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The effects of
exercise on cognitive function in older adults
A
number of studies in the past few years have provided evidence that
physical exercise can ameliorate the effects of aging on the brain,
in terms both of preventing or postponing dementia, and reducing the more
normal age-related decline in cognitive function (see Effects of exercise on cognitive function
for recent reports). The reasons for the effect are
almost certainly multiple, for example:
-
Exercise
has clear effects on cardiovascular fitness, and many recent studies
have provided converging evidence that there is an association between
cardiovascular fitness and mental fitness - "what's good for the
heart is good for the brain".
-
Exercise
helps control blood sugar levels, and a recent study has found that
those with impaired glucose tolerance tend to have a smaller
hippocampus.
-
Exercise
may increase the flow of oxygen-rich blood to the brain.
-
Exercise
may increase self-confidence, and may reduce anxiety and depression.
Interestingly,
while exercise benefits both genders, there is some evidence that it may
be of greater benefit to women (at older ages). This may be related to
estrogen status. There
is some evidence that, in females, the benefits of exercise depend on the
presence of estrogen. Levels of voluntary physical activity also seem to
depend on estrogen status. This may be behind some of the benefit hormone
therapy can have on older women's cognitive functioning (see Hormone
therapy for recent reports).
But
the undoubted benefits of physical activity for seniors do not imply that
exercise has any effect on memory and learning in younger people. That is
quite a different question. In seniors, the hope is that exercise will
counteract some of the biological wear and tear caused by aging. Does
physical fitness matter at younger age levels?
The effects of
exercise on cognitive function in children and young adults
Unfortunately,
there have been far fewer studies involving young people. However, one
study [1], reported at the 2001 Society for
Neuroscience conference, found that, following a 12 week regimen of
jogging for 30 minutes two to three times a week, young adults
significantly improved their performance on a number of cognitive tests.
The scores fell again if participants stopped their running routine.
In
this particular case, it does not seem that level of fitness is the
primary cause - otherwise, you'd expect test performance not to be so
quickly affected by the cessation of physical activity. The researchers suggested that
increased oxygen flow to the brain might have been behind the improvement
in mental sharpness. Oxygen intake did rise with the joggers' test scores.
Supplemental oxygen administration has been found to significantly
improve memory formation in healthy young adults, as well as improving
reaction time [2].
On the
other hand, preliminary results from a series of studies undertaken
with elementary school children do indicate a strong relationship between
academic achievement and fitness scores. One study found that physically
fit children identified visual stimuli faster. Brain activation patterns
provided evidence that the fit children allocated more cognitive resources
towards the task, as well as processing information faster. [3]
What studies with
non-humans tell us
Rodent studies
have a big advantage over human studies - many subjects ready to hand,
complete control of their environment - and accordingly, it is easier to
receive more direct answers. These studies tell us not simply that exercise can be
beneficial for learning, but why it might be so.
Studies
with mice have made it clear that exercise can:
-
increase levels of BDNF
(brain-derived neurotrophic factor; BDNF helps support and strengthen the
synapses in the brain (the connections between neurons), as well as
helping protect and grow new neurons),
-
stimulate neurogenesis (the
creation of new neurons),
-
increase resistance to brain
insult, and
-
perhaps promote brain plasticity. [4]
However,
while there is no doubt that exercise increases levels of BDNF in the hippocampus, we can’t take it for granted that this is entirely a
good thing. Mice bred for 30 generations to be more active (indeed,
exercise “addicts”), showed high levels of BDNF and grew more neurons
in the hippocampus, and yet performed terribly when attempting to navigate
around a maze. Researchers suggested that too much exercise may cause the
brain to “max out” in the production of BDNF and neurons, and this may
prevent learning. Alternatively, the highly active mice may simply have been too
focused on running to concentrate on anything else! [5]
The
point is that at the moment, we don’t know for sure what the
significance of the exercise-induced increase in BDNF and neurogenesis is.
It may be that high levels of exercise place stress on the hippocampus,
damaging or killing neurons. The increased levels of BDNF and neuron
production may simply be attempts to counteract the damage done. All
that's certain is that exercise provokes a lot of activity in the
hippocampus, in particular in that particular region of the hippocampus
called the dentate
gyrus.
Having
said that, let's note that this is the first study to demonstrate a case
of neurogenesis that is not associated with learning improvement.
In general, the production of new neurons is associated with
improvement in learning and memory. It would be unwise, therefore, to take
these findings as indicating the reverse. What they do suggest is that we
cannot assume that such an association always occurs, and that in the case
of exercise, it may well be that you can have too much of a good thing! It
does seem clear, from this and other studies, that there is a direct
association between amount of exercise and BDNF level.
On
the subject of whether you can have too much exercise, it's worth noting
that a human study found that, while moderate aerobic exercise for up to
an hour improved performance on particular cognitive tasks, too much
exercise had a deleterious effect. [6]
Brain
regions affected by exercise
Notwithstanding
the (understandable) emphasis placed on the hippocampus, a critical region
for learning and memory, human studies have implicated many parts of the
brain. Specifically, one
study of seniors found that executive functions were particularly
improved by exercise - executive functions are primarily located in the
prefrontal cortex. Another
study of seniors found reduced grey and white matter in the frontal,
temporal, and
parietal cortexes of those who were less physically fit. In similar vein, another
study of
seniors found differences in the middle-frontal and
superior parietal
regions of the brain as a function of aerobic fitness.
Interestingly,
in the possibly first study to look at higher cognitive funtion during
exercise (sustained, moderate), it was found that functions dependent on
the prefrontal cortex were impaired, but not those requiring little
prefrontal activity. [7]
Exercise
and diet
Exercise
should not, of course, be considered entirely without reference to diet. The
effect of exercise on cardiovascular fitness and blood glucose levels is a
counterweight to the effect diet has had in inducing impaired glucose
tolerance and cardiovascular problems. A number of rodent studies
have found that a high-fat diet impairs learning and memory. Rodent
experiments have also found that exercise can reverse the decrease in BDNF
levels in the hippocampus resulting from a high-fat diet, and prevent the
deficit in spatial learning induced by such a diet. [8]
The question might
therefore arise, if the diet has been healthy, is exercise beneficial?
Interestingly, a very recent study involving older beagles found that both
a diet enriched with antioxidants and a stimulating environment were
helpful in preventing or reducing age-related cognitive decline. That is,
each were good, but both was best. This doesn't directly answer the
question, of course, but it does seem likely that both diet and exercise
are important factors in physical and mental health.
Physical exercise and
mental exercise
The beagle study used
what is termed an "enriched" environment - typically this
involves opportunities for social interaction and mental stimulation, as
well as physical activity. A mouse study endeavored to separate the
components of such an enriched environment, in order to see whether all
were necessary to achieve the observed increased neuron production in the
dentate gyrus. Interestingly, they found that voluntary wheel
running was in itself sufficient to achieve the level of neurogenesis
achieved in typical enrichment conditions. [9]
This is intriguing,
but as much as anything else it points to the limitations of rodent
studies as models for human behavior. A number of human studies, again,
mainly with older adults, point to the value of mental stimulation in
protecting against cognitive decline (see The role of mental stimulation in aiding
memory). Interestingly, one such study
found ballroom dancing was apparently of (surprising) value in protecting
against age-related cognitive decline - it was suggested that there was an
intellectual component to it lacking in other physical activities. But
perhaps, if I may speculate, we should consider more seriously that
activities that combine intellectual and physical (and perhaps social)
attributes might be best of all.
It does seem clear
that, while both mental stimulation and physical exercise might both help
cognitive function, they do so in quite different ways, for different
reasons.
Recommendations
An
analysis of 18 studies [10] on the effects of exercise on cognitive function in
older adults concluded that:
-
exercise
programs involving both aerobic exercise and strength training
produced better results on cognitive abilities than either one alone
-
more
than 30 minutes of exercise per session produce the greatest benefit
Caveat:
Not everyone benefits equally from exercise
It does seem clear
that older adults benefit more from exercise than younger people, as far
as cognitive function is concerned. It also seems that older women,
especially those on hormone-replacement therapy, receive greater cognitive
benefits from exercise than men.
Generalisations aside,
it is as well to remember the findings of a very recent study showing
that, while most people benefit (physically) from exercise, the degree of
benefit is hugely variable between individuals, and some people don’t
benefit at all! [11]
References
Bouchard,
C. 2004. Reported at the Australian Health and Medical Research Congress
in Sydney, Australia.
[11]
http://www.newscientist.com/news/news.jsp?id=ns99996735
Colcombe,
S. & Kramer, A.F. 2003. Fitness effects on the cognitive function of
older adults: A meta-analytic study. Psychological Science, 14,
125-130. [10]
Cotman,
C.W. & Berchtold, N.C. 2002. Exercise: a behavioral intervention to
enhance brain health and plasticity. Trends in Neurosciences, 25 (6),
295-301. [4]
Dietrich,
A. & Sparling, P.B. 2004. Endurance exercise selectively impairs
prefrontal-dependent cognition. Brain
and Cognition, 55 (3), 516-524. [7]
Hillman,
C. & Buck, S. 2004.
Physical
Fitness and Cognitive Function in Healthy Preadolescent Children. Presented at the annual meeting of the Society
for Psychophysiological Research in Santa Fe, N.M., Oct. 20-24.
http://www.eurekalert.org/pub_releases/2004-10/uoia-pfc101904.php
Kubota et al. 2001.
cited in http://nootropics.com/exercise/index.html
[1]
Johnson,
R.A., Rhodes, J.S., Jeffrey, S.L., Garland, T.Jr. & Mitchell, G.S.
2003. Hippocampal brain-derived neurotrophic factor but not neurotrophin-3
increases more in mice selected for increased voluntary wheel running. Neuroscience,
121 (1), 1-7.
Molteni,
R., Wu, A., Vaynman, S., Ying, Z., Barnard, R.J. & Gómez-Pinilla, F.
2004. Exercise reverses the harmful effects of consumption of a high-fat
diet on synaptic and behavioral plasticity associated to the action of
brain-derived neurotrophic factor. Neuroscience, 123 (2), 429-440.
[8]
Rhodes,
J.S., van Praag, H., Jeffrey, S., Girard, I., Mitchell, G.S., Garland,
T.Jr. & Gage, F.H. 2003. Exercise increases hippocampal neurogenesis
to high levels but does not improve spatial learning in mice bred for
increased voluntary wheel running. Behavioral Neuroscience, 117(5),
1006-1016. [5]
Scholey,
A.B., Moss, M.C., Neave, N. & Wesnes, K. 1999. Cognitive
Performance, Hyperoxia, and Heart Rate Following Oxygen Administration in
Healthy Young Adults. Physiology & Behavior, 67 (5),
783-789. [2]
Tomporowski, P.D.
2003. Effects of acute bouts of
exercise on cognition. Acta
Psychol (Amst), 112, 297-324. [6]
van Praag,
H., Kempermann, G. & Gage, F.H. 1999. Running
increases cell proliferation and neurogenesis in the adult mouse dentate
gyrus. Nature Neuroscience, 2 (3),
266-70. [9]
van Praag H,
Christie BR, Sejnowski TJ, Gage FH 1999. Running
enhances neurogenesis, learning, and long-term potentiation in mice. Proc
Natl Acad Sci U S A, 96, 13427-31.
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