCR for page 32
32 Nutrient Requirements of Cats
ation the recommended daily ME allowances per kilo-
gram body weight in Table 1 for the range of body
weights in Figure 1. The E concentrations used for the
dry matter in dry-type, semimoist, and canned cat foods
were 3.2, 4.1, and 4.0 kcal/g, respectively. As fed these
values would be 2.9, 3.0, and 1.1 kcal/g, respectively.
Specific calculations should be made for cat foods with
different composition. It should be emphasized, how-
ever, that these estimates are just guides, and a healthy
cat will normally regulate energy intake in relation to its
needs provided palatable food is available.
GUIDELINES FOR FORMULATING
DIETS FROM NATURAL INGREDIENTS
Diets based upon natural ingredients often have nu-
trient bioavailabilities less than that found in purified
diets. Intact proteins require enzymic digestion in the
gut, B vitamins must be released from their bound
forms, and both of these groups of nutrients may not be
completely hydrolyzed from all natural feedstuffs.
Trace elements may be rendered poorly available be-
cause of binding to phytate or fiber. While nutrient re-
quirements listed in Table 2 are appropriate for purified
feline diets, formulated diets from natural ingredients
require adjustment factors to assure nutrient adequacy.
Guidelines discusser] below are intended to bridge the
gap between minimum nutrient concentrations in puri-
fied diets and those thought necessary for typical com-
mercial diets. The factors suggested have generally been
derived from studies on species other than the cat. Ad-
justment of some of these factors may be necessary at a
later date when bioavailability data become available
for the cat.
Protein Amino Acids
Protein digestibility and hence amino acid bioavaila-
bilities generally will not exceed 90 percent for natural
ingredients commonly used in cat foods. Some pro-
teineacous ingredients, however, may be no more than
50 percent digestible, particularly those rich in collagen
protein (Baker and Parsons, 1985~. Moreover, excessive
heat processing procedures sometimes used in preparing
commercial products may enhance or lower amino acid
bioavailability. Considering the array of ingredients
used in typical feline diets, some of which are high and
some low in digestibility, an average correction factor of
1.3 is suggested. Thus, the total minimum concentra-
tions of each essential amino acid necessary in a practi-
cal-type diet can be estimated by multiplying the puri-
fied diet requirement in Table 2 by 1.3.
Vitamins
Little information is available on the bioavailability
of fat-soluble vitamins. Requirements listed in Table 2
are expressed on the basis of a specific form of each fat-
soluble vitamin, i.e., retinal, cholecalciferol, cY-to-
copherol, and phyIloquinone. Other forms or isomers
may be present in natural ingredients and may, in turn,
contribute vitamin bioactivity to the cat. Information
concerning this can be found in the text, but if more
specific and detailed information is desired, other refer-
ences should be consulted.
Vitamin E deserves special mention when dealing
with diets based upon natural ingredients. Diets con-
taining a high level of polyunsaturated fatty acids
(PUFA, e.g., in fish oil) may lead to a three- to four-fold
increase in the vitamin E requirement. Diets low in
PUFA and containing supplemental selenium and/or
antioxidant, on the other hand, may not necessitate the
30 mg/kg cz-tocophero} activity listed in Table 2. Based
upon the available information at this time and the high
digestibility of triglycerides, it would appear that, with
the exception of vitamin E for diets high in PUFA, the
levels of fat-soluble vitamin activity listed in Table 2 are
suitable for both purified and practical-type diets.
B vitamins exist both in bound and free forms. Thus,
they are not always completely available when con-
sumed by animals. Niacin and folacin are examples of
two B vitamins that are poorly available.
Intestinal B-vitamin biosynthesis likely furnishes
some bioavailable B-vitamin activity to cats, although
information does not exist in cats to quantify the extent
of this contribution. Unlike clogs, rabbits, and rodents,
however, cats engage in minimal coprophagy, and
therefore they would be expected to derive less B-vita-
min activity from intestinal synthesis than would occur
for the other species mentioned.
Work done with other species suggests that a general
bioavailability correction factor of 1.6 should be ap-
plied to the B-vitamin requirements listed in Table 2
when extrapolating them to animals consuming practi-
cal-type diets (e.g., Molitoris and Baker, 1976; Yen et
al., 1976; Anderson et al., 1978; Southern and Baker,
1981~. Thiamin is a special case, however, because it is
extremely heat labile. Thus, while multiplying require-
ment values in Table 2 by 1.6 will likely suffice for prac-
tical-diet B vitamins in general, the reader is referred
to pages 24 and 31 for special precautions concerning
thiamin.
Minerals
Other than phosphorus, little definitive information
exists on the bioavailability of essential macro-elements.
OCR for page 33
Nutnent Requirements of Cats 33
Feline diets are generally high in phosphorus, ant] a ma-
jority of that present exists as meat- or bone-derived
phosphorus, which, relative to CaHPO4 (an accepted
standard), is close to 100 percent bioavailable. Plant-
derived phosphorus, on the other hand, is largely bound
to phytate and as such should be considered no more
than 30 percent bioavailable. Because of this, dry cat
foods for kittens during the first 4 weeks postweaning
may require up to 8 g total phosphorus per kilogram of
dry diet. Other than this adjustment, requirements for
the other macro-elements (Ca, Mg, K, Na, CI) listed in
Table 2 may be applied to both purified and practical-
type diets.
Essential trace elements contained in natural ingredi-
ents are not fully bioavailable to the animals consuming
them. Unfortunately, few data exist to quantify their
bioavailability in cats. Evidence with other species,
however, suggests that both phytate and fiber can bind
trace elements and thereby render them less available.
Phytate binding of trace elements is, moreover, en-
hanced by excess dietary calcium. The binding constant
for zinc is particularly high such that the minimal re-
quirement for zinc may be up to 4 times higher when
determined with a soy-based diet than with a casein-
based cliet. The zinc requirement listed in Table 2 was
set on the assumption that a soy-protein purified diet
was fed.
Factors other than phytate and fiber can affect trace-
element bioavailability. Thus, recent evidence with
chicks has shown that a feed ingredient like menhaden
fish meal possesses mineral unavailability factors
(Halpin and Baker, 1986~. Fish meals are assumed to
contain neither phytate nor fiber, but contain measur-
able levels of neutral detergent fiber, probably mostly
chitin and chitin-like materials (D. H. Baker, University
of Illinois, personal communication, 19861. Whether
animal products other than fish contain factors reducing
trace-element availability is unknown.
The difficulty in assigning guidelines for extrapolat-
ing trace-element requirements from purified diets to
practical diets is multifaceted. Not only may trace ele-
ments be less available in diets based upon natural ingre-
dients than in those based upon purified ingredients, but
factors present in the natural ingredients may further
lower the bioavailability of inorganic sources of the
trace elements. Moreover, mineral-mineral interactions
can come into play. Classic examples of these are excess
dietary phosphorus lowering iron bioavailability and
excess dietary zinc lowering the bioavailability of both
iron and copper. A low ratio of dietary inorganic cat-
ions relative to anions may result in excessive spillage of
cations (e.g., K) in the urine. For example, high-protein
diets, perhaps by virtue of sulfate production and excre-
tion in the urine, increase the potassium requirement of
cats (Hills et al., 1982~. This phenomenon was taken
into consideration, however, in setting the minimal po-
tassium requirement for growth of the kitten. It seems
clear that mineral additions to a diet cannot be made
indiscriminantly. The kind of dietary (organic) ingredi-
ents, the source of each inorganic mineral supplement,
and mineral-mineral interactions must all be considered
carefully before fortifying practical-type diets with
sources of mineral elements. To provide a margin of
safety to protect against unavailability factors present in
diets based upon natural ingredients, an adjustment fac-
tor of 1.3 is suggested for iron, copper, and iodine. * A
correction factor of 1.5 is suggested for zinc and manga-
nese. Selenium probably requires no correction, because
food-borne selenium exists primarily as seleno-
methionine whose selenium moiety is essentially totally
available relative to sodium selenate or sodium selenite.
*Some colorants, e.g., erythrosin, contain considerable quantities
of iodine which is wholly unavailable and yet analytically shows abun-
dant iodine.
Representative terms from entire chapter:
natural ingredients
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