Our genes have evolved a bit over the last 50,000 years of human evolution, but our diets have evolved a lot. That's because civilization has transitioned from a hunter-gatherer lifestyle to an agrarian and, more recently and incompletely, to an industrialized one. These days, many of us are living in an information-intensive, symbol-analyzing, button-pushing, fast-food-munching society. This transformation has been accompanied by consequential twists and turns regarding what we eat, and how and when we eat it.
Toss in antibiotics, sedentary lifestyles, and massive improvements in public sanitation and personal hygiene, and now you're talking about serious shake-ups in how many and which microbes we get exposed to - and how many of which ones wind up inhabiting our gut.
In a review published in Cell Metabolism, Stanford married-microbiologist couple Justin Sonnenburg, PhD, and Erica Sonnenburg, PhD, warn that modern civilization and its dietary contents may be putting our microbial gut communities, and our health, at risk.
[S]tudies in recent years have implicated [dysfunctional gut-bug communities] in a growing list of Western diseases, such as metabolic syndrome, inflammatory bowel disease, and cancer. ... The major dietary shifts occurring between the hunter-gatherer lifestyle, early Neolithic farming, and more recently during the Industrial Revolution are reflected in changes in microbial membership within dental tartar of European skeletons throughout these periods. ... Traditional societies typically have much lower rates of Western diseases.
Every healthy human harbors an interactive internal ecosystem consisting of something like 1,000 species of intestinal microbes. As individuals, these resident Lilliputians may be tiny, but what they lack in size they make up in number. Down in the lower part of your large intestine dwell tens of trillions of single-celled creatures - a good 10 of them for every one of yours. If you could put them all on a scale, they would cumulatively weigh about four pounds. (Your brain weighs three.)
Together they do great things. In a Stanford Medicine article I wrote a few years back, "Caution: Do Not Debug," I wrote:
The communities of micro-organisms lining or swimming around in our body cavities ... work hard for their living. They synthesize biomolecules that manipulate us in ways that are helpful to both them and us. They produce vitamins, repel pathogens, trigger key aspects of our physiological development, educate our immune system, help us digest our food and for the most part get along so well with us and with one other that we forget they’re there.
But when our internal microbes don't get enough of the right complex carbohydrates (ones we can't digest and so pass along to our neighbors downstairs), they may be forced to subsist on the fleece of long carbohydrate chains (some call it "mucus") lining and guarding the intestinal wall. Weakening that barrier could encourage inflammation.
The Sonnenburgs note that certain types of fatty substances are overwhelmingly the product of carbohydrate fermentation by gut microbes. These substances have been shown to exert numerous anti-inflammatory effects in the body, possibly protecting against asthma and eczema: two allergic conditions whose incidence has soared in developed countries and seems oddly correlated with the degree to which the environment a child grows up in is spotlessly hygienic.