From FIBER FUELED by Will Bulsiewicz, published by AVERY, an imprint of Penguin Publishing Group, a division of Penguin Random House, LLC. Copyright © 2020 by Will Bulsiewicz.

When I graduated medical school in 2006, we knew next to nothing about the gut microbiota. But since then, new research tools became available and we have discovered that our gut contains a shockingly broad, expansive community of microorganisms that live inside us in harmony, in balance, and with a purpose.

Believe it or not, there are up to five different types of microorganisms residing within you right now. They include bacteria, yeasts, parasites, viruses, and archaea. It’s hard to fathom, because they are invisible, but they are as alive as you and I are. They have personalities (some are grumpy), unique skill sets, and even their own dietary preferences. And there are 39 trillion of them! (You only have 30 trillion human cells, so you are actually more microbial than human.)

There is a direct connection between your gut microbiota and your immune system. They exist in close proximity to one another.

The human gut is interconnected in a way that draws comparisons to the planet Earth. Your gut microbiome is just as much an ecosystem as the Amazon rainforest or the Great Barrier Reef. All life forms – plants, animals, microbes, even the snakes, sharks and parasites – contribute to the balance and harmony in the ecosystem. Biodiversity makes the ecosystem strong, resilient and adaptable no matter what you throw at it. When things are working the way they’re supposed to in our gut, we have a diverse, abundant community of microbes living in harmony in our colon.

They live off what we feed them. Every single bite, all three pounds of food per day, flows downstream to your gut microbes. Our food is also their food. But they are picky eaters! Not every microbe eats the same thing. Each dietary choice you make will empower a specific group of microbes, while others will languish. If you permanently remove a food group, the microbes that thrive on that food will starve into extinction. In other words, you are selecting the winners and losers in your gut. Their life cycle is so quick that the food choices you make in twenty-four hours will alter the evolution of fifty generations of microbes. It doesn’t take days or even weeks to change your microbes; it takes only one bite. But here’s the good news: You control what you bite into, and therefore you control the makeup of your microbiome.

When we opt for plant foods, the healthy bacteria (let’s call them probiotic) will reward us by molding the fiber and polyphenols into something that reduces inflammation and promotes health and balance. But the opposite can also be true. Unhealthy food feeds unhealthy microbes, and they punish us by creating compounds that inflame our body, such as TMAO. Whatever you choose, the end result of your dietary choices is a unique mix of microbes, as individual as a fingerprint, and that has tremendous influence over your health or risk for disease.

Anything that you put into your mouth will be processed by these microbes, including drugs. This helps to explain why the same drugs can have a lifesaving effect in one person and a life-threatening effect in another. A healthy, diverse gut microbiome does so much more than just process our drugs and our food. It operates as the command center for the five axes of human health: our metabolism, immune function, hormonal balance, mood and cognition and even our genetic expression. That’s pretty much everything that matters.

There is a direct connection between your gut microbiota and your immune system. They exist in close proximity to one another. In fact, just a single layer of cells, invisible to the naked eye, separates your gut microbes from seventy percent of your immune system. Yes, the majority of your immune cells are in your gut. Although technically separate, your gut microbes and immune system are completely intertwined. Evidence has shown us that the microbiota help foster proper development of immune cells, identify invaders, get immune cells to the needed location, and then enhance their infection-fighting power. If you disturb the gut microbes, you should expect it to also disturb your immune system. But a healthy gut microbiota translates into an empowered, strong immune system that optimally functions to identify a threat when it exists – infectious or even malignant – and eradicate it.

Evidence of the gut-immune relationship comes from the autoimmune and allergic epidemic. These conditions are indeed exploding around the world. From 1960 to 2000 the incidence of asthma increased at least tenfold in the Western world. A similar, disturbing rise in autoimmune diseases has occurred in parallel. Since the 1950’s the rates of type 1 diabetes, multiple sclerosis, and Crohn’s disease have each risen 300 percent or more. In each of these conditions, you will find that the balance and harmony of the gut microbiota has been disturbed. Some call it “leaky gut”, but I call it dysbiosis. The gut microbes are not only responsible for immune health. Dysbiosis, has been associated with a number of emerging cancers, metabolic conditions, weight gain, neuropsychiatric disorders, endocrine and hormonal disruptions and more.

The power that our gut microbes wield to induce disease or protect us from it can be intimidating, but we are not helpless victims. We have the power of science on our side, which has shown us that we can use diet and lifestyle to rebalance our gut microbes. So where do we begin if we want to heal our guts to reverse or prevent illness? Is it probiotics? Bone broth? Well, if you were sitting across from me in my office, I’d be going straight for my secret weapon – fiber.

Unfortunately, we’ve been brainwashed into thinking fiber is that boring orange drink our Grandma had so she could poop. It’s so much more! Fiber is the preferred food of our gut microbes. When we feed them fiber, they thrive and become more capable of doing their job. Remember, our gut microbes have an important job when it comes to digestion, immunity, our weight balance, hormones, mood and brain function. We want these microbes functioning in tip top shape so they can go to work on behalf of our health. To do this, we have to fuel them with the ultra-premium stuff – FIBER! So where do we find this magic salve? Plants! Plants have a total monopoly on this nutrient. So if you want it, there’s only one way to naturally get it: from plants!