In medicine, the hygiene hypothesis
states that a lack of early childhood exposure to infectious agents, symbiotic
microorganisms (e.g., gut flora or probiotics), and parasites increases
susceptibility to allergic diseases by suppressing natural development of the
immune system.
Research
led by Dr Molly Fox at Cambridge's Biological Anthropology division has
found that sanitised environments in developed nations might actually cause the
immune system to develop poorly, exposing the brain to the inflammation
associated with Alzheimer's disease.
Previous
research has shown that in the developed world, dementia rates doubled every
5.8 years compared with 6.7 years in low income, developing countries; and that
Alzheimer's prevalence in Latin America, China and India are all lower than in
Europe, and, within those regions, lower in rural compared with urban settings.
"Exposure
to microorganisms is critical for the regulation of the immune system. Aspects
of modern life -- antibiotics, sanitation, clean drinking water, paved roads
and so on -- lead to lower rates of exposure to these microorganisms that have
been "omnipresent" for the "majority of human history," write
the researchers.
This
lack of microbe and bacterial contact can lead to insufficient development of
the white blood cells that defend the body against infection, particularly
those called T-cells -- the foot soldiers of the immune system that attack
foreign invaders in the bloodstream.
Deficiency
of anti-inflammatory ("regulatory") T-cells has links to the types of
inflammation commonly found in the brain of those suffering with Alzheimer's
disease, and the researchers' proposal that Alzheimer's risk is linked to the
general hygiene levels of a nation's population is reinforced by their analysis
of global Alzheimer's rates.
While
childhood -- when the immune system is developing -- is typically considered
critical to the 'hygiene hypothesis', the researchers say that regulatory
T-cell numbers peak at various points in a person's life -- adolescence and
middle age for example -- and that microorganism exposure across a lifetime may
be related to Alzheimer's risk, citing previous research showing fluctuations
in Alzheimer's risk in migrants.
"A
better understanding of how environmental sanitation influences Alzheimer's
risk could open up avenues for both lifestyle and pharmaceutical strategies to
limit Alzheimer's prevalence. An awareness of this by-product of increasing
wealth and development could encourage the innovation of new strategies to
protect vulnerable populations from Alzheimer's."
###
The
above story is based on the September 4, 2013 release by University of
Cambridge.
The
results of the study are newly published by the journal Evolution, Medicine and
Public Health:
Fox
M, Knapp LA, Andrews PW, Fincher CL. Hygiene
and the world distribution of Alzheimer's Disease. Evolution, Medicine, and Public Health, 2013; DOI: 10.1093/emph/eot015
Click HERE if video does not appear on your screen. In this video, Molly Fox discusses her research.
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