Modern life in a developed nation has plenty of perks. We drink clean water and bathe with it as often as we like. We use flush toilets. Our insides are free of parasites, and children almost never die of infectious diseases. There are downsides, too: stress, obesity, depression. But we recognize these as tradeoffs for our contemporary lifestyle.
There's one catch: A recent paper says that no part of this tradeoff is a coincidence. Eliminating certain microorganisms from our environment, the authors suggest, has thrown our immune systems, finely calibrated by millions of years of evolution, off kilter. The results include ailments ranging from multiple sclerosis to mental illness.
This idea may remind you of the "hygiene hypothesis," the theory that overly hygienic modern living makes us more susceptible to allergies and asthma. The hygiene hypothesis originally focused on childhood infections: kids who were allowed to get sick now and then, it said, would have properly developed immune systems and wouldn't react to harmless allergens. But the theory has now expanded to include many kinds of microorganisms that live around and inside us, not just those that can make us sick. These companions are euphemistically called the "old friends."
The old friends historically included three groups of microorganisms, the authors say: bacteria from mud and unclean water or food that pass through our bodies harmlessly; the mostly-bacterial microorganisms that live in our gut; and--sorry--worms. More on them in a moment. We still have a host of microbes living inside us, though doubtless a different complement than what was there in earlier centuries, since our environment and diet affect which species colonize us. As for the other two groups of old friends? We don't eat much dirt these days, and we avoid worms if at all possible (though people in developing nations don't have that luxury).
Numerous studies, according to the authors, have shown that the old friends have a positive impact on our immune systems. You might expect your body to object to so many inhabitants. But in fact, those inhabitants trigger the release of anti-inflammatory agents in our bodies. Perhaps the microorganisms learned this trick over the course of evolution, to protect themselves; or perhaps we learned that it wasn't worth fighting them. Either way, our gut microbes and other microorganisms seem to dampen our immune systems and discourage inflammation.
Inflammation is the body trying to protect itself. But it's not always necessary or productive. Many illnesses stem from chronic inflammation, and it's even been linked to obesity. Asthma, Crohn's disease, multiple sclerosis, and type I diabetes all stem from an immune system that overreacts--whether it's to allergens, foods, or the body's own cells.
Depression, though the steps of its development are less easy to trace, has also been linked to inflammation. The authors think depression might be another overreaction by the body's defenses; instead of an allergen, the trigger is some psychological stressor. There are genes that can make a person more vulnerable to depression, or to any of these other conditions (including obesity), but genes aren't fate--a person's environment always plays a role.
Given all these factors, the authors suggest that since parting ways with our old friends in the middle of the 20th century, we have paid a price. We had grown dependent on these microorganisms to "train" our immune systems, and in their absence, our bodies are liable to overreact. Depending on our genetic vulnerabilities, this can lead to various illnesses. And sure enough, certain conditions related to a jumpy immune system have "increased dramatically in the developed world" since 1950 or so, including asthma, hay fever, type I diabetes, and multiple sclerosis. Depression seems to have increased, too, though it's hard to separate a real increase from a cultural change that encourages it to be diagnosed.
The next step in testing this hypothesis would be to treat depressed patients with the old friends themselves, or with some medication derived from them. There hasn't been much research in this area yet. One intriguing study showed that lung cancer patients receiving chemotherapy were significantly less depressed and anxious after being treated with Mycobacterium vaccae, one of the dirt bacteria.
There has been more interest, surprisingly, in the worms. "Helminthic therapy," in which doctors infest patients with parasitic worms, is being used to treat autoimmune conditions, IBS, and even food allergies. You'd have to be a pretty good sport to voluntarily take on parasitic worms. But these patients are on the cutting edge; while modern living means cleanliness, dirt and worms might be the future.
Thanks to Emily D. for the tip!