Assessment of nutritional interventions for modification of age-associated cognitive decline using a canine model of human aging

Publishing Authors : Joseph A. Araujo, Christa M. Studzinski, Elizabeth Head, Carl W. Cotman & Norton W. Milgram

Date Published : 11/03/05

 

Introduction
Advances in medicine and technology have resulted
in an increase of human life-span over the last century.
This increased longevity, however, is associated with
an increased prevalence of age-related cognitive disorders,
the most frequent being Alzheimer’s disease.
Given the impact of cognitive impairment on quality
of life and cost to society, there is a pressing need for
identifying interventions that can halt, reverse, or ideally
prevent, progressive cognitive decline. One strategy
for systematically evaluating interventions for,
and mechanisms of, cognitive decline consists of animal
models. The ideal animal model should demonstrate
a decline in cognitive function with associated
brain pathology consistent with that present in humans.
The most widely studied animal models, thus far,
are rodents and nonhuman primates. More rece
we have characterized a canine model of human
aging that provides some unique features for evaluating
interventions for age-related cognitive decline
(Adams et al. 2000a). First, dogs develop age-related
cognitive decline consistent with that reported in
humans (Adams et al. 2000a). Second, humans and
dogs show several parallels with respect to ageassociated
brain pathology, such as amyloid-beta
(Ab) deposition, ventricular enlargement and vascular
changes (Cummings et al. 1996a). Third, in many
respects the level of cognitive function seen in dogs is
comparable to that in primates, and in the case of
social cognition, the dog appears uniquely linked to
humans (Hare et al. 2005). Last, dogs and humans
have similar nutritional needs. Our work has facilitated
the development of the canine model by linking
cognitive changes that occur in canine aging with
neuropathological changes. This justifies using the
dog to evaluate nutritional interventions, and we have
now examined several.
The present review addresses the utility of the
canine model for evaluating nutritional interventions.
We first discuss parallels between canine and human
aging with respect to age-associated cognitive decline
and neuropathology. We then highlight the
effectiveness of a nutritional-based intervention consisting
of a combination of antioxidants and mitochondrial
cofactors. We also discuss preliminary
studies using a proprietary blend of docosahexaenoic
acid (DHA), an omega-3 polyunsaturated fatty acid,
and an array of pig brain phospholipids.
Age-associated cognitive decline in dogs
We have used both cross-sectional and longitudinal
studies to examine the effects of age on canine
cognitive function. Our results indicate that cognitive
function declines with age in dogs, but that the
decline is domain, or task, specific and sensitive to
previous experience. Procedural learning and memory,
as well as simple discrimination learning, are
generally insensitive to aging (Milgram et al. 1994).
By contrast, tests of executive function, such as discrimination
reversal learning (Milgram et al. 1994;
Tapp et al. 2003a), and working memory (Adams
et al. 2000b; Head et al. 1995), such as a delayednonmatching-to-position
(DNMP) task (Chan et al.
2002), are highly sensitive to aging. In other instances,
age sensitivity depends on prior test experience.
Naı¨ve young and aged dogs do not differ in their
ability to learn relatively simple size discrimination
tasks (Milgram et al. 1994). Once both groups have
repeated experience on discrimination learning tasks,
however, age differences emerge in that aged dogs
require more trials to learn the task than do young
dogs (Milgram 2003). As in other species, we have
demonstrated that a treatment of environmental enrichment
in the form of physical exercise, cognitive
testing and kennel mates can improve learning ability
in aged dogs (Milgram et al. 2004, 2005). Complex
discrimination tasks, such as an oddity problem (where
animals are required to respond to the different object
of three for reward), are sensitive to age when
objects are increasingly similar (Cotman et al. 2002).
Thus domain-specific cognitive decline is apparent in
canine aging and is modified by previous experience.
Using a cross-sectional approach, we have established
that tasks dependent on frontal lobe function, in
particular tests of executive function (Milgram et al.
1994; Tapp et al. 2003a) and visuospatial working
memory (Chan et al. 2002), are notably sensitive to
age (Adams et al. 2000b; Head et al. 1995). In a
study of 109 dogs (manuscript in preparation), a
statistically significant impairment in both acquisition
of the DNMP using a 5-s delay and memory capacity
were detected as early as six years of age (Figure 1)
(Araujo 2004; Araujo et al. 2004a). Aged dogs also
demonstrate increased variability in which some
show little or no impairment (successful agers) while
at the other extreme are those incapable of learning
(demented) (Adams et al. 2000b). In aged dogs that
perform equivalently to young dogs on the DNMP
(successful agers), increasing memory load by adding
an additional position results in a memory deficit
(Tapp et al. 2003b). Longitudinal testing indicates a
similar pattern, but also reveals marked decline in
individual dogs that may span several cognitive domains
(Adams et al. 2000a). These data are consistent
with age-associated increases in individual cognitive
performance in humans and with the hypothesis that
age-associated cognitive dysfunction is a progressive
process, such as the likely progression to Alzheimer’s
disease observed in patients diagnosed with mild
cognitive impairment (Larrieu et al. 2002). Further,
aged dogs demonstrate early deficits in executive
function and visuospatial working memory, which
are both early features of age-associated cognitive
dysfunction in humans (Bartus 2000; Stuss et al.
1996; Flicker et al. 1984).
28
Veterinarians also have identified an aging syndrome
in pet dogs termed Cognitive Dysfunction
Syndrome (CDS) (Landsberg and Ruehl 1997; Bain
et al. 2001; Ruehl et al. 1995) that is typically diagnosed
because of behavioral changes such as inappropriate
house-soiling, alterations in activity levels,
reduced interaction with family members, and wandering
(Landsberg and Ruehl 1997). It is unclear to
what extent CDS is related to neuropsychological
deficits; however, evidence from laboratory-based
noncognitive behavioral testing suggests a link. In
particular, dogs impaired on neuropsychological tests
are more likely to demonstrate increased activity levels
compared to age-matched cognitively normal dogs
(Siwak et al. 2001, 2003). Impaired dogs, when compared
to young dogs or cognitively intact older dogs,
spend less time close to or in contact with humans
(Siwak et al. 2001). These results suggest that noncognitive
behavioral changes also occur with severe
cognitive impairment, which may partially model the
noncognitive behavioral changes seen in human
dementia.
The age-related behavioral impairments observed
in the laboratory and the clinic likely represent a
continuum in severity of cognitive decline. For example,
service dogs, which are required to provide a
high level of learned skills, may demonstrate agerelated
behavioral deficits at an earlier age than the
average pet. While we have linked the cognitive
deficits observed in the laboratory to pathological
changes in the aging brain (see below), the behavioral
signs associated with CDS are also likely due to a similar
pathology (Cummings et al. 1996a, b; Landsberg
and Ruehl 1997). Thus, nutritional interventions that
reduce the development of brain pathology should
have important benefits for pet dogs.
Brain aging in the dog
The aged canine brain exhibits several features that
also occur in human pathological brain aging. In the
early 1900s, abnormal pyramidal neuron sprouting
was described (Lafora 1914). By the 1950s, researchers
observed BAlzheimer’s-like^ plaque pathology
(Dahme 1962, 1967, 1968; Osetowska 1966), which
is a result of Ab protein deposition (Cummings et al.
1996c). More recent studies have found that dogs
have reduced levels of endogenous antioxidants (Head
et al. 2002), cortical atrophy (Tapp et al. 2004a),
ventricular enlargement (Tapp et al. 2004a; Su et al.
1998), myelin degeneration (Ferrer et al. 1993), and
accumulation of degraded proteins (Borras et al.
1999). However, the canine and human neuropathology
is not identical. Patients with Alzheimer’s disease
develop mature neurofibrillary tangles, which
result from the intracellular accumulation of hyperphosphorylated
tau protein. Dogs also show hyperphosphorylated
tau, but mature neurofibrillary
tangles are not seen (Wegiel et al. 1998; Papaioannou
et al. 2001). Although the absence of mature neurofibrillary
tangles limits the dog as a model for late
Figure 1. Errors to acquire the DNMP task at a 5-s delay (a) and
memory capacity (b). In a study of 109 dogs, we divided the dogs
into six age groups that included puppies (G1 year), young (1Y2.99
years), adult (3Y4.99 years), middle-aged (6Y7.99 years), old
(8Y9.99 years), and senior (10Y11.99 years). In addition to a
significant effect of age [P G 0.05] on acquisition, the data
indicated that middle-aged dogs were impaired significantly
compared to both young and adult dogs. An identical effect was
seen on memory capacity. This indicates that both acquisition of
the DNMP and memory capacity are impaired early in canine
cognitive decline.
29
stage Alzheimer’s disease, the collective data indicate
that canine brain aging models early stage pathology
(Table 1).
Extensive work has been conducted to better understand
the Ab pathology observed in the aging dog.
Ab is naturally deposited in the canine brain with
increasing age, forming diffuse plaques (Cummings
et al. 1996b; Head et al. 1998, 2000; Satou et al.
1997). The predominant species of Ab is the longer,
more toxic, 42 amino acid protein, which is identical
in sequence to that of the human (Cummings et al.
1996a,c). At later stages in the pathology, the shorter
40 amino acid fragment also accumulates in plaques
and blood vessel walls as seen in Ab angiopathy
(Prior et al. 1996; Walker 1997). The distribution of
plaques is region specific; the prefrontal cortex accumulates
plaques more consistently and at an earlier
age than the parietal, entorhinal and occipital cortices,
which is consistent with the deposition pattern
seen in humans (Head et al. 2000). Ab accumulates
first in the deeper cortical layers and subsequently in
the superficial cortical layer (Satou et al. 1997).
Unlike humans, Ab accumulation is not observed in
layer 1 of the canine cortex, but a diffuse band is
observed in the outer molecular layer of the hippocampus
in both species (Cotman et al. 2002).
Both cognitive dysfunction and cortical atrophy
are linked to Ab deposition in the dog (Tapp et al.
2004a; Cummings et al. 1996b). As mentioned previously,
Ab typically is deposited early in the prefrontal
cortex of both dogs (Head et al. 2000) and
humans (Braak and Braak 1997). Cognitive tasks
thought to be dependent on the prefrontal cortex are
impaired by age more consistently and earlier in the
dog (Tapp et al. 2003a, b, 2004a, b). Further, the
frontal lobes selectively atrophy in aged dogs prior to
overall brain atrophy (Tapp et al. 2004a). This
pattern of frontal lobe-based deficits is consistent
with humans, where frontal lobe volume is correlated
with impairment in executive function (GunningDixon
and Raz 2003) and is highly sensitive to age
(Jernigan et al. 2001; Resnick et al. 2000).
Nutritional interventions in the aging dog
Antioxidants and mitochondrial cofactors
According to the oxidative stress hypothesis, aging
is linked to an accumulation of oxidative damage
caused in part by a decrease in mitochondrial function
and a reduction of endogenous metabolic strategies
to counteract the increase in oxidant species
(Ames et al. 1993; Beal 1995; Shigenaga et al. 1994).
The central nervous system is particularly sensitive
to oxidative stress because of high metabolic rates
and reduced antioxidant defenses compared to other
tissues (Halliwell 1992). This may be responsible for
cognitive decline and associated neuropathology
(Beal 1995). To examine this hypothesis, we conducted
a three-year longitudinal study in both young
and aged dogs using two diets: (1) a diet enriched
with a broad spectrum of antioxidants and mitochondrial
cofactors and (2) an isocaloric control diet
(Table 2). During the course of the study, cognition
was monitored and half of the animals were sacrificed
at study completion to determine the effects of
the diet on brain pathology.
After treatment wash-in, we tested the subjects
on an allocentric discrimination task, the landmark
task (Milgram et al. 2002a), in which subjects were
required to utilize the location of an external landTable
2. Antioxidants and mitochondrial cofactors included in the
test diets.
Ingredient Control diet
(ppm)
Enriched diet
(ppm)a
D,L-Alpha-tocopherol acetate 120 1,050
L-Carnitine G20 260
D,L-alpha-lipoic acid G20 128
Ascorbic acid as Stay-C G30 80
a
1% inclusion of each of the following was in the enriched diet
(1:1 exchange for corn): spinach flakes, tomato pomace, grape
pomace, carrot granules, and citrus pulp.
ppm: parts per million.
Table 1. Common features of Alzheimer’s disease in the aged
rodent and canine models.
Features of Alzheimer’s disease Rat Dog
Ab pathology j +
Ab angiopathy j +
Neurofibrillary tangles j j
Markers of oxidative damage + +
Cortical atrophy j +
Ventricular enlargement j +
Memory deficits + +
Noncognitive behavioral alterations + +
Commercial application for interventions j +
Cognitive decline associated with Ab pathology j +
+ Present; j absent.
30
mark to determine the location of reward. The rationale
for developing an allocentric test was based
on the known propensity of Alzheimer’s disease patients
to become lost, even in familiar surroundings
(Cogan 1979), thereby exhibiting deficits in allocentric
spatial ability. Consequently, we hypothesized
that performance on this task would be sensitive to
aging and would be improved with the dietary intervention.
For the landmark task, subjects were required
to respond to one of two identical coasters
based on the proximity of the coaster to an external
landmark, a yellow peg, which could be placed at
various distances from each coaster (Milgram et al.
1999). The further the landmark from the rewarded
coaster, the more difficult the task. Aged dogs on the
enriched diet learned the initial discrimination with
fewer errors and on average were more likely to pass
the learning criteria of subsequently more difficult
levels than those on the control diet. By contrast,
young dogs on the enriched diet did not differ significantly
from young animals on the control food.
We also examined remote memory for the landmark
task by retesting the subjects approximately seven
months later. Aged dogs showed poorer retention of
the task compared to young dogs. There was, however,
a strong tendency for aged dogs on the enriched
diet to relearn the landmark task with fewer errors
than aged animals on the control diet.
We also examined the effect of the diet on a set
of complex discrimination learning problems using
an oddity discrimination task (Cotman et al. 2002;
Milgram et al. 2002b). In this task, the subjects were
presented with three objects, two of which were
identical and the other was the Bodd^ object. Dogs
were required to approach and displace the odd object
to obtain a food reward. Using a set of oddity
problems, we were able to examine performance under
various difficulty levels by using stimuli that
were increasingly more similar. Thus, when the objects
were more similar in appearance, dogs typically
committed more errors to acquire the rule. Young
dogs on the enriched diet did not differ compared to
young dogs on the control diet at any difficulty level.
We also did not see statistically significant effects of
the diet on the easiest oddity discrimination problems
in aged dogs. When the difficulty level was increased,
however, aged dogs on the enriched diet
learned the discrimination with fewer errors than the
aged dogs on the control diet. The results of this
experiment are important for several reasons. First, it
demonstrates that the difficulty of a discrimination
task is important when examining age differences,
which may explain reports of both age-dependent
and independent effects on discrimination learning
and reversals in nonhuman primates (Voytko 1999).
Second, it demonstrates that using multiple difficulty
levels is more effective for assessing the effects of an
intervention than a single discrimination task; a single
discrimination task may not have revealed an
effect if the difficulty level was either too easy or too
difficult. Lastly, it demonstrates that the enriched diet
is effective in attenuating age-related impairments in
complex discrimination learning.
The effects of the diet on visuospatial memory and
long-term retention using a DNMP task were examined
also (manuscript in preparation) (Araujo et al.
2004b). We examined both relearning and memory
capacity, defined as the longest progressive delay a dog
could remember the location of an object within a standardized
number of test sessions. The dogs were trained
initially at baseline and were tested after one and two
years on treatment. The subjects on both the control and
enriched diet did not differ at baseline and after one
year of treatment. By the second year of treatment,
however, the subjects on the enriched diet demonstrated
increased memory capacity (Figure 2) and improved
relearning (Figure 3). Additionally, the pass frequency
for the two groups differed only after two years such
that the diet-enriched group passed the relearning
criterion at over twice the frequency as the control
group. These results suggest that the antioxidant
treatment resulted in maintained cognitive function
whereas the control group showed cognitive decline.
Finally, the longitudinal study assessed changes in
discrimination and reversal learning (Milgram et al.
Figure 2. Memory capacity over three years. There was a trend for
increasing memory capacity in dogs on the combination antioxidant
and mitochondrial cofactor diet compared to dogs on the
placebo diet. This difference was most apparent after the subjects
had been on their respective treatment conditions for two years.
31
2004, 2005). Dogs received a different discrimination
and reversal problem at baseline, and after one and
two years of treatment. As the study progressed, the
enriched diet group maintained and even improved
discrimination and reversal learning ability. By contrast,
the control group showed deterioration in these
cognitive abilities, especially reversal learning. The
positive effects of the diet on reversal learning were
even more robust when combined with environmental
and cognitive enrichment. These experiments demonstrate
that the effects of the enriched diet on certain
cognitive tasks may only become apparent after
several years of testing, possibly due to attenuation,
or reduced rate, of progressive cognitive decline.
The diet was also examined in veterinary clinical
trials and found to have beneficial effects on signs associated
with CDS. To our knowledge, a cocktail intervention
similar to the one used in our studies has
not been examined in human aging. Although some
positive results have been reported in clinical trials
examining the effects of Vitamins E and C (Di
Matteo and Esposito 2003) and antioxidant supplements
(Grodstein et al. 2003; Jama et al. 1996;
Engelhart et al. 2002; Helmer et al. 2003; Zandi et al.
2004) on several aspects of aging (McDaniel et al.
2003; Martin 2003; Martin et al. 2002), the studies
reported here suggest that a cocktail intervention
consisting of several antioxidants and mitochondrial
cofactors may be more effective, possibly due to a
synergistic effect. Similarly, studies in humans
(Grodstein et al. 2003; Zandi et al. 2004; Masaki
et al. 2000) and rodents (Joseph et al. 1998, 1999;
Bickford et al. 2000) suggest that combination antioxidant
therapy may be more effective than single
component supplementation.
The results of the cognitive portion of the study
indicate that a diet enriched with antioxidants and
mitochondrial cofactors can improve cognition in aged
dogs and likely attenuate the progressive nature of
cognitive decline. This suggests that the diet may have
both short- and long-term effects on brain pathology
presumed to be responsible for cognitive impairment.
To examine the effects of the diet on brain pathology,
half the animals were sacrificed at the end of the study
and their brains were harvested for pathological
assessment [procedure described in Head et al.
(2000)]. Overall, there was a significant reduction of
Ab load. Ab loads were decreased in deep cortical
layers by 27%, 75%, 69.5% and 84.1% in the frontal,
entorhinal, parietal and occipital cortices, respectively
(Head et al. 2004). Thus, the cognitive portion of the
study not only predicted efficacy in the veterinary
clinic, but also predicted an effect of the dietary
intervention on brain aging. Further studies are being
conducted to elucidate the mechanism by which the
diet could be reducing Ab accumulation.
Proprietary blend of docosahexaenoic
acid and phospholipids
The rationale for studying DHA is based on the
hypothesis that decreasing membrane fluidity conFigure
3. Long-term retention of the DNMP task at a 5-s delay. At baseline, the two groups of dogs did not differ from one another. Significant
differences emerged two years following treatment such that the dogs on the antioxidant and mitochondrial cofactor combination diet
reacquired the task with significantly [P G 0.05] fewer errors than the control group. The control group was performing no different than at
baseline after two years on treatment.
32
tributes to declining cognitive ability in aged subjects.
As humans age, neuronal membranes may
become more rigid and, as a result, are less able to
release neurotransmitters and respond to chemical
messengers (Sharma et al. 1993). DHA is an omega3
polyunsaturated fatty acid found in membrane
phospholipids and a deficiency in this fatty acid
may play a role in Alzheimer’s disease. A low intake
of fish (Morris et al. 2003), the major dietary source
of DHA, and low serum DHA levels (Kyle et al.
1999) are both linked to an increase in likelihood of
developing Alzheimer’s disease, whereas a high intake
of fish is related to a decreased risk (Morris
et al. 2003). Similarly, Alzheimer’s patients and
humans with mild cognitive impairment have lower
plasma levels of DHA and a larger n-6/n-3 fatty acid
ratio (Conquer et al. 2000). In Alzheimer’s patients,
cholestryl ester-DHA levels, a biomarker for DHA,
is negatively correlated with the severity of dementia
(Tully et al. 2003). DHA also is thought to
possess antioxidant properties (Gamoh et al. 2001;
Hashimoto et al. 2002); thus supplementation should
serve to replace oxidized DHA present in phospholipids.
Furthermore, DHA readily toggles between
several conformations, thereby allowing the membrane
phospholipids to adapt to the changes in receptor
or ion channel conformations (Huber et al.
2002; Koenig et al. 1997). DHA also may have
beneficial anti-inflammatory effects [reviewed in
Horrocks and Yeo (1999)] and has been shown to
have positive effects in rodent models of Alzheimer’s
disease (Hashimoto et al. 2002; Calon et al. 2004).
A pilot study (manuscript in preparation) examined
the benefits of supplementing dogs with a
proprietary blend of DHA and pig brain-derived
phospholipids (provided by Nutramax Laboratories,
Edgewood, MD). In this pilot study, 15 senior dogs
(of at least 12 years of age) were administered either
a placebo capsule or a capsule containing the
proprietary blend of DHA and pig brain-derived
phospholipids (including phosphatidylserine, phosphatidylethanolamine
with plasmalogens, phosphatidylcholine,
phosphatidylinositol, sphingomyelin,
phosphatidic acid and other lysolipids). After approximately
100 days of prestudy treatment, the
animals that survived were maintained on their
respective treatments and tested on the DNMP. The
results indicated a trend towards improved visuospatial
memory in the treatment group (Figure 4); when
compared to their cognitively matched control
animal (used to ensure equivalent cognitive groups
at baseline), the treatment animals had maximal
memory scores that were on average 30 s longer in
all but two of seven instances. We also monitored
changes in the dogs’ serum fatty acid profile. As
expected, the dogs provided with the proprietary
blend showed a dramatic increase in DHA levels
[P G 0.001] and a corresponding decrease in docosapentaenoic
acid levels [P G 0.001], which is the
omega-6 polyunsaturated fatty acid equivalent of
DHA that typically accumulates in the presence of a
DHA deficiency. Analysis of canine brain changes in
DHA levels is currently underway. Finally, the dogs
on treatment developed fewer health-related problems
than controls [chi-square, P = 0.038] (Table 3).
These data support conducting a larger study to better
characterize the supplement’s ability to improve
cognition and quality of life in aged dogs. In humans,
fish oil trials are currently in progress (Clinicaltrials.gov
2004). Phospholipid trials in humans,
particularly phosphatidylserine, have shown minimal
benefits (Amaducci 1988).
Figure 4. DNMP Maximal Memory Scores. Animals on treatment
had a maximal memory score that was on average 30 s longer than
their cognitively equivalent control animal. Higher scores were
indicative of larger memory capacity.
Table 3. Health status of animals during treatment with either
placebo capsule or proprietary blend of DHA and phospholipids.
Health status a Placebo Treatment
Healthy 3 7
Chronic illness 2 0
Euthanized 3 0
a
The number of animals that were considered healthy, suffered
from a chronic illness and that required euthanasia due to chronic
illness are depicted.
33
Implications for assessment of
nutritional interventions
The dog is a particularly useful model for studying
nutritional interventions because dogs provide a
natural model of progressive age-dependent cognitive
decline with associated neuropathology that
partially models the human condition. Alternative
animals models include rodents and nonhuman
primates. Because of their short life span and low
cost, rodents are widely used in initial screening of
cognitive-modifying interventions, but they are limited
in their ability to model complex human cognitive
abilities (Thomas 1996). Furthermore, aged
rodents do not naturally model human Ab neuropathology,
although transgenic mice that overexpress
APP and deposit Ab are useful in this respect. Aged
nonhuman primates are more suitable for studying
age-associated cognitive decline than rodents, but are
expensive to obtain, have long life spans, and can be
difficult to work with compared to rodents and dogs.
In addition, aged nonhuman primates predominantly
express the shorter, less soluble, 40 amino acid
species of Ab (Gearing et al. 1996). Thus, the aged
dog offers some advantages over other models,
particularly for assessing interventions intended to
prevent, attenuate, or reverse the cognitive decline
associated with Ab pathology (Table 1).
The importance of the present studies extends
beyond their significance for human cognitive disorders;
the studies also demonstrate that nutrition can
affect aging and associated behavioral changes in
companion animals. For example, a prescription diet
based on the composition of the antioxidant and
mitochondrial cofactor diet tested in our laboratory is
now marketed for treatment of CDS (Prescription
diet\ canine b/d\). Furthermore, future studies of pet
dogs may provide us with both retrospective and
prospective data on the effects of nutrition on aging by
monitoring conversion rates to CDS. These studies
may prove particularly valuable because dogs typically
are fed the same food daily, thereby reducing the
variability inherent in human nutritional studies.
Our work is not the first to demonstrate that enriched
diets can improve cognitive deficits in animals;
positive results also are reported in rodent models
(Joseph et al. 1998, 1999; Bickford et al. 2000).
Nonetheless, the use of nutritional strategies remains
controversial because of both positive (Grodstein et al.
2003; Jama et al. 1996; Engelhart et al. 2002; Helmer
et al. 2003; Zandi et al. 2004; Martin 2003; Martin et al.
2002) and negative (Laurin et al. 2004; Lindeman
et al. 2000; Luchsinger et al. 2003; Mendelsohn et al.
1998) results in human trials (McDaniel et al. 2003;
Martin 2003; Martin et al. 2002; Mendelsohn et al.
1998). The findings in the longitudinal canine
enriched-diet study are uniquely important because
of the possibility that the combination of antioxidants
and mitochondrial cofactors work synergistically,
both by improving mitochondrial function and by
compensating for the reduction in neuronal metabolic
strategies for reducing the impact of free radicals.
This combination resulted in both decreased Ab
deposition and improved cognitive status. Similarly,
the DHA and phospholipids supplement targeted
several potential mechanisms. Collectively, this data
strongly suggests that future human trials may benefit
from an increased focus on nutritional cocktails, as
opposed to single nutrient supplementation. Although
we are unaware of human studies using such a wide
spectrum of antioxidants and mitochondrial cofactors,
antioxidant combinations may be superior to
single component supplements for human aging
(Grodstein et al. 2003; Zandi et al. 2004; Masaki
et al. 2000). Alternatively, the present results also
may be due to nutritional limitations in the normal
canine maintenance diet. In either instance, the
results indicate that nutrition is an important factor
in age-associated cognitive dysfunction. Considering
that age-related cognitive impairment and approximately
95% of Alzheimer’s disease cases are sporadic
in nature, i.e., not attributable to genetic causes,
a more thorough examination and appreciation of
nutritional interventions is warranted.
Conclusion
The present review summarizes recent work in our
laboratory on cognitive-modifying effects of longterm
nutritional interventions in aged dogs. The dog
demonstrates cognitive decline and brain pathology
consistent with that observed in pathological cognitive
decline in humans, such as that seen in early
Alzheimer’s disease. Therefore, the dog provides an
alternative to rodent and nonhuman primate models
for both screening interventions and examining
mechanisms of pathological aging. We have found
the canine model to be particularly useful for
examining links among interventions, Ab pathology,
34
and cognition. Thus far, we have examined or are in
the process of studying several nutritional interventions,
including antioxidants, mitochondrial cofactors,
phospholipids, and DHA. The results suggest
many or all of these may prove beneficial in both
humans and pet dogs. One possibly important
implication is that nutrition cocktails are likely to
be more effective in human clinical trials than
individual supplements. Collectively, the results of
our studies indicate that dogs, both in the laboratory
and likely as pets, provide a very powerful tool for
examining the effects of nutritional interventions on
age-associated cognitive dysfunction and related
brain pathology.
Acknowledgements
Funding provided by NIH/NIA and U.S. Department
of the Army. Hill’s Pet Food and Nutrition, Topeka,
KS, also supported some animals for the cognitive
testing portion of the antioxidant study. Nutramax
Laboratories, Inc., Edgewood, MA, partially supported
the DHA and phospholipid work. We kindly
thank Dr. Henderson for his comments on this
manuscript.
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