For many years, it was believed that the main function of the large intestine was just to absorb water and dispose of waste, but “[n]owadays it is clear that the complex microbial ecosystem in our intestines should be considered as a separate organ within the body,” and that organ runs on a MAC, microbiota-accessible carbohydrates. In other words, primarily fiber.

One reason we can get an increase of nearly two grams of stool for every one gram of fiber is that the fiber fermentation process in our colon promotes bacterial growth. The bulk of our stool by weight is pure bacteria, trillions and trillions of bacteria, and that was on a wimpy, fiber-deficient British diet. People who take fiber supplements know that a few spoonfuls of fiber can lead to a massive bowel movement, because fiber is what our good gut bacteria thrive on. When we eat a whole plant food like fruit, we’re telling our gut flora to be fruitful and multiply.

From fiber, our gut flora produce short-chain fatty acids, which are an important energy-source for the cells lining our colon. So, we feed our flora with fiber and then they turn around and feed us right back. These short-chain fatty acids also function to suppress inflammation and cancer, which is why we think eating fiber may be so good for us. When we don’t eat enough whole plant foods, though, we are in effect starving our microbial selves, as I discuss in my video Gut Dysbiosis: Starving Our Microbial Self. On traditional plant-based diets, we get lots of fiber and lots of short-chain fatty acids, and enjoy lots of protection from Western diseases like colon cancer. In contrast, on a standard American diet filled with highly processed food, there’s nothing left over for our gut flora. It’s all absorbed in our small intestine before it even makes it down to the colon. Not only may this mean loss of beneficial microbial metabolites, but also a loss in the beneficial microbes themselves.

Research shows the biggest issue presented by a Western diet is that not leaving anything for our bacteria to eat results in dysbiosis, an imbalance wherein bad bacteria can take over and increase our susceptibility to inflammatory diseases or colon cancer, or maybe even lead to metabolic syndrome, type 2 diabetes, or cardiovascular disease.

It’s like when astronauts return from space flights having lost most of their good bacteria because they’ve had no access to real food. Too many of us are leading an “astronaut-type lifestyle,” not eating fresh fruits and vegetables. For example, the astronauts lost nearly 100 percent of their lactobacillus plantarum, which is one of the good guys, but studies reveal most Americans don’t have any to begin with, though those who eat more plant-based are doing better.

So it’s use it or lose it. If people are fed resistant starch, a type of MAC found in beans, within days the bacteria that eat resistant starch shoot up and then die back off when you stop. Eating just a half can of chickpeas every day may “modulate the intestinal microbial composition to promote intestinal health” by increasing potentially good bacteria and decreasing pathogenic and putrefactive bacteria. Unfortunately, most Americans don’t eat beans every day or enough whole grains, enough fruits, or enough vegetables. So, the gut flora—the gut microbiota—of a seemingly healthy person may not be equivalent to a healthy gut flora. It’s possible that the Western microbiota is actually dysbiotic in the first place just because we’re eating such fiber-deficient diets compared to populations that may eat five times more fiber and end up with about 50 times less colon cancer.

This is one of the reasons I recommend three daily servings of legumes (beans, split peas, chickpeas, and lentils) in my Daily Dozen checklist.

The microbiome connection may explain the extraordinary results in the study I featured in my video Is It Worth Switching from White Rice to Brown?.

More on the musical fruit:

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For more on bowel health, check out:

In health,

Michael Greger, M.D.