Let Them Eat Dirt. B. Brett Finlay

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СКАЧАТЬ and that humans traded in that healthier lifestyle for food security and more babies (not a bad deal, actually!). Certain nutritionists have extrapolated from this a recommendation that, in order to promote health, all modern humans should eat the way hunters and gatherers did, but this has been debunked by top evolutionary biologists based on the fact that humans have adapted genetically to the challenges that farming generated (see the Caveman Diet, page 30).

      What these two major events in human history teach us is that changes in lifestyle are accompanied by changes in our microbiota, and that these microbial changes might affect our health for better (e.g., cooking food and decreasing infections) or worse (e.g., agriculture and less microbial diversity). Whether we like it or not, we are married to microbes for life, in sickness and in health, for richer or for poorer.

Bugs “R” Us

      Our microbes are part of what make us human, but our current way of living and eating, especially in the Western world, has exerted further changes in our microbiota and in our biology. In the past hundred years, and especially the last thirty years, humans have learned to process foods to make them tastier, more digestible, and more shelf-stable than ever before. On top of this, our push to clean up our world in order to fight infectious diseases, including the use of antibiotics, has further shifted the composition and diversity of our microbial communities. Double-punching our microbiota like this has induced huge changes in our intestinal environments and, as we will learn in the following chapters, on many other aspects of our bodies’ normal functions.

      In order to appreciate how the microbiota influences our health, it is important that we discuss certain basic biological concepts about our microbiota and the organ most of them call home, the human intestine. The human microbiota consists of bacteria, viruses, fungi, protozoa, and other forms of microscopic life. They inhabit our skin, oral and nasal cavities, eyes, lungs, urinary tract, and gastrointestinal tract—pretty much any surface that has exposure to the outside world. Another term that is frequently used is microbiome, which refers not only to the identity of all the microbes living within us, but also to what they do. A total of 10¹⁴ microbes are estimated to live in the human body and, as mentioned, the intestinal tract is the biggest reservoir of microbes, harboring approximately 10¹³ bacteria. It is this community that influences us, their host, the most. In fact, unless otherwise noted throughout this book, when we use the term microbiota, we are referring to the intestinal microbiota. Although bacteria are approximately twenty-five times smaller than human cells, they account for a significant amount of our weight. If we were to get rid of our microbiota we would lose around three pounds, or about the weight of our liver or brain! A single bowel movement is 60 percent bacteria numbering more than all the people on this globe, a deeply disturbing fact for germophobes.

      For microbes, the gastrointestinal system is a fabulous place to live. It’s moist, full of nutrients, and sticky (allowing microbes to adhere to it), and in many sections it completely lacks oxygen. Although it seems counterintuitive that any life-form would favor a place without oxygen, an enormous number of bacterial species either prefer or require such a place, as this world evolved for billions of years without oxygen. Microbes living without air are called anaerobes and our gut is packed with them.

      About 500–1,500 species of bacteria live in the human gut; the types and numbers vary according to the different sections of the gastrointestinal system. Starting from the top down, the mouth harbors a diverse and complex microbiota—the tongue, cheeks, palate, and teeth are all covered in a dense layer of bacteria known as a biofilm. For example, the dental plaque that dentists remove from our mouths is one of these biofilms. The stomach, on the other hand, is not the best place for microbes, as it is as acidic as battery acid. Still, a few bacterial species have adapted to live under such conditions. Farther down are the small and large intestines, where the number of microbes continues to increase until we reach the very end of the large intestine. Oxygen follows the opposite pattern, as it gradually decreases towards the lower portions of the gut, allowing strict anaerobes (those that die when exposed to the slightest bit of oxygen) to flourish in the large intestine. The differences in living conditions within the small and large intestines determine the number and the types of bacteria that reside in each portion of the gut. For example, the slightly acidic and oxygenated environment in the upper small intestine allows for bacteria that are tolerant to these conditions, such as the bacteria we often eat in our yogurt, known as Lactobacilli. Unlike the upper small intestine, the large intestine, also known as the colon, moves or churns its contents very slowly and produces a lot of mucus, allowing for many more bacteria to grow, especially those that use mucus for food.

      Another characteristic of the human microbiota is its variability between individuals. Although about one-third of bacterial species are shared between all humans, the rest of them are more specific, making our microbiome unique like a fingerprint. Similarities in microbiota are highly dependent on diet and lifestyle, and to a lesser extent, on our genes. For example, identical twins (who share all of their genes) can have very different microbiotas if one is a vegetarian and the other eats meat. Family members, including husbands and wives who are not genetically related, tend to have similar microbiotas due to a shared living environment and diet. Humans also have striking similarities with the microbiotas of several species of apes, but only those that are omnivores like us. Mountain gorillas, for example, have a microbiota much more closely related to pandas, because they both spend their days leisurely eating bamboo.

      Once established in our intestine, microbial communities are very stable. Only drastic changes, such as adopting a vegan lifestyle or moving to a completely different part of the world, will significantly alter your microbiota. Going on antibiotics for a week to treat an infection will also affect your microbiota, but only temporarily in most cases. It will generally bounce back to something resembling its pre-antibiotic state after you finish the treatment and go about your old way of eating. However—and this is a big however—the microbiota takes about 3–5 years from the time we’re born to become a fully established community, and during this period it’s very unstable, especially during the first few months of life. Any drastic changes to it have a very high chance of altering the microbiota permanently. In fact, it is the early colonizers of the intestinal microbiota that have a major influence on the type of microbiome we have later in life. Thus, a short-lived event like a C-section may have long-lasting consequences, since a baby born this way starts with a very different microbiota than a baby born vaginally. The potential health outcomes and impact of this type of event during early life has major implications for later health and disease, as discussed in later chapters.

Immune Cell School

      Given the strong associations between early-life alterations to the microbiota and immune diseases later in life, we might ask: What exactly are microbes doing to us when we’re babies that is so important? As mentioned in the previous chapter, microbes help us use food that we can’t digest properly, and they also fight off bacteria capable of causing us harm. We’ve known about these roles for decades, but they are just the tip of the iceberg. As soon as we’re born and begin getting colonized with bacteria, bacteria kick-start a series of fundamental biological processes in our body. One of them is the maturation of the immune system, the network of cells and organs that defend us from diseases.

      Before scientists started unraveling the role of the microbiota in immunity, every doctor and scientist was taught that we’re born with an immature immune system that gets trained in a small organ called the thymus. Here, immune cells known as T cells—the strategists of our immune system—are taught who is a friend and who is a foe. This training boot camp lasts for a few years only, until the thymus disappears, and all our immune cells have acquired this knowledge. Immunologists deciphered a complex series of mechanisms showing exactly how this occurs, but they couldn’t explain one big question: How does the thymus teach immune cells which kinds of bacteria are beneficial and which ones aren’t? After all, since we’re covered head to toe (also inside and out) with microbes, mostly good ones, how СКАЧАТЬ