Published on: September 7, 2013
by Nick McDermott for Daily Mail:
People in wealthy countries are at ‘greater risk’ as they have less contact with bacteria. Study looked at the link between cleanliness and Alzheimer’s in 192 countries.
Countries with clean drinking water have 9% higher Alzheimer’s rates and countries with less infectious disease have 12 per cent more Alzheimer’s.
An obsession with being too clean and hygienic could lead to a higher risk of dementia, researchers have warned.
Their study pinpointed a significant relationship between a nation’s cleanliness and the number of Alzheimer’s patients. Countries which can afford better sanitation have higher rates of the disease.
The researchers suggested the ‘hygiene hypothesis’ was behind the difference. This is the theory that an excessively clean lifestyle leaves our immune systems out of balance and unable to combat many germs.
It has already been linked to the rise in the number of allergies, such as asthma and eczema.
One in three Britons over 65 will develop dementia. Alzheimer’s and other forms of the condition blight the lives of more than 800,000, with 500 new cases each day.
Lead researcher Dr Molly Fox, from Cambridge University, said: ‘The hygiene hypothesis is well-established. We can now add Alzheimer’s to this list of diseases. ‘There are important implications, especially in developing countries as they increase in sanitation.’
The study of health data from 192 nations found those with a relatively low risk of infection had more patients with Alzheimer’s.
Likewise, better sanitation and urban living were linked with a higher incidence of the disease, irrespective of life expectancy. Taken together, these factors accounted for 42.5 per cent of the variation in rates of Alzheimer’s between countries.
Dr Fox said changes in diet, life expectancy and healthcare could not explain the differences.
The researchers found that exposure to germs throughout an individual’s lifetime, not just early on, may affect the risk of dementia.
A lack of contact upsets the development of white blood cells, particularly those called T-cells, which are a key part of the immune system.
This imbalance has been linked to the types of inflammation found in the brains of those with Alzheimer’s, said the report in the journal Evolution, Medicine and Public Health.
Dr Fox added: ‘We need to find the right balance of exposure.’
A spokesman for the Alzheimer’s Society said: ‘It is an interesting theory. However, it is always difficult to pin causality to one factor.’ The best way to cut risk is to eat healthily, exercise, not smoke and keep blood pressure and cholesterol in check, he added.
Countries where more than three-quarters of the population are located in urban areas, such as the UK and Australia, had 10 per cent higher rates of Alzheimer’s compared to countries, such as Bangladesh and Nepal, where less than one-tenth of people inhabit urban areas.
Overall, differences in levels of sanitation, infectious disease and urbanisation accounted for 33 per cent, 36 per cent and 28 per cent of the variation in Alzheimer’s rate between countries.
Previous research has shown that Alzheimer’s affects fewer people in Latin America, China and India than it does in Europe. Even within those regions, prevalence is lower in urban than in rural areas, according to the new findings.
The hygiene hypothesis is based on the assumption that lack of contact with ‘dirt’ in the form of bacteria and other infectious agents upsets the development of white blood cells, key elements of the immune system.
In particular, T-cells are said to be affected. T-cells have a variety of functions, including attacking and destroying foreign invaders and marshalling other parts of the immune system.
Some, known as ‘regulatory’ T-cells, reign in the immune system when it starts to get out of control. Dysfunctional regulatory T-cells can lead to inflammation and autoimmune disorders.
Regulatory T-cell deficiency is linked to the type of inflammation commonly found in the brains of people with Alzheimer’s disease.
‘Exposure to microorganisms is critical for the regulation of the immune system,’ wrote the researchers.
They added that, since increasing global urbanisation beginning at the turn of the 19th century, the populations of many of the world’s wealthier nations have increasingly very little exposure to the so-called ‘friendly’ microbes due to ‘diminishing contact with animals, faeces and soil.’
‘The increase in adult life expectancy and Alzheimer’s prevalence in developing countries is perhaps one of the greatest challenges of our time,’ said Dr Fox.
‘Today, more than 50 per cent of people with Alzheimer’s live in the developing world, and by 2025 it is expected that this figure will rise to more than 70 per cent.
‘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.’
The hygiene hypothesis is normally thought to be most relevant in childhood, when the immune system is still developing. But in the case of Alzheimer’s, exposure to microbes across a person’s lifetime might be important, say the scientists.
This is because regulatory T-cell numbers peak at various points in life, for example at adolescence and middle age.
The results of the study are published by the journal Evolution, Medicine and Public Health.
Picture: Hero Images/Corbis
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