Eat dirt

Posted 12 March 2012 by Matt Young

A recent article in Natural History magazine does not exactly advise eating dirt, but rather examines what the author calls "the hygiene hypothesis," that is, the hypothesis that the increasing prevalence of asthma and other autoimmune diseases is the result of excessive cleanliness. Along the way, the author, Druin Burch, illuminates in some detail just how good science works. The gist of the hygiene hypothesis is that the more exposure people get to the germs and parasites with which they coevolved, the less likely they are to develop autoimmune diseases like hayfever. The prevalence of allergic and autoimmune diseases has increased in the last 50 years or more, and, moreover, such diseases are more common among "the richer citizens of the world, [for whom] exposure to many common microorganisms is being delayed." Indeed, children who go to nursery school or grow up on farms are less likely to contract allergies or autoimmune diseases. It is possible that, paradoxically, an immune system that does not have enough to do looks for something to attack; sometimes that something is an allergen, and sometimes is the host's own body. Burch notes, however, that correlation does not prove causation, though he does not express it exactly that way. He argues,

There have certainly been lots of articles on the subject, including lots of peer-reviewed studies that seek to look at original data. I hesitate to term them all "science," since many are descriptive: they design and carry out no experiment and attempt to falsify no hypothesis. They fall into the sort of technically proficient studies of natural history that most of us think of as science but which are really just the groundwork for it. They develop theories about the way the world works; science only happens when those theories get properly tested.

And later on,

The most important thing to note, though, is that what started off inconclusive has stayed that way.The association between infections and allergies was first suggested as long as half a century ago--although originally the idea was that infections caused such diseases, not prevented them. Allergies and autoimmune diseases have risen and parasites have dropped away at the same time as a host of other things have changed--democracy has spread, satellites have been launched, mobile phones have been developed, and a raft of modern politicians elected. If you wanted, you could show an association between any of those aspects of contemporary life and the increase in rates of allergies and autoimmune disorders. Associations are easy to spot, even plausible ones--but proving causation is hard. Might early exposure to bugs and parasites be entirely unrelated to allergies and autoimmune diseases? Quite possibly. It's been suggested that the spread of detergents in the world (which lines up chronologically with every other modern phenomenon) disrupts the mucus lining the inside of our intestines, with some diseases being caused as a result.

And finally he confesses that he "had the strong impression that research into the hygiene hypothesis had shown that dirt was good for us," but found that he was "mistaken"; the evidence is "clearly weak." Possibly, he had that impression because "[t]he hygiene hypothesis is the sort of story that's attractive to mass media"; I certainly had that impression for that reason (though the hypothesis seems somewhat stronger to me than to Burch). I cannot entirely agree that descriptive studies that formulate no hypotheses are not science; if that were so, then taxonomy might not be considered a science. But it is important that 50 years of careful epidemiological work is not conclusive. Of more concern to me here, however, is the fact that the very scientists who work on the hygiene hypothesis remain unconvinced. Specifically, they do not formulate a hypothesis and then break their necks to support it. Rather, they look at the evidence as dispassionately as anyone can, and they stand ready to discard the hypothesis if it is falsified. Such behavior is in stark contrast to any of several ideologies I could name.

14 Comments

DS · 12 March 2012

"They develop theories about the way the world works; science only happens when those theories get properly tested."

Actually, they develop hypotheses about the way the world works; theories only happen when those hypotheses get properly tested and are not falsified.

harold · 12 March 2012

I cannot entirely agree that descriptive studies that formulate no hypotheses are not science

I'm shocked that anyone would claim that such work is not science. You have to have a hypothesis before you can test one. Observation precedes hypothesis formation. If smallpox had not already been described before viruses were discovered, for example, it would have been awfully difficult to construct and test the hypothesis that a virus causes smallpox. There is some fairly strong support for the hygiene hypothesis, because in parts of Europe where farming populations live among otherwise similar non-farm families, the children raised on farms develop relatively fewer allergies. (That's "relatively", one branch of my family has been plagued by asthma and hay fever for generations despite being farmers.) One thing that seems to be notably lacking is an animal model. We can never do an experiment of deliberately exposing human children to parasites and microbes to see if they develop fewer allergies (if they survive), for obvious ethical reasons. Lacking a strong animal model, epidemiology will have to do. Epidemiological methods can often eventually provide evidence beyond reasonable doubt. I suppose exposure to antigens, in the absence of live pathogens, might also be tried on an experimental basis. Purely speculating here, one can imagine an "idle hands to the devil's work" mechanism with respect to the inflammatory/immune system. There could be some tendency for some minimum level of activity. Note that autoimmune-related conditions also are unequivocally sometimes post-infectious, e.g. rheumatic fever, Guillaume-Barre syndrome, post-viral Bell's palsy, etc. This fact does not in any way exclude the possibility that lack of sufficient antigen exposure during early development may predispose to different immune-related pathologies. If this idea stands up, the solution would be controlled introduction of sufficient antigens to reduce risk of allergic conditions, not abandonment of hygiene, of course.

Dave Lovell · 12 March 2012

harold said: Purely speculating here, one can imagine an "idle hands to the devil's work" mechanism with respect to the inflammatory/immune system. There could be some tendency for some minimum level of activity.

From a control engineering point of view I intuitively feel something like this is likely, though I would describe it differently. Clearly any immune system requires energy expenditure on the part of an individual, so there ought to be pressure to minimise this when it is not needed. But how can an organism determine the necessary level of immune response? In a real environment the default is surely that something must be having a go at infecting, so immune system response must be wound up until at least something is found and the organism knows it is working. Otherwise it is a bit like trying to tune a radio with the volume turned down.

Just Bob · 12 March 2012

Might there be some ambiguous cause-effect relationship? Perhaps a family line that is genetically predisposed to allergies has, over several generations, tended to move away from farming, so that the population left on the farm tends to be less allergic, and those in urban areas more so. And of course that effect wouldn't obviate the possibility that early and frequent exposure to allergens and pathogens does also contribute to resistance.

Dang, science can be so complicated.

Maybe I'll become a creationist.

............ Nah.

Richard B. Hoppe · 12 March 2012

My great grandmother used to say, "You have to eat a peck of dirt before you die." That settles it for me. :)

harold · 12 March 2012

Just Bob said: Might there be some ambiguous cause-effect relationship? Perhaps a family line that is genetically predisposed to allergies has, over several generations, tended to move away from farming, so that the population left on the farm tends to be less allergic, and those in urban areas more so. And of course that effect wouldn't obviate the possibility that early and frequent exposure to allergens and pathogens does also contribute to resistance. Dang, science can be so complicated. Maybe I'll become a creationist. ............ Nah.

That's plausible. However, an argument against this is that the allergies aren't farm specific, they just occur less in farming families. A certain branch of my family happens to have a lot of males with a similar allergy pattern (generally, hay fever, with asthma at a lower frequency), but no females with these problems. The sample is small but there could be something X-linked going on. But even so, being on the farm only trivially raises the risk of hay fever (which is mainly just annoying anyway, and quite treatable). And city air doesn't protect against asthma. There's certainly no reason why a peanut allergy would cause a Swiss person to move away from their farm.

harold · 12 March 2012

Richard B. Hoppe said: My great grandmother used to say, "You have to eat a peck of dirt before you die." That settles it for me. :)

That's a decent amount of dirt http://en.wikipedia.org/wiki/Peck

jlesow · 12 March 2012

This is not exactly new. My father was a state health officer and among the first to mandate polio vaccination. He mentioned the rarity of paralytic polio in India and was rather lax about his kids' hygiene; at least he never lectured about our play habits. This would have been in the 50s.

He may have been stricter with my older brother, and he did suffer from allergies.

Matt Young · 12 March 2012

Actually, they develop hypotheses about the way the world works; theories only happen when those hypotheses get properly tested and are not falsified.

Yes, technically I guess that is so. A theory in science means a well supported exposition; usually we think of a theory as explaining a vast number of experiments or observations, as in electromagnetic theory or the theory of evolution. But Burch was writing for a lay audience, for whom theory lies somewhere along the continuum between hunch or speculation and hypothesis, so I let it go.

Robert Byers · 13 March 2012

This comment has been moved to The Bathroom Wall.

Dave Luckett · 13 March 2012

Byers, had you ever considered actually checking the empirical data derived from experimental observation about this and other matters? Rather than come up with utter screaming nonsense like "the other colds are just stronger cold virus's" (sic) - whatever that vacuosity is supposed to mean.

I mean, finding out facts by looking up the actual research is a radical suggestion, but it just might work.

apokryltaros · 13 March 2012

Dave Luckett said: Byers, had you ever considered actually checking the empirical data derived from experimental observation about this and other matters? Rather than come up with utter screaming nonsense like "the other colds are just stronger cold virus's" (sic) - whatever that vacuosity is supposed to mean. I mean, finding out facts by looking up the actual research is a radical suggestion, but it just might work.

Byers would not be considered an idiot here and at Pharyngula if he had considered checking facts, rather than pull nonsense out of his butt for Jesus.

DS · 13 March 2012

Matt Young said:
Actually, they develop hypotheses about the way the world works; theories only happen when those hypotheses get properly tested and are not falsified.
Yes, technically I guess that is so. A theory in science means a well supported exposition; usually we think of a theory as explaining a vast number of experiments or observations, as in electromagnetic theory or the theory of evolution. But Burch was writing for a lay audience, for whom theory lies somewhere along the continuum between hunch or speculation and hypothesis, so I let it go.

Yes of course. It's a minor point that would detract from the overall picture. However, if we don't want creationists to get away with sloppy use of language in order to attack evolution, the least we should do is model the proper use of scientific terms ourselves. Unfortunately, a word with seven letters is just easier to use than a word with ten letters. Still, a wise man once said that the price of freedom is eternal vigilance. Once again, I was right. :)

Tenncrain · 13 March 2012

Byers, since you have been moved from this thread, a reply awaits you here in the Bathroom Wall.