Better hygiene could increase risk of Alzheimer’s

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. 

Populations in high income, more urbanised and highly industrialised countries have higher rates of Alzheimer’s disease. A new study from the University of Cambridge has suggested that this could be because better hygiene in these areas has greatly reduced contact with bacteria and viruses, leading to poorly developed immune systems that leave the brain more exposed to the inflammation associated with Alzheimer’s disease.

Dr Molly Fox, lead author of the study, said: "The ‘hygiene hypothesis’, which suggests a relationship between cleaner environments and a higher risk of certain allergies and autoimmune diseases, is well established. We believe we can now add Alzheimer’s to this list of diseases.”  

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. The researchers concluded that hygiene was positively associated with risk of Alzheimer's disease. 

James Pickett, head of research at the Alzheimer's Society, who was not involved in the research, said: "We have known for some time that the numbers of people with Alzheimer’s varies 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.

"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."

Meanwhile, the NHS has responded to the story by releasing a statement. "Media reports suggesting that being clean causes Alzheimer's disease cannot be backed up because the study can’t prove cause and effect," it says. 

"The causes of Alzheimer’s disease are largely unknown, with genetic factors and age being the most established risk factors. If there is a link between a country’s economics and sanitation and risk of Alzheimer’s disease, it could be due to confounding from other environmental and sociodemographic factors, rather than being a direct effect of sanitation and pathogen exposure alone."