Can all our efforts at cleanliness and fighting germs be contributing to the growing epidemic of Alzheimer’s? A new study suggests it might.
The findings, published in the journal Evolution, Medicine and Public Health, come from researchers in Britain, who undertook an analysis of Alzheimer’s rates around the world. They found that the prevalence of Alzheimer’s was highest in wealthy nations, where better hygiene and strict sanitation measures are in place.
"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, the lead author of the study, who conducted the research at Cambridge University's Biological Anthropology division.
According to the hygiene hypothesis, exposure to a variety of bacteria, viruses worms and other pathogens is not always bad. While many germs can make you sick, others can prime immune system cells and spur the development of a healthy immune system.
Various studies suggest that exposure to various germs helps to prevent allergies, asthma and autoimmune disorders like Type 1 diabetes and inflammatory bowel disease. Lack of exposure, on the other hand, can increase the likelihood of such disorders. Children who grow up in households with pets, other studies have shown, are less likely to develop allergies.
For the current study, the researchers tested whether exposure to such germs might explain some of the variation in rates of Alzheimer’s in 192 countries. They found that countries with higher levels of sanitation had higher rates of Alzheimer's.
For example, in countries where all people have access to clean drinking water, such as in the U.K. and France, Alzheimer's rates are 9 percent higher than in 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 percent higher rates of Alzheimer's compared with countries with high rates of infectious disease, such as China and Ghana.
Alzheimer’s was also more common in more urbanized countries, where sanitation tends to be better.
The findings are consistent with earlier research, which has shown that Alzheimer's prevalence is higher in Europe, for example, than in Latin America, China and India.
"Exposure to microorganisms is critical for the regulation of the immune system," the researchers wrote. In recent decades, less contact with soil and animals, combined with the development of antibiotics, clean drinking water, paved roads and other factors, means the populations of many of the world's wealthier nations have increasingly very little exposure to “good” microbes that can stimulate the immune system and fight inflammation.
While no one is suggesting we spend more time in the dirt, "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,” said Dr. Fox.
The “hygiene hypothesis” is interesting and may be important for understanding some diseases involving the immune system. However, in the opinion of Fisher Foundation scientists, studies such as the one reported here should be viewed with some skepticism. There are many potential reasons why reports of Alzheimer’s disease rates differ with regard to wealthy versus poorer nations. For one thing, it is likely that the prevalence of Alzheimer’s is under-reported in poorer nations, where people have less access to doctors, medical specialists and the very expensive tests necessary to diagnose probable Alzheimer’s. Hygiene and exposure to microbes are only two of many differences that distinguish wealthy versus poorer countries. Again, hygiene may turn out to be an important factor but we need to consider more variables before we come to conclusions about cause-and-effect.
Source: Molly Fox, Leslie A. Knapp, Paul W. Andrews, Corey L. Fincher: “Hygiene and the World Distribution of Alzheimer's Disease.” Evolution, Medicine and Public Health, doi: 10.1093/emph/eot015. Volume 2013 Issue 1 2013 First published online: August 11, 2013.