Alzheimer’s may be linked to improved hygiene, say scientists

LONDON — 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 per cent, 36 per cent and 28 per cent 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 greater degree of sanitation and lower prevalence of pathogens had a higher burden from the disorder. Countries with 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 agents might stall the proper development of important elements of the body’s immune system such as white blood cells. The team suggest 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.”

Dr 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.” THE GUARDIAN