[enkephalin]

> I'm not sure what the research is?

at least for infants/children, the research is in favor of introducing bacteria to the immune system.

Exposure to microbes during early childhood is associated with protection from immune-mediated diseases such as inflammatory bowel disease (IBD) and asthma.

...

These results indicate that age-sensitive contact with commensal microbes is critical for establishing mucosal iNKT cell tolerance to later environmental exposures.

http://www.sciencemag.org/content/336/6080/489.abstract#aff-...

also some interesting info on children delivered by c-sections (which prevents newborns from getting into contact with the diverse vaginal flora):

CS was associated with a lower total microbial diversity, delayed colonisation of the Bacteroidetes phylum and reduced Th1 responses during the first 2 years of life.

http://gut.bmj.com/content/early/2013/07/09/gutjnl-2012-3032...

there's much more info to be found on the topic. these are just the ones i recently stumbled upon.

[glaugh]

The book "Epidemic of Absence" is the best summary of the research on this, to my knowledge.

[rcthompson]

So, clearly there are disadvantages to underexposure to bacteria (hygiene hypothesis etc.), but are they so bad that they outweigh the risks of allowing bacterial exposure? In other words, is there any evidence that the optimal level of exposure to bacteria (i.e. the level that minimizes the sum of the risk of pathogenic infection and the risk of cleanliness-related problems like allergies and autoimmunity) is nonzero?

[jblow]

Left to its own devices, a baby will stick pretty much everything into its mouth.

If you believe that evolution generates behavior to maximize survival value, this fact about babies seems relevant.

[rcthompson]

Left to its own devices in a modern America, a human will typically become obese from overeating and eating unhealthy foods (guess how I know!). That doesn't make this behavior adaptive. Humans didn't evolve under an environment of plenty, so their behavior in such an environment does not necessarily maximize survival value. Ditto for an environment where parents don't pay attention to what their infants put in their mouths.

In summary, it's a long, long shot to claim that babies being willing to eat anything constitutes evidence that bacterial exposure is beneficial.

[enkephalin]

consider that babies rely solely on instincts, whereas our behavior as we get older, depends greatly on factors such as upbringing, our experiences and expectations, media exposure etc.

[rcthompson]

I don't see how that's relevant or detracts from my point.

[Shivetya]

Always remembered stories about kids who grew up with animals did better long term, both emotionally and physically.

[ljf]

As someone that grew up on a farm (and a data point of 1), I am pretty damn healthy and very rarely ill.

My wife on the other hand grew up in a spotlessly clean house, and has a range of allergies, asthma, coeliac disease, allergic to asprin and anaesthetics.