Alzheimer's may be linked to better hygiene, say scientists
Improvements in hygiene could
partly explain increased rates of
Alzheimer's disease seen in many
developed countries, according to
research into the link between
infections and the condition.
The researchers studied the
prevalence of the
neurodegenerative disease across
192 countries and compared it with
the diversity of microbes in those
places.
Taking into account differences in
birth rate, life expectancy and age
structure in their study, the
scientists found that levels of
sanitation, infectious disease and
urbanisation accounted for 33%,
36% and 28% respectively of the
discrepancies seen in Alzheimer's
rates between countries.
In their report which was published
in the journal Evolution, Medicine
and Public Health, the researchers
concluded that hygiene was
positively associated with risk of
Alzheimer's disease. Countries with
a greater degree of sanitation and
lower prevalence of pathogens had
a higher burden from the disorder.
Countries with a greater degree of
urbanisation and wealth also had
higher Alzheimer's burdens.
Whether hygiene causes the pattern
is not yet clear – cleanliness or
infectious disease might be
associated with some other factor –
but the team does have a
speculative hypothesis for how the
two factors might be linked.
Exposure to micro-organisms –
good and bad – is important for the
body to develop proper immune
responses.
The researchers' "hygiene
hypothesis" suggests that as societies
have become cleaner, the reduced
level of contact with bacteria and
other kinds of infectious agent
might stall the proper development
of important elements of the body's
immune system such as white blood
cells. The team suggests that
developing Alzheimer's might be
linked to autoimmune disease, in
which the body's immune system
attacks itself.
"Alzheimer's disease (AD) shares
certain etiological features with
autoimmunity," the researchers
wrote in the journal Evolution,
Medicine and Public Health.
"Prevalence of autoimmunity
varies between populations in
accordance with variation in
environmental microbial diversity.
Exposure to micro-organisms may
improve individuals'
immunoregulation in ways that
protect against autoimmunity, and
we suggest this may also be the case
for AD."
James Pickett, head of research at
the Alzheimer's Society, who was
not involved in the research, said it
was well known that the
prevalence of Alzheimer's disease
varied between countries. "That
this discrepancy could be the result
of better hygiene is certainly an
interesting theory and loosely ties
in with the links we know exist
between inflammation and the
disease," he said.
"However, it is always difficult to
pin causality to one factor and this
study does not cancel out the role
of the many other lifestyle
differences such as diet, education
and wider health which we know
can also have a role to play. One in
three people over 65 will develop
dementia. The best way to reduce
your risk is to eat a healthy diet,
exercise regularly, not smoke and
keep your blood pressure and
cholesterol in check."
