Exercise is brain food: The effects of physical activity on cognitive function

Abstract

This commentary reviews selected biomedical and clinical research examining the relationship between physical exercise and cognitive function especially in youth with disability. Youth with physical disability may not benefit from the effects of exercise on cardiovascular fitness and brain health since they are less active than their non-disabled peers. In animal models, physical activity enhances memory and learning, promotes neurogenesis and protects the nervous system from injury and neurodegenerative disease. Neurotrophins, endogenous proteins that support brain plasticity likely mediate the beneficial effects of exercise on the brain. In clinical studies, exercise increases brain volume in areas implicated in executive processing, improves cognition in children with cerebral palsy and enhances phonemic skill in school children with reading difficulty. Studies examining the intensity of exercise required to optimize neurotrophins suggest that moderation is important. Sustained increases in neurotrophin levels occur with prolonged low intensity exercise, while higher intensity exercise, in a rat model of brain injury, elevates the stress hormone, corticosterone. Clearly, moderate physical activity is important for youth whose brains are highly plastic and perhaps even more critical for young people with physical disability.

Full text

Developmental Neurorehabilitation, July 2008; 11(3): 236–240
INVITED COMMENTARY
Exercise is brain food: The effects of physical activity
on cognitive function
MICHELLE PLOUGHMAN
Clinical Research, Rehabilitation Program, Eastern Health Authority, St. John’s, Newfoundland and Labrador,
Canada
(Received 15 January 2008; revised 29 January 2008; accepted 5 February 2008)
Abstract
This commentary reviews selected biomedical and clinical research examining the relationship between physical exercise
and cognitive function especially in youth with disability. Youth with physical disability may not benefit from the effects of
exercise on cardiovascular fitness and brain health since they are less active than their non-disabled peers. In animal models,
physical activity enhances memory and learning, promotes neurogenesis and protects the nervous system from injury and
neurodegenerative disease. Neurotrophins, endogenous proteins that support brain plasticity likely mediate the beneficial
effects of exercise on the brain. In clinical studies, exercise increases brain volume in areas implicated in executive
processing, improves cognition in children with cerebral palsy and enhances phonemic skill in school children with reading
difficulty. Studies examining the intensity of exercise required to optimize neurotrophins suggest that moderation is
important. Sustained increases in neurotrophin levels occur with prolonged low intensity exercise, while higher
intensity exercise, in a rat model of brain injury, elevates the stress hormone, corticosterone. Clearly, moderate
physical activity is important for youth whose brains are highly plastic and perhaps even more critical for young people with
physical disability.
Keywords: Neurotrophin,neuroplasticity,brain-derived neurotrophic factor,BDNF,rehabilitation
Este comentario revisa investigaciones biome
´dicas y clı
´nicas selectas que examinan la relacio
´n entre el ejercicio fı
´sico y la
funcio
´n cognitiva, particularmente en jo
´venes con discapacidad. Los jo
´venes con alguna discapacidad fı
´sica pueden no
beneficiarse de los efectos del ejercicio en el cerebro y en el acondicionamiento del sistema cardiovascular, ya que son menos
activos que jo
´venes sin discapacidad. En modelos animales, la actividad fı
´sica estimula a la memoria y al aprendizaje,
promueve la neuroge
´nesis y protege al sistema nervioso de las lesiones y de las enfermedades neurodegenerativas. Las
neurotrofinas son proteı
´nas endo
´genas que apoyan a los procesos de plasticidad cerebral, y posiblemente median los efectos
bene
´ficos del ejercicio sobre el cerebro. En estudios clı
´nicos el ejercicio aumenta el volumen cerebral en a
´reas implicadas en
el procesamiento ejecutivo, mejora la cognicio
´n en nin
˜os con para
´lisis cerebral, y aumenta la habilidad fone
´mica en nin
˜os en
edad escolar que tienen dificultad para la lectura. Los estudios que examinan la intensidad del ejercicio necesaria para
optimizar a las neurotrofinas sugieren que la moderacio
´n es importante. El ejercicio prolongado de baja intensidad produce
aumentos sostenidos en los niveles de las neurotrofinas, mientras que en un modelo experimental en ratas con lesio
´n
cerebral, el ejercicio de alta intensidad mostro
´una elevacio
´n de la corticoesterona, que es la hormona del estre
´s.
Evidentemente la actividad fı
´sica moderada es importante para los jo
´venes cuyos cerebros son altamente pla
´sticos, y tal vez
crı
´tica para jo
´venes con discapacidad fı
´sica. Tı
´tulo abreviado: El ejercicio es alimento cerebral.
Palabras clave:Neurotrofina,neuroplasticidad,factor neurotro
´fico de origen cerebral,BDNF,rehabilitacio
´n
Introduction
It is immediately recognized that exercise promotes
good health of the cardiovascular and musculoskele-
tal systems, however the field of exercise
and cognitive function is rapidly growing.
Unfortunately, youth with physical disability are
twice as likely to watch more than 4 hours of television
per day than young people without disability [1].
Correspondence: Michelle Ploughman, Clinical Research, Rehabilitation Program, Eastern Health Authority, L. A. Miller Centre, 100 Forest Road, St. John’s
Newfoundland and Labrador, Canada A1A 1E5. Tel: þ1709 777 2099. E-mail: mploughm@mun.ca
ISSN 1751–8423 print/ISSN 1751–8431 online/08/030236–5 ß2008 Informa UK Ltd.
DOI: 10.1080/17518420801997007
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As a result, inactive youth with disability not only
have lower cardiovascular and musculoskeletal fit-
ness, but do not avail of the cognitive benefits of
exercise.
Studies in ageing humans show that endurance
exercise is protective against cognitive decline,
especially executive planning and working memory
[2–4]. In both humans [5–7] and primates [8],
exercise increases attention and performance on
cognitive tasks. In a rat model of stroke, running
exercise promotes neuronal dendritic branching and
enhances relearning of forelimb motor skill [9].
Currently there are three hypotheses explaining how
exercise may affect executive control. Exercise may
increase oxygen saturation [2] and angiogenesis [10]
in brain areas crucial for task performance. Kramer
et al. [2] found that walking exercise increased the
rate of oxygen consumption in healthy older adults
that was associated with improved reaction time and
enhanced performance in tests of executive function-
ing. The second hypothesis suggests that exercise
increases brain neurotransmitters, such as serotonin
and norepinephrine, facilitating information proces-
sing [11–13]. Increased levels of arousal, detected by
brain electroencephalogram (EEG), have been
measured in persons exercising at less than 70%
of their maximum oxygen capacity (considered
within the moderate training zone) [5, 7, 14].
The third, and probably most well-studied hypoth-
esis, is that exercise upregulates neurotrophins such
as brain-derived neurotrophic factor (BDNF),
insulin-like growth factor (IGF-I) and basic fibro-
blast growth factor (bFGF) that support neuronal
survival and differentiation in the developing brain
and dendritic branching and synaptic machinery in
the adult brain (for review see [15]). It is clear that
youth with physical disability, with brains ripe for
new learning and sometimes with concomitant
cognitive challenges, may require physical activity
even more than adults.
The important neurotrophins
To understand neurotrophins and how they support
the nervous system, one must understand the effects
of these endogenous substances on the neurons
themselves. Neurotrophins are proteins that have
classically been identified as mediators of neuronal
survival and differentiation during development.
Each neurotrophin regulates specific populations of
neurons during development; however, more
recently, neurotrophins have been shown to maintain
the viability of neurons in adulthood and protect and
restore neurons in response to injury and ageing.
Neurons are described as being ‘plastic’; the efficacy
of synaptic transmission is adaptable and neuro-
trophins serve as activity-dependent modulators of
synaptic plasticity [15]. Neurotrophins regulate
target genes which may encode structural proteins,
enzymes or neurotransmitters that result in
modification of neuronal morphology and function.
This ability for neuronal plasticity allows one to form
and retain memories and learn in all dimensions;
spatially, cognitively and motorically. The neurotro-
phin brain-derived neurotrophic factor (BDNF) has
emerged as a key mediator of synaptic plasticity in the
memory centre of the brain, the hippocampus [16].
Synaptic signalling and responsiveness are enhanced
within seconds of BDNF administration to rat
hippocampal neurons [17, 18]. BDNF also augments
the number of synapses and enhances axonal
branching within the cortex [17], thereby increasing
the potential synaptic contact sites [18]. When the
critical expression of BDNF is blocked within the rat
brain, the animals show impairments in memory and
learning [19, 20]. Importantly, physical activity in
rats increases BDNF, as well as genes that are
members of synaptic vesicle trafficking machinery
and parts of signalling pathways whose activity affects
synaptic function [21].
Exercise and neurogenesis
It was once believed that the adult brain was
incapable of producing new neurons. It is now
known that neurogenesis occurs in the hippocampus
and in the layer of cells surrounding the lateral
cerebral ventricles (the subventricular zone) and,
moreover, that exercise stimulates this proliferation
[22]. These cells are sometimes referred to as
endogenous stem cells. In a recent study [23],
examining the benefit of stem cells in a rat model
of stroke, exercise and enriched environment stimu-
lated migration of transplanted stem cells to the
injury site and enhanced sensorimotor recovery.
Physical activity may increase baseline neuronal
activity or neurotrophic support, providing the
necessary signals for these cells to integrate into
neuronal networks. Stem cells (both endogenous
and transplanted) and the influence of physical
activity in the developing brain is a promising area
of research that could benefit children with physical
and cognitive impairment.
Exercise enhances cognitive function
In rats, 1 week of voluntary exercise increases BDNF
and enhances performance on the Morris water
maze, a test of spatial memory in which rats must
remember the location of a submerged platform
[24]. These findings have been confirmed by others
in both the normal animal brain [25, 26] and the
brain altered by injury [9, 27]. There is evidence that
Exercise is brain food 237
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the exercise-cognition phenomenon exits in humans
as well, especially in young adults. Aerobic fitness in
children is associated with higher measures of
neuroelectric responsiveness (P3 amplitude in brain
evoked potentials), faster cognitive processing speed
[28] and better performance in a test of executive
control [29]. A meta-analysis confirmed the positive
relationship between physical activity and cognitive
and academic performance in school aged children
[30]. Even though level of physical activity can be
confounded by other factors such as IQ and socio-
economic status, these findings are convincing.
In a 2-year follow-up of a prospective randomized
controlled trial, Reynolds and Nicolson [31] deter-
mined that a 6 month home-based sensorimotor
programme enhanced school and reading perfor-
mance in 36 children with reading difficulty (some
with dyslexia). Daily home exercises focused on
cerebellar/vestibular challenge including balancing
on one leg, spinning, bouncing, standing on a
wobble board, tandem walking and throwing and
catching balls. Students demonstrated improve-
ments in reading accuracy, phonemic skill, verbal
working memory and reduction in inattention
symptoms as well as accelerated gains in standard
school performance tests. These findings suggest
that the benefits of exercise are not simply cardio-
vascular. The enrichment provided by physical
activity has a broad effect on seemingly unrelated
executive processing centres. Furthermore, a recent
study by Verschuren et al. [32] showed that a
45 minute programme (twice per week for 8 months)
not only improved aerobic capacity, strength and
function but significantly improved cognition
and quality of life in people with cerebral palsy
aged 7–20 years. Even more interesting is that 4
months after completing the programme, partici-
pants maintained the cognitive benefits while fitness
measures returned to baseline. These studies sup-
port the concept of an exercise-cognition interaction
in children with disabilities. Programmes that
increase physical activity and fitness in youth with
disability will also likely improve executive function.
Future studies examining the implementation of
physical activity programmes should not only mea-
sure physical fitness but include cognitive, emotional
and quality of life outcome measures as well.
Exercise protects the nervous system
Physical activity attenuates the memory and cogni-
tive decline associated with normal ageing and in
pathological conditions such as Alzheimer’s Disease
[33, 34]. Exercise has also been recently shown to
increase brain volume in healthy exercising adults. In
a study using magnetic resonance imaging (MRI) to
examine brain volume, 59 people aged 60–79 were
randomly assigned to aerobic or non-aerobic exer-
cise groups (1 hour three times a week for 6
months). Adults exercising aerobically showed
increased brain volume in frontal lobe regions
implicated in higher order processing, attentional
control and memory [35].
In a rodent model of stroke, 2 weeks of voluntary
running (0.8 km per day) preceding cerebral stroke
resulted in improved survival and sparing of neurons
in multiple brain regions [36]. Carro et al. [27] have
demonstrated convincingly that moderate exercise
(1 km per day) improves functional recovery and
saves neurons in a number of rodent injury models.
They examined hippocampal degeneration (vulner-
able in Alzheimer’s disease), brainstem injury and
hereditary cerebellar degeneration. Lesioned animals
that ran before brain injury had improved function
and spared neurons and animals running for 5 weeks
following injury, improved to 90% of controls.
Exercise maintained function and preserved
Purkinje cells in the cerebellum and prevented
ataxia in hereditary cerebellar degeneration.
Although research in the neuroprotective effects
of exercise in humans, especially young people,
is scarce, findings in biomedical research are
compelling. One wonders if people who are inactive
are less protected against neurological injury
and neurodegenerative disease.
How much exercise is enough?
If exercise is beneficial for the brain, the next logical
question is ‘How much exercise is enough?’ A recent
meta-analysis of 37 studies (1306 children, young
adults and older adults) suggests that although most
studies support that exercise has a positive effect on
cognitive performance, cardiovascular fitness (VO
2
Max) alone does not explain these benefits [37].
Exercise effects on executive function are not dose-
responsive, meaning that better fitness does not
necessarily lead to larger cognitive gains. In fact,
smaller gains in fitness are associated with larger
cognitive effect sizes. Studies in children with
reading difficulties also show that children received
cognitive benefits from a programme designed to
challenge balance, timing and co-ordination, rather
than cardiovascular fitness [31]. This suggests that
physical activity levels that benefit cognition may
not necessarily be as intense as those levels required
to increase cardiovascular fitness. However, some
studies support that intense rather than moderate
exercise [12, 13] enhances neurotransmitter levels
and improves executive performance. These con-
flicting findings may reflect differences in exercise
paradigms and outcome measures. The specific
238 M. Ploughman
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exercise-cognition effect threshold is not known and
warrants further study.
Our research, using an adult rat model of
stroke, examined varying parameters of running
exercise to optimize the effects of exercise on brain
neurochemistry. BDNF and other markers of
neuroplasticity within brain tissue were examined
after 30 and 60 minutes of either running or walking
on a motorized wheel along with 60 minutes on a
voluntary running wheel [38, 39]. Telemetry was
used to determine the intensity of exercise to help
translate findings to clinical practice. Although 60
minutes of motorized exercise resulted in a robust
and immediate increase in hippocampal BDNF,
long-term, lower intensity (as measured by heart
rate), voluntary exercise resulted in more prolonged
upregulation of BDNF (2 hours). The implication
for clinical rehabilitation is that frequent, intermit-
tent, low intensity training (that is realistic to
provide) as well as a single bout of running exercise
can increase hippocampal levels of BDNF creating a
favourable ‘neuroplastic milieu’ during recovery
from stroke. Even though exercise enhanced pro-
teins that may be beneficial to recovery of function
after stroke, serum corticosterone, a stress hormone,
was found to be elevated in response to all methods
of exercise but more so with the more intense
exercise regimens. Since corticosterone has been
implicated in downregulation of BDNF [40], this is
a concern and warrants caution acutely after brain
injury. For the clinician, the old adage ‘everything in
moderation’ appears to apply when prescribing
physical activity for brain health.
Clinical considerations
It is believed that in most societies today people are
less active than in previous generations. The effect of
this inactivity on brain health is yet to be determined.
Hillman et al. [41] state that physical activity during
childhood may optimize cortical development pro-
moting lasting changes in brain structure and
function, but for youth with disability, the greatest
challenge is to find ways to increase physical activity
[42]. Kang et al. [43] examined barriers to exercise in
146 youth, aged 12–19, with physical disabilities
attending wheelchair basketball camp. These young
people identified mainly logistical barriers including
lack of time, pain or discomfort, lack of a place to
exercise with peers, weather and people’s misconcep-
tions of their abilities. Adults administered the same
tool complained more of psychological barriers to
exercise (lack of time, motivation, self-discipline).
These barriers may be overcome by an individually
tailored approach to exercise prescription such as
PEP-for-youth [42] and health providers must
emphasize, for their young clients, the multiple
benefits of physical activity on brain and body health.
Declaration of interest: The author reports no
conflicts of interest. The author alone is responsible
for the content and writing of the paper.
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