Impact of nutrition on canine behaviour: current status and possible
*, B. Beerda
, W. H. Hendriks
, A. F. B. van der Poel
and M. W. A. Verstegen
Animal Nutrition Group, Animal Sciences Group, Wageningen University and Research Centre, PO Box 338,
6700 AH Wageningen, The Netherlands
Animal Production Division, Animal Sciences Group, Wageningen University and Research Centre, PO Box 65,
8200 AB Lelystad, The Netherlands
Each year, millions of dogs worldwide are abandoned by their owners, relinquished to animal
shelters, and euthanised because of behaviour problems. Nutrition is rarely considered as one of
the possible contributing factors of problem behaviour. This contribution presents an overview of
current knowledge on the inﬂuence of nutrition on canine behaviour and explores the underlying
mechanisms by which diet may affect behaviour in animals. Behaviour is regulated by
neurotransmitters and hormones, and changes in the availability of their precursors may inﬂuence
behaviour. Tryptophan, the precursor of serotonin, may affect the incidence of aggression, self-
mutilation and stress resistance. The latter may also be inﬂuenced by dietary tyrosine, a precursor
to catecholamines. As diet composition, nutrient availability and nutrient interactions affect the
availability of these precursors in the brain, behaviour or stress resistance may be affected. PUFA,
especially DHA, have an important role as structural constituents in brain development, and
dietary supply of n-3 and n-6 PUFA could modify aspects of the dopaminergic and serotonergic
system and, consequently, cognitive performance and behaviour. Finally, persistent feeding
motivation between meals can increase stereotyped behaviour and aggression and decrease
resting time. This feeding motivation may be altered by dietary ﬁbre content and source. At
present, few studies have been conducted to evaluate the role of nutrition in canine (problem)
behaviour through the above mentioned mechanisms. Studies that explore this relationship may
help to improve the welfare of dogs and their owners.
Dogs: Food: Nutrients: Behaviour
The domestic dog (Canis familiaris) is believed to have
evolved from the grey wolf (C. lupis) as a separate species at
least 15 000 years ago and it is thought to be the ﬁrst animal
species to be domesticated by humans
. At the present
time, as a result of selective breeding, approximately 400
distinct dog breeds are recognised worldwide, representing
a large variation in body size and weight, with the latter
ranging from 1 to 90 kg. Initial functions of dogs such as
hunting, shepherding and guarding have diminished
gradually in importance in favour of the dog’s role as a
companion to humans
. Though most human – dog relation-
ships are fulﬁlling, each year a large number of animals are
abandoned by their owners or relinquished to animal
. Aggression toward people and animals, running
away, destructive behaviour, disobedience, house soiling
and excessive barking are unwanted behaviours that make
owners relinquish or abandon their dogs
. Although only
20 % of the dogs in the US shelters are assigned by their
owners for euthanasia
, a further 40 % of dogs admitted are
. Of the sheltered dogs that are purchased by new
owners, approximately 20 % are returned to shelters
large proportion of these animals are euthanised
number of dogs and cats euthanised annually in the USA is
estimated to be between 5 and 17 million
, with 3– 6
million as a result of behaviour problems
. Strategies that
combat problem behaviours in dogs will greatly beneﬁt
animal welfare. The behaviour of individual dogs is
controlled by numerous factors and from studies in humans
it can be derived that nutrition plays a role also. For
example, diets rich in vitamins and minerals may decrease
anti-social behaviour in schoolchildren
tation of vitamins, minerals and essential fatty acids
Abbreviations: DHA, docosahexaenoic acid; EPA, eicosapentaenoic acid; GLP-1, glucagon-like peptide-1; ISF, insoluble ﬁbre; LNAA,
large neutral amino acids; ME, metabolisable energy; PUFA, polyunsaturated fatty acid; PYY, peptide tyrosine tyrosine; SF, soluble ﬁbre;
VFA, volatile fatty acids.
*Corresponding author: Ir G. Bosch, fax þ31 317 484260, email Guido.Bosch@wur.nl
Nutrition Research Reviews (2007), 20, 180–194
qThe Authors 2007
decreased anti-social behaviour, including violence, of
young adult prisoners
. Dietary effects on behaviour have
been investigated for anti-social aspects, but also for
behavioural changes related to ageing and, in this, dogs have
been used as a model for humans. Dogs develop similar
cognitive deﬁcits and neuropathology as can be seen in
ageing humans and elderly suffering from dementia
Milgram and co-workers initiated a series of experiments
with young and aged beagle dogs to study dietary
interventions on age-related cognitive decline. Results
showed that canine food enriched with antioxidants and
mitochondrial cofactors decreased the rate of cognitive
decline in aged beagle dogs under laboratory conditions and
improved age-related behavioural changes in older pet dogs
held in home situations (for reviews, see Roudebush et al.
). These ﬁndings demonstrate clearly that
canine behaviour can be inﬂuenced by dietary components.
The present review presents an overview of our current
knowledge on the inﬂuence of dietary macronutrient
composition on the behaviour of dogs and explores
the underlying mechanisms by which diet may affect
behaviour. Findings from food– behaviour studies in dogs
and other mammals are integrated to assess in what
way problem behaviour in dogs may be reduced through
Effects of dietary amino acids and protein content on
After ingestion, proteins are enzymically degraded and
absorbed in the small intestine mainly as tripeptides,
dipeptides and free amino acids. After hydrolysis of the
peptides in the enterocytes, the free amino acids are
transported through the portal vein to the liver. Amino acids
are important constituents required for the synthesis of
enzymes and other proteins, and used as precursors for the
synthesis of neurotransmitters and hormones
example, serotonin, catecholamines, acetylcholine and
histamine are metabolites from tryptophan, tyrosine, choline
and histidine, respectively
. These neurotransmitter pre-
cursors (except for choline) are amino acids and are natural
dietary constituents. Behaviour results from signal detec-
tion, transmission and processing in the (central) nervous
system, which is accomplished and modulated by chemical
messengers such as neurotransmitters and hormones.
Changes in neurotransmitter precursors such as tryptophan
and tyrosine are, therefore, likely to inﬂuence behaviour.
The amount and timing of food intake, diet composition and
digestibility are all factors that determine the availability of
different amino acids, i.e. precursors of chemical messen-
gers. Consequently, such factors may inﬂuence behaviour.
The effects of tryptophan and tyrosine on behaviour will be
discussed as these could be relatively potent modulators; for
similar reports on choline, histidine and threonine, we refer
Findings and mechanisms in different mammals
Tryptophan. A diet high in tryptophan has been shown to
reduce mouse killing by rats
, reduce aggression in
, enhance exploratory behaviour in female
and reduce self-injurious behaviour in rhesus
. In contrast to the observed reductions in
aggression in some experimental conditions, dietary
supplementation of tryptophan has also been shown to
increase territorial aggression in male mice
tryptophan may also inﬂuence the resistance or tolerance to
stress and, therefore, change the behavioural stress
response. Koopmans et al.
reported enhanced recovery
after social stress as measured by lower plasma cortisol and
noradrenaline concentrations in pigs fed a surplus of dietary
tryptophan compared with pigs fed diets containing a
‘normal’ concentration of tryptophan. In addition, sup-
plementation of dietary tryptophan reduced plasma cortisol
concentrations during a stress-inducing mental arithmetic
task in healthy stress-vulnerable humans
. It was, therefore,
suggested by Markus et al.
that tryptophan supplemen-
tation above normal dietary concentrations could improve
the ability of an individual to cope with stress. The effects of
dietary tryptophan on stress resistance involve different
pathways. In rats a variety of stressors, such as
immobilisation, foot shock, and hypothermia, increase
brain tryptophan and serotonin turnover
26 – 29
humans show decreased plasma tryptophan concentrations
in comparison with normal subjects
. It appears that
initially stressors stimulate serotonin turnover, which over
time may deplete serotonin (precursor) supplies and result in
decreased serotonin (precursor) concentrations.
Quantitatively the most important pathway for tryptophan
metabolism, after protein synthesis, is the kynurenine
pathway which is responsible for over 90 % of tryptophan
. In humans, normally 1 % of the available
tryptophan is converted to serotonin which is mainly present
in the gastrointestinal tract
. The ﬁrst and rate-limiting step
in the synthesis of serotonin is the hydroxylation of
tryptophan to 5-hydroxytryptophan by the enzyme trypto-
phan hydroxylase (Fig. 1). Tryptophan hydroxylase is
normally about half saturated with tryptophan
quently, an increase in tryptophan in the brain, which
increases serotonin synthesis and serotonergic neurotrans-
, can maximally double serotonin synthesis. The
second step in the synthesis of serotonin is the
decarboxylation of 5-hydroxytryptophan to serotonin
which is stored in vesicles in the nerve terminal were it is
held before release. When serotonin is released into the
synaptic cleft, serotonin can bind to different subtype
receptors (for reviews, see Barnes & Sharp
). Via binding to these different receptors, serotonin
can produce many different effects on post-synaptic cells
inﬂuencing various parts of the brain involved in controlling
a variety of physiological functions including hormone
releases, cardiovascular functioning, pain, appetite, and in
general mood and behaviour
35 – 37
Tryptophan transport across the blood–brain barrier and
metabolism is in part affected by animal factors such as
, social status
level of arousal
. The availability of dietary tryptophan to
the brain is largely dependent on the composition of
the ingested diet. Tryptophan is found in nearly all
protein-containing foods where it is found in a lower
concentration compared with the other large neutral amino
acids (LNAA) tyrosine, phenylalanine, leucine, isoleucine
Impact of nutrition on canine behaviour 181
. For access into the brain, tryptophan shares the
same carrier as other LNAA for transport across the blood –
. Central tryptophan concentrations can either
be increased by increasing plasma tryptophan or by
lowering plasma concentrations of LNAA
. As trypto-
phan is normally present in only small concentrations in
dietary protein compared with other LNAA, the consump-
tion of a meal high in protein will decrease the ratio of
tryptophan to other LNAA
and thereby potentially lower
The fraction of unbound tryptophan as compared with
that bound to albumin is another factor that may inﬂuence
tryptophan availability to the brain
approximately 80– 90 % of all tryptophan molecules in the
blood are bound to serum albumin
. It has been suggested
that the majority of the albumin-bound tryptophan is
available for passage across the blood– brain barrier
possibly the concentration of circulating free tryptophan
may be especially important
. According to Chaouloff
three factors affect circulating free and bound tryptophan
concentrations: (i) the rate of lipolysis because blood non-
esteriﬁed fatty acids displace tryptophan from its binding to
; (ii) the activity of tryptophan 2,3-dioxygenase,
the rate-limiting enzyme in tryptophan detoxication through
the kynurenine pathway – activation (inactivation) of this
enzyme decreases (increases) circulating blood tryptophan
; (iii) uptake into peripheral and central tissues.
Carbohydrate-induced insulin rises facilitate the uptake of
most LNAA into skeletal muscle, but not tryptophan bound
. Consequently, the ratio of tryptophan
relative to LNAA increases. This results in a competitive
advantage of tryptophan over LNAA for uptake at the
blood– brain barrier. However, as little as 2– 4 % of the
energy of a meal as protein seems to prevent this increased
availability of tryptophan
Tyrosine. In rats, a high-tyrosine diet prevents adverse
behavioural and neurochemical effects (for example,
immobility during a swim test, depletion of brain
noradrenaline) of various acute stressors including
, restraint and tail-shock
57 – 59
. Human studies
also suggest beneﬁcial effects of tyrosine under conditions
of stress (for reviews, see Lieberman
Tyrosine, which can be synthesised from phenylalanine,
is the direct precursor for the catecholamines dopamine,
noradrenaline and adrenaline
. Dopamine can be
synthesised from tyrosine in neurons in two steps. The
ﬁrst and rate-limiting step is the conversion of tyrosine to
dihydroxyphenylalanine by the enzyme tyrosine hydroxy-
lase. In rats, central tyrosine hydroxylase is approximately
75 % saturated with tyrosine
. In the second step,
dihydroxyphenylalanine is decarboxylated to dopamine
which can be used as an endproduct (neurotransmitter) in
neurons or further converted to noradrenaline or adrena-
. Like tryptophan, tyrosine competes with other
LNAA at the blood–brain barrier for entry into the
and is taken up into skeletal muscle under the
inﬂuence of insulin
. In diets, tyrosine is typically
available in much higher concentrations compared with
tryptophan and high-protein meals will typically raise
tyrosine concentrations in the brain, but will lower the
concentration of tryptophan
. Catecholamines play a key
role in a variety of behavioural, neuroendocrine and
cardiovascular responses during stress
. Increases in
Fig. 1. Effects of dietary characteristics on tryptophan uptake by the central nervous system and synthesis of serotonin from brain tryptophan
(adapted from Grimmett & Sillence
with modiﬁcations). ( ), Factors that may ultimately decrease brain tryptophan; 5-HTP,
5-hydroxytryptophan; NEFA, non-esteriﬁed fatty acids.
G. Bosch et al.182
brain tyrosine have little or no effect on catecholamine
, but the situation may be different during
stress when brain noradrenaline turnover increases and
noradrenaline concentrations decrease
. An enhanced
noradrenergic activity is part of a normal adaptive stress
. In stressed rats (tail-shock), ingestion of a
high-tyrosine diet reversed the post-stress decline in brain
noradrenaline and attenuated behaviour changes, i.e.
decreased locomotion, standing on hind legs, hole-poking
in a novel open ﬁeld
. This suggests that a high-tyrosine
diet may be beneﬁcial during severe stress, as it prevents
depletion of the substrate required for catecholamine
synthesis in times of high catecholaminergic activity and
Findings in dogs
Studies on the effects of tryptophan or tyrosine on behaviour
in dogs seem to be limited to one. DeNapoli et al.
formulated diets with high or low protein content
(approximately 310 or 190 g crude protein/kg, respectively)
and with or without tryptophan supplementation (1·45 g/kg)
in order to provide varying tryptophan contents and
tryptophan:LNAA ratios (Table 1). Each of the four diets
was fed in random order for 1 week to thirty-three privately
owned dogs that displayed a high territorial aggression,
dominance aggression or hyperactivity. There was no effect
of dietary protein or tryptophan content on the behavioural
scores within each group of problem behaviour. However,
when the groups of dogs were analysed as one study
population a lower territorial aggression score was obtained
for dogs fed the high-tryptophan diet compared with dogs
fed the low-tryptophan diet, but only when fed a low-protein
diet. In addition, dogs fed the high-protein diet without
tryptophan supplementation showed a higher dominance
aggression score compared with dogs on the other dietary
Three studies in literature have reported that low-protein
diets decreased aggression in dogs, though these were not
performed under controlled experimental conditions. In a
study with seven aggressive golden retrievers held at in-
home living conditions, incidences of aggression as reported
by their owners immediately decreased after the introduc-
tion of a low-protein diet (15–18 % of total energy)
Unfortunately, neither the composition of the experimental
diet nor the composition(s) of the diet(s) before the dietary
intervention were reported. The reduction in aggressive
incidences, however, was only sustained in three dogs; two
dogs deteriorated again in their behaviour and contact
was lost with the remaining two clients. In another study,
twelve dogs that exhibited either high territorial aggression,
dominance aggression or hyperactivity and fourteen control
dogs were fed each of three diets varying in protein content
(180, 250 and 310 g crude protein/kg DM) for 2 weeks at
in-home living situations
. The low-protein diet and
medium-protein diet decreased territorial aggression scores
compared with the high-protein diet. No effects of dietary
protein content in dogs with dominance aggression or
hyperactivity were found. Additional behavioural analysis
of the group of dogs demonstrating territorial aggression
revealed that ﬁve of these dogs showed dominance-related
Table 1. Effect of dietary protein and tryptophan (TRP) content on canine behaviour
Authors Dogs and design Diets* Results
Seven aggressive golden retrievers at in-home living
situations. Measurements were not reported
15 –18 % protein of total dietary energy
based on approximately 20 % meat
and 80 % boiled rice
Seven dogs improved of which three sustained the improvement,
two worsened, and contact was lost with two clients
Twelve territorial aggressive, twelve dominance aggressive,
twelve hyperactive and fourteen control dogs
(age .1 year) fed each diet (Latin square) at
in-home living situations for 14 d. Each day, owners
scored their dogs for territorial aggression, dominance
aggression, excitability and fearfulness
(1) 180 g protein/kg; 1·0 g TRP/kg;
(2) 250 g protein/kg; 1·6 g TRP/kg;
(3) 310 g protein/kg; 1·6 g TRP/kg;
(a) Territorial aggressive dogs showed lower territorial aggression
scores when fed diets 1 and 2 compared with diet 3
(b) Seven territorial aggressive dogs were fearful and showed lower
territorial aggression scores when fed diets 1 and 2 compared
with diet 3; the remaining ﬁve territorial aggressive dogs tended
to be dominant which was not affected by dietary treatment
(c) No changes in behaviour scores of dogs within the dominance
aggressive, hyperactive and control groups
Eleven territorial aggressive, eleven dominance aggressive
and eleven hyperactive dogs (age .1 year) fed each
diet (at random) at maintenance level at in-home living
situations for 7 d. Each day, owners scored their dogs for
territorial aggression, dominance aggression, excitability,
fearfulness and hyperactivity
(1) 186 g protein/kg; 1·8 g TRP/kg;
(2) 188 g protein/kg; 3·0 g TRP/kg;
(3) 308 g protein/kg; 2·4 g TRP/kg;
(4) 315 g protein/kg; 3·7 g TRP/kg;
(a) No changes in behaviour within each behaviour group for any
(b) When all dogs were combined, dominance aggression scores
were higher for dogs fed diet 3 compared with dogs fed
diets 1, 2 and 4
(c) When all dogs were combined, territorial aggression
scores were higher for dogs fed diet 1 compared
with dogs fed diet 2
LNAA, large neutral amino acids (tyrosine, phenylalanine, leucine, isoleucine, valine).
* Values are presented on a DM basis.
Impact of nutrition on canine behaviour 183
territorial aggression, whereas the other seven dogs
showed fear-related territorial aggression. In the latter
dogs, territorial aggression decreased when fed the low-
For adult dogs fed at maintenance, the minimal dietary
tryptophan requirements are currently set at
0·0669 g/1000 kJ (0·28 g/1000 kcal) metabolisable energy
(ME) with a tryptophan:LNAA ratio of 0·061 : 1 and for
tyrosine and phenylalanine the minimal dietary require-
ments are 0·3537 g/1000 kJ (1·48 g/1000 kcal) ME
Association of American Feed Control Ofﬁcials
has minimum dietary requirements for these
nutrients which are slightly higher (0·1099 and 0·4995 g/
1000 kJ (0·46 and 2·09 g/1000 kcal) ME, respectively) in
order to account for the lower digestibility and availability
of nutrients in commercial canine foods compared with
semi-synthetic diets. Nutritional guidelines for humans
and dogs rarely take behaviour into account as a response
criterion, something which has been criticised
minimum quantity of tryptophan in a commercial canine dry
expanded diet that has passed a maintenance AAFCO
feeding protocol has been reported to be 0·0502 g/1000 kJ
(0·21 g/1000 kcal) ME
. The criteria for passing an
AAFCO maintenance feeding protocol however, do not
take into account animal behaviour. It is unknown if the
minimal amount of tryptophan in typical dog foods meets
the requirements of the wide variety of dogs, for example,
from emotionally stable to anxious individuals, under
different conditions, for example, from stress-free to
stressful. Both excessive intake and a deﬁciency of
tryptophan are detrimental to the health of an animal
and are likely to affect behaviour. In horses, a dose of
0·1 mg/kg body weight appears to be too low, causing mild
. In humans, the most common side effect of
overfeeding precursors of neurotransmitters has been
reported to be nausea
. There are currently no requirement
estimates for the maximum amount of tryptophan
in canine food and it remains to be determined how
high-tryptophan diets affect the health of dogs and their
behaviour in the long term.
Effects of dietary lipids on behaviour
Lipids have various functions, such as constituents of
cellular membranes, precursors for chemical messengers
(for example, steroid hormones) and their use as an energy
source or stored in the body as adipose tissue. After adipose
tissue, the central nervous system has the greatest
concentration of lipids
. The structural constituents in the
grey matter of the brain and retinal tissues in mammals are
derived from dietary linoleic acid (18 : 2n-6) and a-linolenic
acid (18 : 3n-3). Both are polyunsaturated fatty acids
(PUFA) and can be metabolised to long-chain PUFA by
sequential alternating enzymic desaturation and elongation.
Linoleic acid can be metabolised to arachidonic acid
(20 : 4n-6) which can be further metabolised to docosapen-
taenoic acid (22 : 5n-6). The enzymic desaturation and
elongation of a-linolenic acid yields eicosapentaenoic acid
(EPA) (20 : 5n-3) which can be further metabolised to
docosahexaenoic acid (DHA) (22 : 6n-3)
Findings and mechanisms in different mammals
There is ample scientiﬁc literature available in which the
effects of both dietary deﬁciency and supplementation of
PUFA on animal performance in cognitive or behavioural
tests are evaluated (for reviews, see Wainwright
McCann & Ames
). For example, the learning ability of
rodents decreased when fed n-3 fatty-acid-deﬁcient
and increased when fed DHA-supplemented
compared with rodents fed diets adequate in n-3
fatty acid concentrations. Other studies, however, did not
ﬁnd affects of dietary n-3 PUFA manipulation on learning
performance as tested with a Morris water-maze in rats
. Dietary PUFA seem to affect animal cognition
but can also cause behavioural changes. Rats fed n-3
PUFA-deﬁcient diets showed increased aggression scores
in a resident intruder test
and increased expression of
stress-related behaviours during several stress tests
compared with male rats fed adequate amounts of n-3
PUFA. Similarly, anxiety was found to be increased in
mice fed a diet deﬁcient in n-3 PUFA
, though others did
not observe any effects of dietary PUFA on anxiety in
The dopaminergic and serotonergic systems in the brain
are known to play important roles in learning, emotions, and
37,86 – 90
, which makes it tempting to assume
that the effects of PUFA on behaviour run through these
systems. Indeed, both systems are known to be inﬂuenced
by PUFA. Rats deﬁcient in n-3 PUFA compared with rats
fed diets with a-linolenic acid showed a reduction in
dopamine concentration in the frontal cortex
91 – 94
increase in dopamine concentration in the nucleus
but no effects in the striatum
. In the
frontal cortex of these animals the rate of dopamine
synthesis and breakdown mediated by monoamine oxidase
was not affected
and the reduced concentrations may
have been linked to the reduced dopaminergic storage
. Changes in dopamine concentrations were
followed by changes in number of D
PUFA-deﬁcient rats had a lower number of D
the frontal cortex
but higher in the nucleus
. Rats fed diets supplemented with EPA
and DHA had an increased dopamine concentration and D
binding possibly as a result of a reduction in monoamine
oxidase activity in the frontal cortex compared with rats fed
adequate amounts of PUFA
As for dopamine concentrations, frontal cortex serotonin
concentrations were increased in rats fed diets sup-
plemented with n-3 PUFA
. In line with this, serotonin in
the frontal cortex was reduced in piglets fed n-3 and n-6
PUFA-deﬁcient formula for 18 d from birth compared with
piglets fed formula supplemented with linoleic acid and a-
linolenic acid and/or arachidonic acid and DHA
ﬁndings in the frontal cortex may not extrapolate to other
brain areas. For example, in the hippocampus of 2-month-
old rats fed an n-3 PUFA-deﬁcient diet extracellular basal
serotonin concentrations were increased
probably due to reduced storage pools
, not due to
decreased activity of monoamine oxidase
. Such effects of
n-3 PUFA deﬁciency on serotonin concentrations are not
found in all studies (for example, Delion et al.
G. Bosch et al.184
In addition to the observed changes in the dopaminergic
and serotonergic systems in different brain regions, physical
properties (for example, ﬂuidity, permeability) of cerebral
membranes may also mediate dietary effects on cognition
. For example, chronic dietary deﬁciency in
n-3 PUFA resulted in low concentrations of n-3 PUFA in the
whereas diets high in EPA and DHA resulted in
high concentrations of EPA and DHA in the brain of
. In addition, dietary a-linolenic acid deﬁciency
induces a more pronounced reduction in DHA concen-
trations in the frontal cortex than in the striatum and
. Besides changes in brain PUFA compo-
sitions, dietary PUFA may alter properties of the neuronal
membrane, such as the activity of membrane-bound
enzymes, receptors and ion channels
. These alterations
may affect neurological functioning and may, therefore, also
contribute to the observed changes in cognitive functioning
Findings in dogs
To the authors’ knowledge, there are at this moment no
scientiﬁc articles available regarding the inﬂuence of n-3 or
n-6 PUFA deﬁciency or enrichment on canine behaviour or
cognitive performance. Since DHA is essential for the
development and function of the brain and retina
supply may affect neurological development in puppies. For
example, low dietary concentrations of DHA during the
gestation or lactation of bitches and dry diets for puppies
depressed their retinal sensitivity
. Although the
immediate connection between the cellular effects of
DHA and visual sharpness and cognitive abilities in
receiving dietary DHA still needs more support
seem to emphasise the importance of DHA in the diet of
bitches during gestation until weaning and the diet of
puppies in order to ensure optimal neurological develop-
ment. At present, there is no recommended allowance for
DHA for both bitches in gestation and lactation or puppies,
but the recommended allowance for a-linoleic acid is 3·35 g/
1000 kJ (0·8 g/1000 kcal) ME
. A diet high in a-linolenic
acid fed from breeding throughout lactation increased a-
linolenic acid concentration in milk but failed to do this for
. In a recent study, puppies converted a-linolenic
acid to DHA during the ﬁrst month of weaning but little
conversion of a-linolenic acid to DHA occurs after
. It seems that the capacity of puppies to
synthesise DHA from dietary a-linolenic acid or other n-3
fatty acid precursors is active for only a short time during the
neonatal period and is decreased thereafter. The amount of
dietary a-linolenic acid for sufﬁcient synthesis of DHA and
the amount of DHA required for optimal neurological
development in puppies still remain to be determined.
Whether the provision of sufﬁcient DHA for optimal
neurological development in dogs also results in changes in
the dopaminergic and serotonergic systems and subsequent
effects in cognitive abilities or behaviour in later life remains
to be conﬁrmed.
Concerning commercial dog food, it seems likely that in
dogs deﬁciencies of PUFA are rare as long as fat oxidation
during process and storage of the food is limited
of PUFA, particularly the n-3 family, are nowadays higher in
commercial dog food compared to foods of several years
(Delton-Vandenbroucke et al., 1998). However, the
amount and ratio between n-6 and n-3 fatty acids may differ
considerably between commercially available diets. The
n-6:n-3 fatty acid ratio of twelve commercial dry dog foods
was found to differ considerably, ranging from 17:1 to
Effects of dietary carbohydrates on behaviour
Feeding of mammals is a discontinuous process in which
periods of food consumption are interspersed with periods
. Food intake behaviours are controlled by
feelings of hunger
, but may be modulated
by psychological and social factors
. Numerous central
and peripheral signal molecules are involved in the
regulation of eating (for reviews, see Bray
, de Graaf
and Strader & Woods
). The rate and site of
degradation of nutrients largely determines the postprandial
physiological state of an animal and in this way the extent
and duration of satiety and, therefore, behaviour. There is a
wide variety of carbohydrates with different physical and
chemical properties. These properties can affect the rate and
site of degradation of these carbohydrates
. In single-
stomached animals, degradable carbohydrates may be
digested with endogenous enzymes in the ﬁrst part of the
gastrointestinal tract, or fermented by micro-organisms that
colonise predominantly the last part of the gastrointestinal
tract. Products derived from digestible carbohydrates are
mainly monosaccharides. The digestion of starch and
absorption of monosaccharides are primarily responsible for
the ﬂuctuations in the postprandial blood glucose concen-
trations that subsequently may modify tryptophan avail-
ability in the brain when protein intake is low (see section on
Findings and mechanisms in different mammals: Trypto-
phan), and inﬂuence mood in at least humans (for a review,
). The indigestible carbohydrates are often
referred to as dietary ﬁbre, which contains non-starch
polysaccharides, resistant starch and non-digestible oligo-
saccharides. The fermentation endproducts of dietary ﬁbre
are volatile fatty acids (VFA; acetic, propionic and butyric
acid), lactate, alcohol and the gases methane, hydrogen and
. Apart from the fermentability, other
physical and chemical properties of dietary ﬁbre include
solubility, ability to bind water and affect viscosity, and
possible interactions with the digestion and absorption of
starch, protein and fat. In addition, the duration of satiety
experienced by animals between meals may be affected by
carbohydrates, which in turn may reduce the behavioural
side effects of a high feed motivation.
Findings and mechanisms in different mammals
The effects of dietary carbohydrate sources (i.e. ﬁbrous
ingredients) on animal behaviour have been relatively well
studied especially in pigs, where non-lactating sows were
fed energy-restricted diets in order to prevent excessive lipid
deposition and reduced reproduction performance. Com-
monly diets for sows are formulated to meet the daily
nutrient requirements for maintenance and reproduction.
However, the latter may not result in a sufﬁcient level of
Impact of nutrition on canine behaviour 185
satiety between meals and is believed to be an important
reason for a persistent high feeding motivation throughout
the day contributing to the development of stereotyped
. In order to reduce stereotyped behaviour in
sows, diets high in ﬁbrous ingredients (sugarbeet pulp, oat
hulls, soyabean hulls, wheat bran) can be fed
resulting in an increased time of sows laying down
increased resting time, less time spent on foraging and
and reduced posture changes 8 and 10 h after
. The latter authors compared sows fed a high-
and a low-fermentable carbohydrate diet (for further
examples, see Meunier-Salau
). The relationship
between dietary ﬁbre content and stereotyped behaviour has
also been documented in horses. A large survey among
trainers of race horses in Sweden revealed a negative
correlation between the amount of roughage provided and
the incidence of stereotyped behaviour (cribbing or wind-
sucking, weaving, box-walking) or wood-chewing in
. Wood-chewing may be related to a ‘ﬁbre
deﬁciency’ in the diet and represent attempts to increase
dietary ﬁbre intake
126 – 128
. The effect of ﬁbrous ingredients
on behaviour is not generic for all ﬁbre sources; for
example, solvent-extracted coconut meal and soyabean
hulls as a dietary ﬁbre source do not appear to affect
physical activity in pigs
, whereas sugarbeet pulp silage
. Since sows which are fed low amounts of feed were
shown to be more active compared with sows fed large
amounts of feed
it has been suggested that hunger is most
likely the cause of the increased physical activity
The variety in physical and chemical properties of
different ﬁbrous ingredients results in differences between
these ﬁbres in creating and maintaining satiety and
preventing feelings of hunger. The biological mechanisms
behind the satiating properties of dietary ﬁbre are still not
fully understood, but several dietary ﬁbre characteristics
seem to be important. First, ﬁbres with a high water-binding
capacity may increase the volume and weight of the gastric
contents when liquids are available. The weight or volume
may stimulate stretch receptors that can induce gastric
signals of satiation
. Second, gastric emptying can be
affected either directly by dietary ﬁbres high in intragastric
or indirectly through the stimulation of the
release of glucagon-like peptide-1 (GLP-1) (a potent
inhibitor of gastric emptying
). Stimulation of GLP-1
production can be mediated through carbohydrate fermenta-
tion in the distal part of the gastrointestinal track
through the production of VFA (mainly acetate) which
stimulates the release of peptide tyrosine tyrosine
137 – 139
. The effects of GLP-1 and PYY in delaying
gastric emptying are referred to as the ‘ileal brake’
mechanism which results in a moderate and stable ﬂow of
nutrients from the stomach into the small intestine
decrease in postprandial gastric-emptying rate will, conse-
quently, prolong gastric distension and gastric signals of
137 – 139
. This mechanism was studied by Moran
in rhesus monkeys where intramuscular injections
of PYY reduced gastric emptying and resulted in a decrease
in food intake. In addition, there are indications that PYY in
the brain reduces appetite in humans
, although this is still
a subject for debate
. Third, ﬁbrous dietary ingredients
may increase small-intestinal transit time
, possibly also
by stimulation of PYY which is found to suppress intestinal
. An increase in small-intestinal transit time: (i)
prolongs contact between nutrients and intestinal receptors
involved in maintaining satiety
and postpones feelings
; (ii) results in the slowing down of starch
digestion and subsequent absorption of glucose, thereby
maintaining more stable postprandial glucose and insulin
concentrations in the blood
. A transient decline in blood
glucose level preceded meal initiation in rats
and caused a delay in the decrease in blood
glucose concentrations. This may prolong satiety and
postpone hunger and meal initiation (for a review, see
Campﬁeld & Smith
). Finally, fermentation of carbo-
hydrates may yield VFA which leads to a higher level of
satiety by (i) PYY-mediated reduction of gastric emptying
and (ii) becoming a source of energy (mainly acetate)
at times when glucose supply from the small intestine is
decreasing, which stimulates longer-term sati-
As suggested previously, hunger is most likely the cause
for the observed behavioural effects seen in sows
or appetite is correlated with the peripheral concentration of
, a twenty-eight amino acid peptide synthesised
predominantly in the stomach
. For example, a rise in
blood ghrelin concentration is associated with meal
initiation in humans
. Supplementation of short-chain
oligofructose (average degree of polymerization of 4·5) in a
diet for 3 weeks decreased energy intake and lowered
ghrelin concentrations in rats compared with rats fed the
control diet without fructan supplementation. However, rats
fed a diet supplemented with long-chain oligofructose
(average degree of polymerization of 25·0) showed a
decrease in energy intake but not in ghrelin concentrations
compared to rats fed the control diet
. It is suggested that
the lower blood ghrelin concentrations may contribute to a
decrease in appetite during fasting
. Whether these
results were accompanied with changes in behaviour (for
example, food-seeking behaviour) requires further
investigation. Fig. 2 shows the effects of dietary ﬁbre on
Findings in dogs
‘When we are considering how a dog is behaving, we really
should be considering what is inside the stomach’
, p. 1046). Despite this statement, little
additional research has been conducted on the association
between canine behaviours and satiety or feeding
motivation between meals. To the authors’ knowledge,
three studies have investigated the effects of dietary ﬁbre on
satiety and feeding motivation in dogs of which only one
also studied canine behaviour and another measured
ad libitum food intake of dogs fed diets varying in ﬁbre
source and content (Table 2). Butterwick & Markwell
overweight dogs (.115 % ideal body weight) six different
moist diets varying in type and amount of ﬁbre on an
energy-restricted basis (45 % restriction of calculated
maintenance energy requirements; ME (kJ) ¼461 £body
). The four experimental high-ﬁbre diets
formulated to vary in soluble ﬁbre (SF) and insoluble ﬁbre
(ISF), i.e. (g/kg DM) 40·8 SF and 13·6 ISF; 112·5 SF and
G. Bosch et al.186
37·5 ISF; 35·7 SF and 202·4 ISF; 24·8 SF and 310·6 ISF, were
compared with two dry control diets (36·5 SF and 14·6 ISF;
45·5 SF and 15·2 ISF). The authors found no differences in
time spent at behaviours related to feeding motivation (i.e.
cumulative time spent at feeding bowl and number of visits
to bowl 30 min after feeding, intake of a meal (canned diet)
provided 3 h after introduction of the test diets) between
dogs fed the different diets. In contrast, Jewell & Toll
ﬁnd effects of ﬁbre content on the satiety of dogs. Dogs with
ad libitum access to dry diets with a medium or high crude
ﬁbre content (135·5 and 223·4 g/kg DM) decreased total ME
intake compared with dogs that had ad libitum access to
low-crude ﬁbre diets (16·3 and 16·4 g/kg DM). When dogs
were offered a subsequent meal, 30 min after the end of the
last meal, energy and DM intake were lower in dogs fed the
high-ﬁbre diet compared with dogs consuming the low-ﬁbre
. Similarly, Jackson et al.
observed that a high-ﬁbre
content in dry diets reduced energy intakes in dogs. These
authors fed dogs in the morning either a diet high in total
dietary ﬁbre (26·7 SF, 263·7 ISF g/kg as fed) or low in total
dietary ﬁbre (18·1 SF, 123·2 ISF g/kg as fed) followed 6 h
later by ad libitum access to a diet containing 23·2 SF, 123·5
ISF g/kg as fed. Average energy intake over the day was
lower (kJ/kg body weight) in the dogs fed the high-ﬁbre diet
in the morning compared with the energy intake of the dogs
fed the low-ﬁbre diet in the morning (273 v. 332 kJ (65·3 v.
79·4 kcal)/kg body weight). The difference in average daily
energy intake was the result of the energy intake in the
morning since there were no signiﬁcant differences observed
in intake of the diet provided in the afternoon between
the high-ﬁbre (181 kJ (43·2 kcal)/kg body weight) and low-
ﬁbre (197 kJ (47·2 kcal)/kg body weight) groups. These
latter two studies showed that high levels of ﬁbrous dietary
ingredients in dogs can increase satiety and reduce energy
intake. This, however, was not conﬁrmed in a study
by Butterwick & Markwell
. The latter may be due to the
energy restriction and the large differences in DM content of
diets between studies. Energy restriction will result in an
increased feeding motivation in dogs to a level that nulliﬁes
the possible effects of ﬁbre on satiety
. DM content of the
moist diets fed to dogs in the study of Butterwick &
ranged between 132 and 168 g/kg whereas
Jewell & Toll
and Jackson et al.
fed dry diets with a
DM content between 908 and 923 g/kg. On an energy basis,
intake of a diet with a high DM content or high energy
density will result in lower weight of the digesta in the
stomach compared with a diet with similar nutrient
composition but lower DM content. A low dietary DM
content will therefore have a higher weight of digesta in the
stomach and will stimulate stretch receptors which affect
satiety in dogs
. Finally, food intake in g DM/kg body
weight was found to be lower in dogs with ad libitum access
to a diet with 15 g short chain fructo-oligosaccharides/kg
DM compared with dogs with ad libitum access to a diet
with 60 g cellulose/kg DM
. The authors suggested that
satiety between diets was altered because of the differences
in fermentability of the ﬁbre sources included in the diets.
Unfortunately, no measurements were made in this study to
elucidate possible mechanisms underlying their observed
difference in food intake.
The mechanisms behind the observed effects of dietary
ﬁbre on inducing and maintaining satiety in humans and
pigs (see previous section) have in part been also observed
in dogs. Stimulation of stretch receptors through infusion
of liquids or ﬁlling a balloon with water placed in the
stomach reduced sham feeding in dogs, indicating that
stimulation of stretch receptors induces satiety in dogs
Gastric emptying was reduced in dogs as ﬁbre (for
example, psyllium, guar gum) content and viscosity of the
, which will prolong gastric distension
Fig. 2. Effects of dietary ﬁbre (DF) on satiety. ( ), Factors that may ultimately increase the residence time of digesta in the designated
segments of the gastrointestinal tract; WBC, water-binding capacity; VFA, volatile fatty acids; GLP-1, glucagon-like peptide-1; PYY, peptide
Impact of nutrition on canine behaviour 187
and gastric signals of satiation. In addition, a study of
Bueno et al.
in which dogs were fed different ﬁbre
sources (wheat bran, cellulose, guar gum), both gastric
emptying and intestinal transit time were affected with the
effect depending on the ﬁbre source included.
A delay in gastric emptying and thus an increase in
intestinal transit time by dietary ﬁbre (alginate) results in
more stable blood glucose concentrations as observed by
Murray et al.
. In dogs fed a diet with a high level of
fermentable ﬁbres (sugarbeet pulp, gum arabic and
fructo-oligosaccharides), intestinal GLP-1 concentrations
were found to be increased compared with dogs fed a diet
with low-fermentable ﬁbre (cellulose) levels
slows down gastric emptying
and intestinal transit
which may result in prolonged gastric ﬁll and delayed
nutrient digestion and absorption. In dogs, the ‘ileal
brake’ mechanism may also result from stimulation of
the release of PYY by fatty acids sufﬁcient to delay gastric
emptying in dogs
As reported above, fermentation of carbohydrates
, which may lead to prolongation of satiety by
becoming a source of energy (mainly acetate) at times
when glucose supply from the small intestine is
. Although dogs have a relatively
small and simple large intestine, dogs are capable
of fermenting a signiﬁcant quantity of dietary non-digestible
. Moreover, the faecal microﬂora of dogs
were found to give similar in vitro organic matter
disappearance results compared with the microﬂora from
humans, pigs and horses
. The latter indicates that
differences between these species in carbohydrate fermenta-
tion capacity are probably dependent on factors other than the
microbial population. The extent of fermentation in the
gastrointestinal tract in an animal largely depends on the time
available for microbial fermentation
. In dogs, a transit
time through the total gastrointestinal tract between 20 and
35 h is considered normal
. The large-intestinal transit of
digesta can take up to 90 % of the total gastrointestinal transit
, presenting a considerable time for large-intestinal
microﬂora to ferment undigested substrates entering from the
ileum. The VFA produced can be used by the hindgut bacteria
for protein synthesis, resulting in an increase in microbial
mass, or absorbed in the large intestine. The contribution of
large-intestinal VFA absorption towards the total energy
maintenance requirements of dogs has been reported to be
approximately 2 –7 %
Table 2. Effect of dietary ﬁbre on food intake and canine behaviour
Authors Dogs and design Dietary ﬁbre content or source* Results
Study 1, two groups of ﬁfteen beagle
dogs were assigned to one of two
maintenance level for 14 d
On day 7, one of two diets was
provided 75 min after ﬁrst meal. On
day 14, the other diet was offered
75 min after ﬁrst meal. After 14 d,
the experimental design was
repeated but each group of dogs
received the other of the two diets
Study 2, identical as study 1 but
with different diets
(1) 16 g CF/kg (study 1)
(2) 136 g CF/kg (study 1)
(3) 16 g CF/kg (study 2)
(4) 223 g CF/kg (study 2)
(a) Energy intake of all dogs was
lower than energy on offer
(b) Dogs fed diets 2 and 4 had lower
daily energy intake than dogs fed
diets 1 and 3, respectively
(c) Energy intake of the second meal
75 min after ﬁrst meal was lower
when dogs were fed diets 2 and 4
compared with dogs fed diets 1
and 3, respectively
Six obese terrier dogs (.115 % of
ideal BW) were fed each of the six
wet diets (6 £6 Latin square) at
45 % of maintenance level for 12 d.
Number of visits to the bowl and
cumulative time spent at the bowl
were observed for 30 min from the
start of the meal. On day 7 and 10,
8 and 11, or 9 and 12, dogs had ad
libitum access to a wet diet that was
provided 180 min after the ﬁrst
meal and food intake was
(1) 7 g CF/kg; 41g SF/kg; 14 g ISF/kg
(2) 13 g CF/kg; 113 g SF/kg; 38 g
(3) 143 g CF/kg; 36 g SF/kg; 202g
(4) 149 g CF/kg; 25 g SF/kg; 311g
(5) 15 g CF/kg; 37 g SF/kg; 15 g
(6) 8 g CF/kg; 46 g SF/kg; 15g ISF/kg
(a) No differences between diets in
daily energy intake
(b) No differences between diets in
(c) No differences between diets in
food intake of the second meal
180 min after ﬁrst meal
Two groups of ﬁfteen miniature
schnauzers and toy poodles were
assigned to one of two dry diets fed
in the morning at 50 % of daily
intake and had ad libitum access to
a control diet in the afternoon
(approximately 6 h later) for 8 d
(1) 95 g CF/kg; 27 g SF/kg; 264 g
(2) 20 g CF/kg; 18 g SF/kg; 123 g
(3) 21 g CF/kg; 23 g SF/kg; 124 g
(a) Dogs fed diet 1 had lower morning
and daily energy intake/kg BW
than dogs fed diet 2
(b) There was no difference in food
intake of diet 3 in the afternoon
between dietary treatments
Twenty-eight adult beagle dogs were
stratiﬁed by BW and assigned at
random to one of four dry diets with
ad libitum access for 35 d
(1) 60 g cellulose/kg
(2) 15 g FOS/kg
(3) 60 g beet pulp/kg
(4) 60 g beet pulp/kg; 20 g gum
talha/kg; 15 g FOS/kg
(a) No differences between diets in
DM intake per d
(b) Dogs fed diet 2 showed lower DM
intake/d per kg body weight
compared with dogs fed diet 1
CF, crude ﬁbre; BW, body weight; SF, soluble ﬁbre; ISF, insoluble ﬁbre; FOS, fructo-oligosaccharides.
* Values are presented on a DM basis except for the data of Jackson et al.
, which are as-fed.
G. Bosch et al.188
not to provide information on the way these values were
derived. In addition, the effect of production and absorption of
acetate as an energy source for body tissues on postprandial
satiety remains to be investigated. The work of Pouteau
on a method to evaluate acetate production and
metabolism using stable isotopes may be the starting point for
further exploration of the importance of carbohydrate
fermentation in the gastrointestinal tract and satiety in dogs.
To our knowledge, there is no information available in the
scientiﬁc literature regarding possible inﬂuences of dietary
ﬁbre on ghrelin concentrations and behaviour in dogs.
However, when dogs are fed one scheduled meal per d,
ghrelin concentrations increase before and decrease rapidly
after the meal to remain relatively constant throughout the
rest of the day
, which may indicate little potency of
ghrelin concentrations to affect canine behaviour through-
out the day.
Nowadays, dry extruded diets for dogs may contain 30 %
or more carbohydrates of which starch is the major
component. Moreover, the non-digestible carbohydrate
fraction in diets can also make up a considerable
. As mentioned previously, ﬁbres differing in
physical and chemical properties have diverse physiological
responses in animals. Nutrient digestion as well as transit
time through the gastrointestinal tract may be inﬂuenced by
the amount and source of ﬁbre included in canine diets. In
the case of a reduction in nutrient digestibility when ﬁbres
are included, it is necessary to increase the concentration of
some nutrients in order to ensure that the nutrient
requirements of the animals are met
. Future canine
research on the behavioural effects of dietary ﬁbre should
account for the fact that different breeds may respond
differently (in terms of satiety). Gastric emptying rate is
inversely related to body weight in dogs of different sizes
Moreover, large-breed dogs have a longer large-intestinal
transit time and increased apparent total dietary ﬁbre
, which may increase the production and the
use of VFA but may increase gastrointestinal discomfort as a
result of enhanced fermentation activity.
The degree of satiety in animals such as pigs has been
shown to affect behaviour, including aggressive and
stereotyped behaviour. Although likely, it is up till now
unknown whether canine behaviour can be affected by
degree of satiety and further research is required. Assuming
that behaviours in dogs are more favourable during times of
satiety than during times of hunger as observed in pigs (for
example, aggression), speciﬁc dietary ﬁbres through their
potential to prolong satiety may assist in preventing
unwanted canine behaviours.
The present contribution provides an overview of current
knowledge on the inﬂuence of dietary macronutrient
composition on canine behaviour. It can be concluded that
little research has been conducted in this ﬁeld although
research in other species indicates that there is potential to
modify behaviour in dogs through nutrition. There is
evidence that dietary composition can modulate animal and
human behaviour through different mechanisms. Dietary
protein may contain the precursors tryptophan and tyrosine
for the respective neurotransmitters serotonin and catechol-
amines. Since bioavailability of both tryptophan and
tyrosine in the brain are dependent on the dietary protein
content and amino acid composition, dietary composition
may have an impact on the behaviour and wellbeing of dogs
under speciﬁc circumstances (for example, stress). How-
ever, before application and extrapolation of the evidence
found in mostly rodent laboratory studies into commercial
canine diets is undertaken, research is required to identify
the optimal and safe dietary inclusion level in combination
with behavioural tests to study the magnitude of effects on
(problem) canine behaviour. The n-3 PUFA have an
important role in the development of the brain, and the
supply of essential fatty acids such as DHA could affect
aspects of the dopaminergic and serotonergic system and,
consequently, cognitive performance and behaviour as
observed in rodents. Most canine studies and dietary n-3
PUFA have been mainly focused on the effect of maternal
intake of different dietary n-3 PUFA during gestation and
lactation on n-3 PUFA in the milk and/or n-3 PUFA intake
on retinal function of puppies. It would be of interest to
examine the DHA required for optimal neurological
development and whether this leads to alterations in
cognitive abilities or behaviour later in life of dogs. In the
literature, studies have been reported which show that,
depending on the physical and chemical properties, certain
dietary ﬁbres induce satiation or prolongation of satiety after
a meal. However, there have been no studies conducted in
which the effect of dietary ﬁbre on physiological satiety
parameters, behaviour (for example, activity) and/or feeding
motivation were studied in dogs. If dietary ﬁbre has short-
term effects that result in prolongation of satiety and a
reduction of hunger between meals, it may help to prevent
unwanted canine behaviours and also promote long-term
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