Not Enough Dirt?

Anyone familiar with the “hygiene hypothesis“?  This theory proposes that by living in an ever-more-sterile environment, we have forced our immune systems to “look for something else to do,” like causing asthma and an array of allergies, or attacking our islet cells, for example.

Dirty_hands The theory goes that children who grow up exposed to multiple bacteria have stronger immune systems — whereas the rising incidence of conditions like Type 1 diabetes, inflammatory bowel disease, multiple sclerosis and others may be, at least in part, the result of lifestyle and environmental changes that have made us too “clean” for our own good. 

And now, the latest extension of this theory says that too little dirt may actually contribute to clinical depression (!)  According to a recent report in The Economist, researchers at Bristol University in the UK experimenting with a treatment for lung cancer may have discovered a way to improve patients’ emotional health by re-introducing some of the missing dirt. No kidding.

Doctors were inoculating patients with something called Mycobacterium vaccae, “a harmless relative of the bugs that cause tuberculosis and leprosy, that had, in this case, been rendered even more harmless by killing it.”   Patients inoculated with this stuff not only experienced fewer symptoms of the cancer, but also felt emotionally more healthy, more vital and sharper in a cognitive sense.

Now they’re trying out the theory on mice.  The hypothesis is that the immune response to M. vaccae induces the brain to produce serotonin (a molecule that acts as a neurotransmitter, or chemical messenger between nerve cells), that is typically lacking in people with depression.

So far, it has worked. The researchers now have stress-free mice! — measured, in case you’re wondering, by dropping them into a tiny swimming pool. Previous research has shown that unstressed mice enjoy swimming, while stressed ones do not. The mice treated with serotonin swam around enthusiastically.

The implications are big:

1) “It opens a new line of inquiry into why depression is becoming more common… No one suggests this is the whole explanation for depression, but it may turn out to be part of it.”


2) “It offers the possibility of treating clinical depression with what is, in effect, a vaccination… Besides cancer, and now depression, it is being looked at as a way of treating Crohn’s disease (an inflammation of the gut) and rheumatoid arthritis.” 

Since diabetes and depression often go hand-in-hand, we may have hit the jackpot here.  Re-introduce bacteria into our lives and we might get better.  Who knew?


11 Responses

  1. Jana
    Jana April 27, 2007 at 1:15 pm | | Reply

    I don’t know about the “too little dirt” theory of the increase in diabetes, etc. but I do think decreased infant and early childhood mortality probably has something to do with it. Those of us with not-so-good immune systems live through things that in times past might have killed us off early on (in my case a life-threatening kidney infection when I was 3 years old, followed by another not-so-serious one when I was 4 and then finally a diagnosis of type 1 diabetes at age 19).

  2. Type 1 Diabetes information
    Type 1 Diabetes information April 27, 2007 at 10:03 pm | | Reply

    Very good article, Five star!

  3. Bernard Farrell
    Bernard Farrell April 28, 2007 at 3:00 am | | Reply


    This is why we have chickens and goats in our backyard. We just love the dirt. :)

  4. Scott
    Scott April 28, 2007 at 10:52 am | | Reply

    The hygiene theory has been around for some time, but so far, there is little to substantiate the theory. As it turns out, some countries such as India have also seen an incidence in T1DM, suggesting that hygiene may not be the answer, but perhaps climate changes or pollution. Unfortunately, there is a relative lack of studies investigating the environment or pollution’s impact on the development of T1DM. Looks like researchers will have plenty to do for a long time to come!

  5. artsweet
    artsweet April 28, 2007 at 8:49 pm | | Reply

    I love being the anti-anecdote.

    I was breastfed for a long long time. (cow’s milk ‘theory’)

    My mom practically rolled me in dirt.

    I have diabetes. And depression (and that one has a strong family history).

    Next theory please?

  6. Kelly
    Kelly April 28, 2007 at 9:04 pm | | Reply

    Hi Amy!

    Very interesting.I just attended an advisory committee meeting for the Celiac Disease Center at Columbia U. this last week…and someone asked a question about the “dirt” theory. The doctors present all agreed that they thought it was weak. *shrug* I guess we’ll see as the research continues.

  7. JasonJayhawk
    JasonJayhawk April 29, 2007 at 2:10 am | | Reply

    I just want to know how a person determine when a mouse really enjoys swimming.

    And if nobody knows, then I want to know where to apply for a money grant to perform this kind of research.

  8. AmyT
    AmyT April 29, 2007 at 2:56 pm | | Reply

    Hey Jason,
    I think they start yelling “Marco!” and “Pollo!” You guys crack me up…

  9. Michelle
    Michelle May 6, 2007 at 7:17 am | | Reply

    My mom would disagree with the dirt theory as (according to her) we live in a pig sty and both my kids still ended up with autoimmune diseases. ;)

  10. Manfred
    Manfred May 14, 2007 at 11:43 pm | | Reply

    There might be something to it. I do office work and lived in a 3rd floor flat for 14 years. Two years ago my life changed and I moved to an island and now have a garden and am outside a lot doing things like digging, cutting and cleaning. I still have my office work but now I am more exposed to dirt. I feel much better (but only after bathing!). Don’t know whether the increase in physical activity or the increased exposure to dirt or a combination of both is effective but something rings true about the hygiene hypothesis.

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