Environment and Health: Could Our Own Bugs Provide the Link?

Healthy bodies may require buggy environs.

Who are we anyhow? A body; a brain, a heart, other organs. There’s skin and bones. And there are thoughts; consciousness. Some might even say a soul. But it turns out that that’s not all. Not by a long shot. We are also our microbiome – the trillions of microbes (affectionately referred to as bugs) that live in our gut, on our skin, and who knows where else. If you were to count cells in your body—you’d find that 90 percent of them belong to microbes rather than you; or at least what you think of as you.  With respect to genes, the number of microbe genes in our body outnumber our “own” genes by a factor of 100.

And a good fraction of these bugs are not just squatters – taking up residence in our bodies and not paying rent.  They’re symbionts, carrying out critical functions for us like digesting our food, fending off infections, helping to build organs like your heart as well as possibly influencing our diet and behavior. One of the most important functions of the microbiome as laid out in the introduction to the journal Science’s special issue on the topic is that “From birth, the microbiota intimately shapes the development and function of the human immune system.” And “is a major contributor to host metabolism through nutrient release” which in turn affects our fitness or overall health.

In one recent study a team led by Miriam Susan LaTuga from the Albert Einstein College of Medicine found a strong correlation between extreme low-birth weight infants and their gut microbiota.  Other studies suggest a possible linkage between obesity and the microbiome. (See for example here and here.)

The Environmental Connection – Autoimmune Diseases

Interesting, but what does this have to do with the environment? Possibly plenty. For one, the chemicals we eat, inhale and touch can affect our own microbiome; for example, by killing some communities and causing others to thrive. And the environment in which we live has its own set of microbiota that influence and interact with our microbiome. The changes wrought in both cases can lead to a cascade in our bodies that ultimately leads to acute or chronic disease.

Let’s take autoimmune diseases. These are the diseases where one’s immune system overreacts and attacks its own body and organs. These diseases include conditions that are problematic but not fatal like eczema, to more serious illnesses like asthma, to diseases that can lead to total debilitation and death such as multiple sclerosis.  The worrisome aspect of these diseases is that their incidence has increased over the past 50 years or so in developed countries like those in Europe (see Figure 1 in [pdf]) and the US.*  Could that rise have some relationship to contemporaneous environmental changes? Scientists are beginning to entertain that as a serious possibility. Check out a few recent studies.

Not Enough Bugs In the City?

Writing last month in Clinical Pediatrics, Ruchi Gupta of Northwestern University’s Feinberg School of Medicine and colleagues observed that geography seems to influence food allergies in young children. They found that children living in cities were more apt to have food allergies than those living in more rural locations. What might explain this pattern? One obvious explanation is that exposure to urban pollutants is the culprit. The authors note another possibility too; that “exposure in early life to certain microbial agents associated with rural living may protect against atopy.” In other words the microbiota found in rural areas suppress allergic response.

Plant Diversity Equals Bug Diversity

This latter hypothesis also finds support in a paper published in the Proceedings of the National Academy of Sciences in early May by Ilkka Hanski of the University of Helsinki and colleagues.  In this work the authors explore what they refer to as the “biodiversity hypothesis;” namely that reduced contact with the diversity of biota normally found in the natural world limits or alters the biodiversity of our own microbiota in a way that promotes disease.  In this study of 118 Finnish teenagers, Hanski at al found that the personal microbiotic diversity on the teenagers’ skin correlated positively with the plant diversity in their environment. And further, the children who tested positive for an allergic sensitivity tended to have a less diverse microbiome. (And in particular, less diversity of one type of bacteria important in suppressing inflammation.) And so, the authors speculate, the more types of plants we come into contact with, the greater our own personal bug diversity and the less susceptible we are to allergies.

And Then There Are Antibiotics

Antibiotics are also changing our microbiome in ways we are just beginning to understand.   Antibiotics can be an effective treatment for illness and are often our only choice to combat deadly bacteria. Their use has also contributed to a precipitous drop in infectious diseases (see Figure 1 in [pdf]).  But some argue that we have overused them — to the point that we are increasingly running out of options for certain antibiotic-resistant illnesses.  And that’s not all; antibiotics play havoc with our microbiome. Every time we take antibiotics for an illness, inadvertently consume them in our food or lather them over our skin with an antibacterial soap, we target not just the bacteria that make us sick, but the ones that keep us healthy, opening up shifts in population that may not be beneficial. For example, there are indications that therapies for ulcers and gastric cancer that target the bacteria Helicobactor pylori can lead to trade offs, as these same bacteria also play a role in limiting obesity which can lead to diabetes.

And there is that intriguing correlation between the drop in infectious diseases and the rise in autoimmune diseases. Perhaps there is a link through our microbiome.  While research points to the alteration of our microbiome as a critical link between the rise in some autoimmune diseases and declining biodiversity in the places we live, our microbiome is being altered by more than environment.

What About Chemicals?

Antibiotic use (as discussed above) is almost certainly a complicating factor, but what about toxic chemicals? Could toxic chemicals be altering our microbiome? Maybe so. For example a review by Suzanne Snedeker and Anthony Hay of Cornell University in Environmental Health Perspectives reports that the makeup of our personal microbiota can make chemicals more or less bioavailable–potentially ratcheting up or ratcheting down a chemical’s toxicity. This also suggests that alterations to our microbiome from chemical exposures have the potential to either increase or decrease the damage we experience.  On the upside researchers are looking into ways that prebiotics and probiotics can have a positive impact on health. (See here and here.) Ongoing research with mice hints that ‘fecal pills’ may one day be a cure for diabetes or liver disease.

In case you were wondering, yep, fecal pills are exactly what they sound like – pills containing fecal matter. For some reason I find the ideal of living in a biodiverse world a little more palatable than downing a few fecal pills with my morning coffee. What about you?


End Note

The overall trends of autoimmune diseases in believed to be increasing, but the incidence of some individual autoimmune diseases is not.

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