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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