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All animals and plants appear to establish symbiotic relationships with microorganisms, and, within us, the trillions of good bacteria in our gut can be thought of as a forgotten, additional organ—metabolizing, detoxifying, and activating many crucial components of our diet.

Health-promoting effects of our good bacteria include boosting our immune system, improving digestion and absorption, making vitamins, inhibiting the growth of potential pathogens, and keeping us from feeling bloated. Should bad bacteria take roost, however, they can produce carcinogens, putrefy protein in our gut, produce toxins, mess up our bowel function, and cause infections.

The symbionts—the good bacteria that live in symbiosis with us—are largely nourished by fruits, vegetables, grains, and beans. Pathobionts, the disease-causing bacteria that may disrupt our microbial balance, instead appear to be fed by meat, dairy, eggs, junk food, and fast food.

Indeed, what we eat determines what kind of bacterial growth we foster in our gut, which can increase or decrease our risk of some of our leading killer diseases.

What happens to our gut flora microbiome when we’re on plant-based versus animal-based diets? Researchers have found that a strict vegetarian diet resulted in reduced intestinal inflammation, suggesting a promotion of homeostatis in our microbiome. In contrast, an animal-based diet has helped contribute to growth of disease-associated species, like Bilophila wadsworthia, which is associated with inflammatory bowel disease, and A. putredinis, found in abscesses and appendicitis, as well as decreases in fiber-eating bacteria.

The human gut has a diverse collection of microorganisms making up around 1,000 species, with each individual presenting with her or his own unique collection. Yet, simply put, the bacteria are what we eat. Eat fiber, and the fiber-munching bacteria multiply, and we get more anti-inflammatory, anti-cancer short-chain fatty acids. Eat less fiber, and our fiber-eating bacteria starve away.

What’s more, there appear to be only two types of people in the world: those who have mostly Bacteroides type bacteria in their gut, and those whose colons are overwhelmingly home to Prevotella species instead. It’s amazing that with so many hundreds of types of bacteria, people settle into just one of two categories. Our guts are like ecosystems. Just like there are lots of different species of animals on the planet, they aren’t randomly distributed. You don’t find dolphins in the desert. In the desert, you find desert species. In the jungle, you find jungle species. Why? Because each ecosystem has different selective pressures, like rainfall or temperature. We now know, when it comes to gut flora, it doesn’t seem to matter where we live, whether we’re male or female, or how old or skinny we may be. What matters is what we eat: Components found more in animal foods like protein and fat are associated with the Bacteroides enterotpye, and those found almost exclusively in plant foods are associated with Prevotella.

If whatever gut flora enterotype we are could play an important role in our risk of developing chronic diet-associated diseases, such as obesity, metabolic syndrome, and certain cancers, can we alter our gut microbiome by altering our diet? Yes. Diet can rapidly and reproducibly alter the bacteria in our gut.

Image Credit: TLFurrer / Thinkstock. This image has been modified.

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