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Why Sanitation May Lead to Alzheimer’s Disease

New research from Britain’s University of Cambridge suggests that the higher rates of Alzheimer’s disease observed in industrialized countries may be due to those populations living in biologically clean environments, with reduced contact with bacteria, viruses and other microorganisms. Previous research, conducted over the past several years, has examined the association between the immune system and Alzheimer's. This new study looks at exposure to environmental challenges, which might strengthen the immune system and reduce the risk of dementia. If true, this would place Alzheimer’s disease alongside allergies and asthma on the list of diseases that might be encompassed by the hygiene hypothesis, a proposed explanation for why allergies and asthma are now epidemic, especially in developed countries. The Cambridge researchers looked at whether “pathogen prevalence” underlies levels of variation in Alzheimer’s rates across 192 countries. They found that countries with higher levels of sanitation had higher rates of Alzheimer’s, noting that countries such as the U.K. and France, where everyone has access to clean drinking water, have nine percent higher Alzheimer rates than Kenya, Cambodia and other countries where less than half the population has access to clean drinking water. The researchers found that differences in levels of sanitation, infectious disease and urbanization accounted respectively for 33 percent, 36 percent and 28 percent of the discrepancy in Alzheimer’s rates between countries. Noting that “exposure to microorganisms is critical for the regulation of the immune system,” the investigators wrote that “the populations of many of the world’s wealthier nations have increasingly very little exposure to the so-called ‘friendly’ microbes which ‘stimulate’ the immune system - due to diminishing contact with animals, feces and soil.” The study was published online in the journal Evolution, Medicine and Public Health on August 11, 2013.

My take? The hygiene hypothesis holds that children who grow up in crowded and dirtier environments are less likely to develop allergies and asthma than youngsters raised in cleaner, more protected environments. The idea is that the developing immune systems of less privileged kids are exposed to lots of germs from an early age and as a result become stronger, better attuned to the world around them, and more protective of health. If a link between immunity and dementia is confirmed, this new research would make an intriguing case for including Alzheimer’s disease under the umbrella of the hygiene hypothesis. However, it is important to remember that the hygiene hypothesis remains a theory. We will need a lot more evidence from human studies before we could consider it valid.

Molly Fox et al, “Hygiene and the world distribution of Alzheimer's Disease,” Evolution, Medicine and Public Health, doi: 10.1093/emph/eot015