1708 AJVR, Vol 65, No. 12, December 2004
Similar to other species, obesity is the most common
nutritional problem in cats. The prevalence of obe-
sity in cats has increased from <10% in 1973
to 52% in recent studies.
This represents a major
health risk, with obese cats at risk for hepatic lipidosis
and 3.9 times more likely to develop diabetes mellitus,
4.9 times more likely to develop lameness, and 2.3
times more likely to suffer from nonallergic skin dis-
than cats that are not obese.
The cause of the increase in obesity in cats is
unclear, although several risk factors have been identi-
fied, including middle age (3 to 9 years
or 4 to 6
), inactivity and confinement indoors,
types of premium or prescribed diets,
access to food.
Results of several studies
that neutering is a risk factor for obesity in cats, with
neutered cats 3.4 times more likely to become obese
than sexually intact cats.
The mechanism by which neutering causes obesi-
ty has been investigated, and gonadectomy appears to
induce changes in metabolic rate and feeding pattern.
Results of 1 study
indicate that resting metabolic rate
was 28% higher in sexually intact male cats and 33%
higher in sexually intact female cats, compared with
neutered cats, whereas the metabolic rate after food
was withheld was lower in female (but not male) cats
Food intake in male cats is reported-
ly 26% higher and food intake in female cats is 17%
higher 3 months after neutering, compared with food
intake before neutering,
whereas neutered female cats
consistently eat most or all of the food offered to them
and neutered cats of both sexes substantially increase
their food intake when fed ad libitum.
energy intake, combined with reduced energy expendi-
ture due to a reduction in metabolic rate, may be the
cause of obesity after neutering.
The energy requirements of neutered cats are
therefore substantially less than sexually intact cats.
Results of a long-term study
of 60 female cats during
the 12 months before and 12 months after ovariohys-
terectomy indicate that cats that were offered carefully
controlled portions of food (40 to 45 kcal of metabo-
lizable energy (ME)/kg/d) gained only 7.5% of body
weight after neutering (with no change in body com-
position), compared with 3.6% in the previous 12
In that study, cats that were fed ad libitum
gained approximately 31% of body weight after neu-
Results of those studies
indicate that weight
gain in neutered cats is not inevitable and may be con-
trolled by restriction of dietary energy. The purpose of
the study reported here was to evaluate the effect of
dietary fat and energy density on body weight gain,
body composition, and total energy expenditure
(TEE) in neutered and sexually intact cats.
Material and Methods
Cats⎯Twenty-four (12 male and 12 female) sexually
intact European shorthair cats were used in the study. Cats were
sourced from an inbred specific pathogen free colony at 6 to 7
months of age. Cats were permitted to adapt to their new envi-
ronment and a commercially available, nutritionally complete
dry cat food
for approximately 1 month. Cats were housed in
large indoor pens and maintained on a 12-hour light-dark
cycle, with room temperature maintained between 20
At the end of the adaptation period when the mean ±SE
age of cats was 9.1 ±0.5 months, all cats received glycopy-
Received December 12, 2003.
Accepted March 18, 2004.
From the Nutrition Unit, National Veterinary School of Nantes,
44307 Nantes Cedex 3, France (Nguyen, Dumon, Siliart, Martin);
and the Royal Canin Research Center, 30470 Aimargues, France
Address correspondence to Dr. Nguyen.
Effects of dietary fat and energy on body weight
and composition after gonadectomy in cats
Patrick G. Nguyen, DVM, MS; Henri J. Dumon, DVM, MS; Brigitte S. Siliart, DVM, MS;
Lucile J. Martin, DVM, Ph.D; Renaud Sergheraert; Vincent C. Biourge, DVM, PhD
Objective⎯To evaluate the effect of dietary fat and
energy density on body weight gain, body composi-
tion, and total energy expenditure (TEE) in neutered
and sexually intact cats.
Animals⎯12 male and 12 female cats
Procedure⎯Male cats were castrated (castrated
male [CM]) or underwent no surgical procedure (sex-
ually intact male [IM]). Female cats underwent
ovariectomy (spayed female [SF]) or laparotomy and
ligation of both uterine tubes without ovary removal
(sexually intact female [IF]). Cats were fed either the
low-fat (LF) or high-fat (HF) diet for 26 weeks, with the
final allocation consisting of 8 groups: IF-LF, IF-HF, SF-
LF, SF-HF, IM-LF, IM-HF, CM-LF, and CM-HF. Mean
food intake for each group was recorded daily, and
body weight was monitored weekly throughout the
study. Body composition and TEE were measured
before surgery in week 0 and at the end of the study
(week 26) by isotope dilution (double-labelled water).
Results⎯Neutered cats gained significantly more
body fat and body weight (53.80 ±5.79%) than sexu-
ally intact cats (27.11 ±5.79%) during the study. Body
weight gain of neutered cats fed the HF diet was
greater than those fed the LF diet. Following correc-
tion for body composition, TEE was similar in all
groups and no pattern towards increased food intake
Conclusions and Clinical Relevance⎯Weight gain
in neutered cats was decreased by feeding an LF, low
energy-dense diet. To prevent weight gain in cats after
neutering, a suitable LF diet should be fed in careful-
ly controlled meals rather than ad libitum. (
Am J Vet
03-12-1243r.qxd 11/10/2004 11:03 AM Page 1708
AJVR, Vol 65, No. 12, December 2004 1709
(0.01 mg, SC), and 15 minutes later, cats were anes-
thetized with tiletamine (10 mg/kg, IV) and zolazepam
mg/kg, IV). Six male and 6 female cats were neutered, where-
as the remaining cats underwent control procedures. Male
cats were castrated (castrated male [CM]) or underwent no
surgical procedure (sexually intact male [IM]). Female cats
underwent ovariectomy (spayed female [SF]) or laparotomy
and ligation of both uterine tubes without ovary removal
(sexually intact female [IF]). Each group (CM, IM, SF, and
IF) of cats was housed separately in large indoor pens (3
cats/pen) and maintained in the same environmental condi-
tions as before surgery. After surgery, all cats were fed one of
the experimental diets. All experimental protocols were
approved by the Animal Use and Care Advisory Committee
of the Nantes Veterinary School and adhered to European
Diets⎯Two experimental dry diets were formulated by
use of the same ingredients but differed in starch, fiber, and
fat content (Table 1). The low-fat (LF) diet contained 109 g
of fat and 14.4 MJ of ME/kg, whereas the high-fat (HF) diet
contained 206 g of fat and 18.4 MJ of ME/kg. In week 13 of
the study, metabolizable energy content was determined by
measuring the digestibility of the diets in accordance with
the American Association of Feed Control Officials and
assuming an energy loss of 3.6 MJ/g of digestible crude pro-
Study design⎯The study was performed for 26 weeks.
Half of the cats were fed the LF diet, and half were fed the HF
diet. The final allocation resulted in 8 study groups (n = 3) as
follows: IF-LF, IF-HF, SF-LF, SF-HF, IM-LF, IM-HF, CM-LF,
During the study, each group of 3 cats lived together and
were fed together ad libitum with constant access to fresh
drinking water, unless otherwise indicated. All food was
renewed once daily, and food intake was calculated gravimet-
rically; the amount of food remaining uneaten was subtract-
ed from the amount of food offered. Mean daily food intake
for each group was therefore calculated as total intake for the
pen divided by the number of cats in that pen (ie, 3).
Measurements⎯Body weight of all cats was recorded
once weekly, and food intake of each group was recorded
daily. Blood samples from each cat were obtained from a
jugular vein on day 0 and at the end of week 26 for CBC; to
determine concentrations of hemoglobin, urea, creatinine,
glucose, total protein, albumin, sodium, potassium, and
chloride in plasma; and to determine activities of alkaline
phosphatase and alanine aminotransferase in plasma.
Isotope studies⎯Body composition and TEE were
assessed before the surgical procedures were performed and
at the end of week 26 by use of the dilution and elimination
rate of double-labelled water isotopes.
On the day before
these measurements were obtained, urine (cystocentesis) or
blood (jugular venipuncture) samples were obtained for
analysis of background concentration of isotopes.
On the day that measurements were obtained, each cat
was individually housed in a metabolism cage and food and
water were withheld for 2.5 hours, by which time the cat was
assumed to have equilibrated its body water. The 2 isotopes,
deuterium oxide (D
O; 99.9% deuterium-to-hydrogen ratio
administered at a dose of 0.190 ±0.003 g/kg, SC) and water
containing oxygen 18 (
O:O administered at a
dose of 0.177 ±0.003 g/kg, SC), were given to cats separate-
ly (time = 0 hours). Blood samples from each cat were
obtained from a jugluar vein 2.5 hours after administration of
the isotopes (time = 2.5 hours), after which cats were per-
mitted to eat and drink and returned to their groups.
Urine or blood samples were collected 8 and 15 days
after administration of isotopes for isotope dilution analysis.
On the morning of those days, each cat was individually
housed in a metabolism cage for 3 hours for urine collection.
Blood samples were obtained from a jugular vein in cats that
had not urinated after 3 hours. Blood samples were immedi-
ately centrifuged, and plasma and urine were stored at –20
prior to analysis.
Measurement of isotopes in plasma or urine was per-
formed by use of isotope ratio mass spectrometry
Oxygen 18 enrichment in biologic fluids was measured by
use of in vitro
O equilibration with carbon dioxide (CO
as previously described.
H) enrichment in bio-
logic fluids was measured following reduction to hydrogen
gas on zinc at 550
Total energy expenditure (TEE) was determined by use
of the Weir formula
as described previously, assuming that
the respiratory quotient was identical to the food quotient,
which was estimated from food composition and measured
Carbon dioxide production rate
)was calculated from the difference of
rate (loss in water and CO
) and deuterium elimination rate
(loss in water) and corrected with factors, taking into account
the isotopic fractionation occurring inside the body or
between the body and the outside environment. A single
homogenous water pool (equivalent to N
) was assumed.
isotope elimination rate was determined by use of logarithms
for deuterium and K
O), following transposition of
linear regressions of enrichments (atom percent excess).
Isotope pools (N
for deuterium and N
O) were calculat-
ed from the injected doses and enrichments of biologic fluids
2.5 hours after injection of isotopes, assuming the extrapolat-
ed values from linear regressions of isotopic enrichment at
time zero were similar to the plateau enrichment measured 2.5
hours after injection of isotopes. Body composition was deter-
mined as described previously
; total body water and fat-free
mass (FFM; in kg) were calculated from
O enrichment at
time zero and 2.5 hours after injection of isotopes.
Statistical analyses⎯Statistical comparisons were per-
formed by use of ANOVA; values of P<0.05 were consid-
ered significant. Multifactor ANOVA (with diet, sex, and
neuter status as factors and absolute and relative changes in
body weight, FFM, body fat, and TEE corrected for body
weight, metabolic body weight [BW
], and FFM as vari-
ables) was used to identify significant effects and interac-
tions. Although the sample sizes were small (n = 3), a 1-way
ANOVA was used to identify differences between the
groups after post hoc examination by use of the Student
Newman Keuls test. Computer software
was used for all
There were significant differences in week 0 values
for body weight and FFM among groups; therefore, rel-
Table 1—Composition of low-fat (LF) and high-fat (HF) diets fed
to neutered and sexually intact male (IM) and female (IF) cats.
Nutrients LF diet HF diet
Dry matter (g/kg) 920 923
Crude protein (g/kg) 323 333
Ether extract (fat; g/kg)) 109 206
Ash (g/kg) 52 55
Total dietary fiber (g/kg) 113 66
Nitrogen free extract (carbohydrate; g/kg) 403 340
Metabolizable energy (MJ/kg) 14.4 18.4
Diets were formulated in accordance with the American
Association of Feed Control Officials recommendations
and maintenance in cats.
03-12-1243r.qxd 11/10/2004 11:03 AM Page 1709
ative percentage change, calculated as (Value
) X100, during the course of
the study was analyzed. Body weight increased in all
groups during the 26-week study (Figures 1 and 2),
and this increase was significantly greater in neutered
cats (53.8 ±5.1%) than sexually intact cats (27.1 ±
5.9%). There was no significant effect of diet or sex on
body weight. There was no significant difference in rel-
ative percentage change in body weight among the 8
groups of cats during the study, although within a
group, relative percentage change in body weight was
numerically greater in all cats fed the HF diet, com-
pared with cats fed the LF diet (Table 2).
Similar to the observed increase in body weight,
FFM (kg) increased numerically in all groups during
the study. There was no significant effect of diet or sex
on body composition, and there was no significant dif-
ference in the relative percentage change of FFM
among the 8 groups.
Similar to body weight, fat mass (kg) increased
numerically in all groups during the study, and this
increase was significantly greater in neutered cats
(168.4 ±28.2%) than in sexually intact cats
(83.9 ±23.6%) and cats fed the HF diet (176.0 ±
31.8%), compared with the LF diet (76.3 ± 14.7%).
There was no significant effect of sex on body compo-
sition; however, neutered males fed the HF diet (CM-
HF) had a significantly higher increase in fat mass than
all other males and females fed the LF diet (IF-LF and
SF-LF), but not females fed the HF diet (IF-HF and SF-
HF). This increase in body fat in the CM-HF group was
similar to that seen in all females fed the HF diet.
1710 AJVR, Vol 65, No. 12, December 2004
Figure 1⎯Mean body weight (g) in sexually intact (IF) or spayed
female (SF) cats fed a low-fat (LF) or high-fat (HF) diet for 26
weeks. Standard error bars have been omitted for clarity (the
mean of the standard errors was 10% of the mean).
Figure 2⎯Mean body weight (g) in sexually intact (IM) or cas-
trated male (CM) cats fed an LF or HF diet for 26 weeks.
Standard error bars have been omitted for clarity (however, the
mean of the standard errors was 5% of the mean).
Figure 3⎯Mean food intake in IF or SF cats fed an LF or HF diet
for 26 weeks. Standard error bars have been omitted for clarity
(standard error of daily food intake ranged from 1 to 11 g with a
mean value of 6 g).
Table 2—Mean ±SD relative percentage change in body weight
(BW), fat-free mass (FFM; kg/cat), and body fat (kg/cat) in IM and
IF cats, and neutered male (castrated, CM) and female (spayed,
SF) cats fed an LF or HF diet for 26 weeks.
Relative Relative Relative
change in change in change in
Group BW (%) FFM (%) body fat (%)
IF-HF 36.29 22.72 11.14 11.95 156.86 84.51
IF-LF 25.03 9.60 14.34 6.96 65.82 24.82
SF-HF 69.13 11.27 37.09 9.30 178.66 27.53
SF-LF 46.70 5.98 32.10 3.02 94.63 13.81
IM-HF 28.14 ⴞ1.02 16.48 2.51 76.31 10.09
IM-LF 18.96 ⴞ8.52 14.94 5.61 36.55 21.24
CM-HF 55.40 ⴞ8.58 16.51 6.48 292.23 47.85
CM-LF 43.91 ⴞ12.41 28.39 13.02 108.06 44.83
Within a column, values with different superscript letters are
Relative percentage change was calculated as (Value
Figure 4⎯Mean food intake in IM or CM cats fed an LF or HF
diet for 26 weeks. Standard error bars have been omitted for
clarity (standard error of daily intake ranged from 3 to 10 g with
a mean value of 6 g).
Figure 2 for key.
03-12-1243r.qxd 11/10/2004 11:03 AM Page 1710
AJVR, Vol 65, No. 12, December 2004 1711
Mean daily food intake of all groups varied during
the course of the study, which precluded meaningful
statistical analyses (Figures 3 and 4). In absolute
terms, TEE (kJ/d/cat) increased in all groups during
the study. There was a significant interaction between
diet and sex; therefore, although both male and female
neutered cats fed the HF diet had similar increases in
TEE, male cats fed the LF diet had a higher increase in
TEE than female cats fed the LF diet. This interaction
was not detected in sexually intact cats. There were few
differences among the 8 groups, although neutered
male cats fed the LF diet (CM-LF) had a significantly
higher relative percentage increase in absolute TEE,
compared with most other groups.
These differences in TEE may have been caused by
changes in body composition because there was no sig-
nificant effect of diet, sex, or neuter status even after
correction of TEE for body weight, metabolic body
weight, or FFM and there were no differences among
the 8 groups (Table 3). Interestingly, at week 26, the
TEE decreased in SF-LF cats after correcting for meta-
bolic body weight or FFM, compared with all other
groups. Although not directly comparable because of
methods of data collection, a comparison of mean daily
energy intake and expenditure indicated that intake
was greater than expenditure in all cats, particularly in
neutered cats fed the HF diet (Figure 5).
For all cats, results of CBC; concentrations of
hemoglobin, urea, creatinine, glucose, total protein,
albumin, sodium, potassium, and chloride in plasma;
and activities of alkaline phosphatase and alanine
aminotransferase in plasma were within reference
intervals at weeks 0 and 26.
All cats gained weight during the 26-week study.
Because cats were immature (9 months of age) at the
beginning of the study, part of this weight gain was
attributed to growth. In addition, cats were fed ad libi-
tum during the study, which is a feeding practice that
has been identified as a risk factor for obesity.
Additionally, cats were inactive, which also predispos-
es cats to weight gain.
With only 24 cats and 8 treatment groups, the
sample size in each group was small; therefore, the
power of the statistical tests was limited. Another lim-
itation of the study was that mean daily food intake for
groups of cats rather than individual cats, was mea-
sured and recorded.
Similar to results of other studies,
cats gained more body weight during the study than
sexually intact cats and this weight gain was comprised
mostly of fat mass. Relative percentage change in body
weight gain was numerically greater in female than
male cats and this finding is supported by results of 1
but is not supported by results of other stud-
Differences among studies may be a reflection of
differences in methodology, animals sampled, and geo-
Weight gain was lowest in neutered cats fed the LF
diet, compared with those fed the HF diet, indicating
that feeding an LF, low energy-dense diet may be a
practical way to prevent obesity in neutered cats.
Obesity develops when energy intake is greater than
energy expenditure; therefore, an LF diet has been pro-
moted as a means to reduce overall energy intake in
Controversy exists as to the influence of
dietary fat on the prevalence of obesity and the effica-
cy of an LF diet in reducing obesity
; however, this
may be an issue that is specific to humans. Other
species are not influenced by the same social or
lifestyle-bound factors, and it may be argued that when
an LF diet is the only available food, this dietary
approach is likely to be successful in controlling body
weight. Consequently, consumption of HF diets results
Table 3—Mean ±SD total energy expenditure (TEE) corrected for BW, metabolic body weight (BW
), and FFM in sexually IM and IF
cats and neutered male (CM) and female (SF) cats fed an LF or HF diet measured at the beginning (week 0) and end (week 26) of the
Week 0 Week 26
TEE corrected TEE corrected TEE corrected TEE corrected TEE corrected TEE corrected
for BW for BW
for FFM for BW for BW
Group (kJ/kg of BW/d) (kJ/kg of BW
/d) (kJ/kg of FFM/d) (kJ/kg of BW/d) (kJ/kg of BW
/d) (kJ/kg of FFM/d)
IF-HF 221 15 288 17 271 13 213 8 298 5 316 10
IF-LF 263 53 339 67 336 68 261 12 339 8 346 8
SF-HF 242 7 312 16 316 13 211 21 310 24 340 26
SF-LF 288 22 380 30 378 33 231 5 321 3 321 5
IM-HF 205 18 284 25 259 25 242 13 358 20 336 12
IM-LF 229 10 323 12 287 10 254 20 358 28 314 21
CM-HF 248 40 338 49 292 45 222 9 341 10 351 19
CM-LF 171 17 232 17 221 28 236 20 338 28 325 12
Figure 5⎯Comparison of mean ±SD daily energy intake and total
energy expenditure in IM and IF cats and CM and SF cats fed an
LF or HF diet for 26 weeks. Daily energy intake was calculated as
the mean value for each group for 26 weeks. Total energy expen-
diture was calculated as the mean of the values for week 0 and
week 26 for each group.
03-12-1243r.qxd 11/10/2004 11:03 AM Page 1711
in a higher risk for obesity.
Thus, LF, low energy-dense
diets are commercially available and are recommended
for treatment and prevention of obesity in companion
However, providing an LF diet alone may
not be sufficient to completely prevent weight gain in
neutered cats. This is evident from the increased body
weight in neutered cats in the study reported here and
is supported by results of a study
indicating that ad
libitum access to an LF or HF diet results in similar
overall energy intakes. Results of other studies
cate that feeding regimen is likely to be important, sug-
gesting that optimal body weight control in cats sus-
ceptible to obesity would be achieved by a combination
of an LF diet and carefully controlled rations.
There was no effect of either sex or food on TEE,
especially when it was corrected for FFM. At the end of
our study, values were more homogenous than they
were at the beginning, which may have been caused by
a constant expenditure per kilogram of FFM and a sta-
bilization of activity levels because of the limited area
that was available and because cats were no longer
immature. All groups except for 1 had a higher TEE
after correction for FFM at the end than at the begin-
ning of the study. At the beginning of the study, cats
were growing and it was likely that their energy intake
was greater than their energy requirements, compared
with later in the study when growth was complete.
Excessive energy intake results in an increase in diet-
induced thermogenesis and therefore in TEE. In 1
group of cats, TEE corrected for FFM was higher at the
beginning than at the end of the study. Numerically, this
group had the highest TEE during the initial evaluation,
and we believe that this reflected differences in the level
of activity, which had probably not stabilized yet. In the
study reported here, neutering had little effect on TEE
of either sex, even though the sample size was small
and individual responses varied. This finding is sup-
ported by results of 1 study,
which indicated that neu-
tering had no effect on metabolic rate in cats, but is not
supported by results of other studies.
although results of 1 study
indicate that the resting
metabolic rate (measured by indirect calorimetry) of
both neutered female and male cats decreased, results
of another study
of metabolic rate (indirect calorime-
try) after food was withheld only found a neutering-
induced change in females and results of 1 study of
energy expenditure (by use of double-labelled water)
only found differences in male cats.
Results of other
studies report that there was neither a change in activ-
nor decreased roaming
Reasons for these differences are not clear and may
reflect differences in methodology, including activity
levels of cats, in various studies.
changes in metabolic rate should be considered sepa-
rately from voluntary energy expenditure such as exer-
cise; cats in the present study had little opportunity to
increase their activity levels.
The primary cause of body weight gain after neu-
tering may be increased food intake, which has been
reported in other studies
and is supported by
results of studies in rodents, in which castration or
ovariectomy caused hyperphagia that was reversed by
administration of testosterone
study reported here, food intake of cats varied. There
was no pattern towards increased intake in neutered
cats, although there appeared to be a larger discrepan-
cy between energy intake and expenditure in neutered
cats, compared with sexually intact cats of the same sex
eating the same diet. However, the effect of diet was
clearly indicated; increases in body weight and body fat
were greatest in cats fed the HF diet, compared with
cats fed the LF diet. The HF diet was higher in energy
content, and although increased weight gain in cats fed
this diet was likely a consequence of ingestion of extra
energy, this was not confirmed by results of the present
study. The macronutrient profile, especially the carbo-
hydrate content, of the 2 diets was also different, and
an effect of this difference in diet composition cannot
be ruled out. Furthermore, compared with the HF diet,
the LF diet contained approximately twice the amount
of dietary fiber and this may decrease digestion and
absorption of nutrients from the LF diet. The amount
of dietary fiber may also affect food palatability and
therefore energy intake.
Gonadectomy removes estradiol or testosterone,
which have multiple effects within the body, includ-
ing stimulating physical activity and roaming behav-
and acting as satiety signals in the CNS.
Additionally, results of a study
in which the body
weight of neutered cats was held constant indicate
that neutering induces changes in serum concentra-
tions of nonesterified fatty acids and circulating lep-
tin. Results of other studies report changes in plasma
and circulating leptin
although weight gain itself can cause glucose intoler-
ance, increased leptinemia,
and changes in the
serum lipoprotein profile (triglycerides and α-
lipoprotein concentrations increase and cholesterol
and pre-βand β-lipoprotein concentrations
Therefore, the metabolic milieu of
neutered cats is notably different from that of sexual-
ly intact cats and this may feasibly affect the parti-
tioning, utilization, and storage of macronutrients, as
detected in rodents.
Further studies investigating
the influence of hormonal and metabolic changes on
energy use in neutered cats are warranted.
Kitten 34, Royal Canin, Cedex, France.
Robinul-V (glycopyrrolate 20 mg/100 mL), Vétoquinol, Lure, France.
Zoletil 100 (tiletamine 50 mg/mL and zolazepam 50 mg/mL), Virbac,
Leman, Saint-Quentin-en-Yvelines, France.
Optima, Micromass, Manchester, UK.
SPSS 10.0.5, SPSS Inc, Chicago, Ill.
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