Katherine Harmon Courage wants us to think about digestion as a collaborative journey between us and our microbes. In her new book, Cultured: How Ancient Foods Can Feed Our Microbiome, she envisions digestion not as a simple food-in, excrement-out process, but as a series of encounters with varying microbial players that takes place along the winding 30-foot tunnel of our gastrointestinal tract. Along the way, microbes digest the food we can’t, and in return we give them a warm, well-stocked place to live. But a surge in microbiome research over the past two decades has revealed they do much more than simply digest food. They can mediate weight gain, fight off infection, and even alter our mood. Scientists still have much to learn about the identity of these microbes, which are important, and how the beneficial ones work their magic.
What are Microbes and How They Thrive During the Digestion Process?
Courage believes this focus on the microbes themselves is myopic. She views the process of digestion as collaborative because the food we put into our bodies affects the kinds of bacteria that live and thrive there. In her book, she explores the science behind how what we feed our microbes affects our health.
She thinks we can learn how to better work together with our microbial partners by looking to the past. From Greenland to Greece, Courage explores the ancient gut-friendly foods that have become integral parts of many food cultures, and offers suggestions on how to diversify the kinds of foods we feed our microbiome.
We spoke with Courage about the science behind pro- and prebiotics, and what she learned exploring fermented staples across the world. The interview has been edited for brevity and clarity.
A lot of the buzz around the microbiome has been about the microbes themselves, and what they do for us. You focus much of your book on what they eat, the ” prebiotics” we feed them. Why?
It may be less interesting to talk about fiber than about all these new species we’re learning about and infusing into foods, but what we feed our microbes is just as important as what microbes are there.
I think that, from our human perspective, it’s helpful to think about microbes in two broad categories. There are microbes that we have in our guts throughout our lives that are adapted for living there, and then there are the microbes we get from food or supplements. Those latter ones just kind of pass through. They can survive the journey, and can certainly provide benefits along the way, but they aren’t long-term residents of the gut, and they’re not going to have the long-term health impacts that more-permanent residents might have.
What Happens to Microbes if There Not Fed?
When microbes are not fed, they start to eat us. Our lower intestine, which helps us absorb as much as we can from our digested food before we expel it, also makes it easy for things to escape.
When our microbes don’t get enough fiber, they can start eating away the mucus lining protecting this thin layer, which can lead to leaky gut syndrome, which is associated with many poor health outcomes.
Importance of having a diverse diet of fiber
Prebiotic fiber is just any kind of carbohydrate that we can’t digest ourselves that instead passes through out digestive system as food for microbes. There are many different types of fiber that get broken down by different microbes at different stages of digestion. That’s why it’s a good idea to eat a wide variety of foods, and not just focus on a particular supplement here and there. Lots of different kinds of fibers help lots of different microbes thrive and create different beneficial compounds for us. Which is good because we’re learning that generally, a more diverse microbiome is an indicator of health. If you look at people’s guts around the world — and even in the same society — people with more diverse microbiomes tend to be healthier overall.
One kind of fiber that’s gotten a lot of focus is insulin. We’ve actually been adding it to foods for longer than we’ve been looking closely at it, but it’s commonly found in foods like chicory root or sunchokes. It’s a very long carbohydrate chain, which means it takes a bit longer to pass through our system and get broken down by microbes. Research shows that it encourages growth of bifidobacteria, lactobacteria.
Another big one comes from fruits and veggies, called Fructo-oligosaccharides. It’s shorter than insulin and adding it to your diet has been shown to reduce markers of inflammation.
Galacto-oligosaccharides are another form of fiber found in milk, and are broken down in the colon.
I was really surprised to learn about resistant starch as another form of fiber. It comes from more simple carbohydrates that have been cooked and then cooled; think of cold potato or pasta salad. So once those starches are crystallized, they become the type of resistant starch that our bodies can’t break down anymore. Even cold pasta, which you don’t necessarily think of as being healthy, can be a great source of resistant starch.
Do Other Aspects of Our Diet Besides Fiber Affect the Microbiome?
Almost everything we eat has some kind of impact on our microbes. One example I talk about in the book is meat. Really kind of fatty meats like pork can have a negative health impact on us via our microbes, because they produce a metabolite called TMAO, which has been linked to negative health outcomes. But fish oil has been shown to be beneficial — the microbes of mice fed fish oil instead of pork lard produced much fewer TMAOs.
Another exciting area of research is looking at how gene expression in the same microbial strains can change, based on what they’re being fed. Different metabolites get produced not by different microbes, but by the same microbes being fed differently.
