Better hygiene in wealthy nations may increase Alzheimer's risk

New research has found a very significant relationship between a nation's wealth and hygiene and the Alzheimer's burden on its population. High-income, highly industrialised countries with large urban areas and better hygiene exhibit much higher rates of Alzheimer's.

Using 'age-standardised'* data - which predict Alzheimer's rates if all countries had the same population birth rate, life expectancy and age structure -- the study found strong correlations between national sanitation levels and Alzheimer's.

This latest study adds further weight to the hygiene hypothesis in relation to Alzheimer's: that sanitised environments in developed nations result in far less exposure to a diverse range of bacteria, viruses and other microorganisms -- which might actually cause the immune system to develop poorly, exposing the brain to the inflammation associated with Alzheimer's disease, say the researchers.

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, said Dr Molly Fox, lead author of the study and Gates Cambridge Alumna, who conducted the research at Cambridge's Biological Anthropology division.

There are important implications for forecasting future global disease burden, especially in developing countries as they increase in sanitation.

The researchers tested whether pathogen prevalence can explain the levels of variation in Alzheimer's rates across 192 countries.

After adjusting for differences in population age structures, the study found that countries with higher levels of sanitation had higher rates of Alzheimer's. For example, countries where all people have access to clean drinking water, such as the UK and France, have 9% higher Alzheimer's rates than countries where less than half have access, such as Kenya and Cambodia.

Countries that have much lower rates of infectious disease, such as Switzerland and Iceland, have 12% higher rates of Alzheimer's compared with countries with high rates of infectious disease, such as China and Ghana.

More urbanised countries exhibited higher rates of Alzheimer's, irrespective of life expectancy. Countries where more than three-quarters of the population are located in urban areas, such as the UK and Australia, exhibit 10% higher rates of Alzheimer's compared to countries where less than one-tenth of people inhabit urban areas, such as Bangladesh and Nepal.

Differences in levels of sanitation, infectious disease and urbanisation accounted respectively for 33%, 36% and 28% of the discrepancy in Alzheimer's rates between countries.

Researchers said that, although these trends had overlapping effects, they are a good indication of a country's degree of hygiene which, when combined, account for 42.5% of the variation in countries' Alzheimer's disease rates -- showing that countries with greater levels of hygiene have much higher Alzheimer's rates regardless of general life expectancy.

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 -- supporting the new study's findings.

The results of the study are newly published by the journal