Can cleanliness contribute to Alzheimer’s?

Over the centuries, our lives and communities have become much more hygienic. And that’s mostly a good thing. But according to scientists at the Biological Anthropology Laboratory at Cambridge University, UK, there’s a startling downside to all this cleanliness. It seems to be correlated with an increased risk of Alzheimer’s disease (AD).

By comparing data from 192 countries, the researchers discovered that the most industrialized, wealthiest countries with large urban areas and good sanitation have the highest rates of AD. The reason, the researchers believe, has to do with the “hygiene hypothesis.” This is the idea that immune systems that are overly protected against exposure to germs are weaker and less able to ward off disease.

In well-off, highly sanitized countries, our immune systems are “protected” through the widespread use of antibiotics, chlorinated water, antibacterials, and artificial surfaces. All these factors can lead to insufficient development of white blood cells, including the T-lymphocytes, which attack invading microbes.

Previous research has noted deficiency of certain types of T-cells in the brains of people with AD. What’s more, an underdeveloped immune system exposes the brain to the inflammation associated with AD.

This idea is not as new as it may sound. The immune system requires some exposure to microbes in order to develop properly. When exposed naturally to germs early in life, the body develops lifelong natural immunity.

Since the 19th century, many populations have had less and less exposure to “friendly” microbes in soils, animals and manure, which strengthen the immune system, and had always been present throughout earlier human history. But as our exposure to germs has decreased, so has our resilience.

For example, polio became a big problem in the late 19th to mid-20th centuries (before the vaccine was developed) when a large proportion of the population began practicing “better” hygiene. This new level of cleanliness kept people from being exposed to polio as babies, but it didn’t do them any favors later in life. There is even evidence from ancient Egypt that the royal elites who were protected from germs had more problems with viruses such as polio later in life.

Polio is one of many viruses that travel through the environment–in this case infecting humans through contaminated water supplies. It is basically an intestinal virus. But in a small minority of cases, the virus passes into the spinal cord and can cause temporary or permanent paralysis. The vast majority of babies infected in infancy simply experience an acute viral infection. Paralysis or partial paralysis is very rare. (This may be due in part to the “passive immunity” that babies retain from their mothers–especially if they’re also breastfed). That exposure in infancy confers lifelong immunity.

If, on the other hand, better hygiene staves off your exposure until you’re an adolescent or young adult, you have a much greater chance of having it spread into your spinal cord and cause paralysis.

Each person infected with polio will on average pass the infection to several other people. So, we can see how the virus hitting a previously uninfected population of “baby boom” adolescents would have caused an epidemic during the 1950s…as it dramatically did.

But don’t ditch the soap just yet…

No one is suggesting that we stop washing up. In fact, as I’ve explained in previous Daily Dispatches, handwashing is the best way to prevent the influenza virus from spreading.

But don’t use antibacterial soap or hand sanitizer. And it’s important to only use antibiotics when they’re absolutely necessary.

If you want to further support brain function, nature provides a number of options, including berberine and carotenoids such as lutein and zeaxanthin. And dirt–if we believe the new study’s hypothesis.

So, don’t be afraid of getting a little dirty while working in the garden, enjoying nature, or visit a working farm. And let your children or grandchildren play out in the dirt too!


1. Fox, M, Knapp, LA, Andrews, PW, Fincher, CL, Hygeine and the world distribution of Alzheimer’s disease, Evolution, Medicine and Public Health, 2013, DOI: 10.1093/emph/eot015.