You Feed Them, They Feed You
A beginner’s guide to the brain-gut-microbiome system, and why the number of plants you eat matters more than any superfood.
Most of what you eat never makes it far. Proteins, fats and simple sugars (carbohydrates) are broken down and absorbed rapidly in the first part of your small intestine, usually within a few hours of a meal. Some of what’s on your plate gets past that point, though, and that part is the subject of this article.
Fiber and polyphenols (large, health-promoting molecules) which are contained in all plants, pass through your small intestine largely intact. They end up in your colon, which is home to trillions of bacteria and other microbes, the densest microbial community anywhere on the planet. Your body’s own cells can’t do much with these large molecules, but your gut microbes love to feed on them and break them down into compounds your body can actually absorb.
What happens in your colon
When fiber reaches your colon, gut bacteria ferment it. One of the main products is a group of compounds called short-chain fatty acids: acetate, propionate and butyrate. Butyrate has received most attention, as it is not only the main fuel source for the cells lining your colon but also the main inflammation fighting compound in your gut, which helps keep that lining in good shape and keeps it from turning into a leaky gut. The other two travel through your bloodstream and send signals to tissues elsewhere in the body, including muscle and brain.
Polyphenols get a similar treatment. These are the compounds that give berries, fruits and vegetables their color, dark chocolate its bitterness and good olive oil its peppery bite. Researchers estimate that only about 5 to 10% of the polyphenols you eat are absorbed in the small intestine (Cardona et al., 2013). The rest continue on to your colon, where microbes break them down into smaller compounds that your body can absorb and use.
This is why we think about food differently than most nutrition advice does. A lot of the value in plant foods doesn’t come from you digesting and absorbing them directly. It comes from your microbes processing them first and passing the results back to your body.
Meet the brain-gut-microbiome system
Dr. Emeran Mayer has studied how the brain and the gut communicate for more than 45 years. He’s a gastroenterologist and neuroscientist at UCLA, where he founded the Goodman-Luskin Microbiome Center. For most of his career, the research looked at two parties: the brain and the gut. Over the last decade, a third party became impossible to ignore.
Dr. Mayer calls the whole network the brain-gut-microbiome system. Your brain sends signals to your gut through nerves and hormones. Your gut sends signals back the same ways in addition to immune signals. The microbes in between add their own signals or metabolites, and many of those come from what they make out of your food.
The practical point is simple. What you eat doesn’t only feed you. It shapes which microbes thrive in your gut, and those microbes shape a lot of what happens next.
Why variety beats any single superfood
If microbes depend on what you send them, the obvious question is what to send. The wellness industry’s answer tends to be one ingredient at a time: kale one year, turmeric the next, then acai, then whatever’s trending now.
The microbiome doesn’t work that way. Gut bacteria specialize. Some are good at breaking down resistant starch, the kind found in cooked-and-cooled potatoes, green bananas and legumes. One species, Ruminococcus bromii, does this unusually well (Ze et al., 2012). Other bacteria prefer the fructans in onions, garlic and leeks. Others break down the pectin in apples or the beta-glucan in oats. Polyphenols work the same way, with different microbes handling different compounds.
Eat a narrow diet, even a healthy one, and you feed a narrow slice of that community. Eat widely and you give more species something to work with.
The best evidence for this comes from the American Gut Project, one of the largest studies of the human microbiome. Researchers analyzed samples from more than 10,000 people. Participants who ate more than 30 different types of plants per week had more diverse gut microbiomes than those who ate 10 or fewer (McDonald et al., 2018). That held up regardless of whether people called themselves vegan, vegetarian or omnivore. The label mattered less than the variety on the plate.
We should be honest about what that study shows. It’s an association, not a controlled trial, and nobody has proven that 30 is a magic number. But it’s consistent with what we know about how gut microbes specialize. It also gives you something concrete to aim for, which is more useful than being told to “eat more plants.”
Where most people start
Most of us aren’t close. CDC data shows only about 1 in 10 American adults eat the recommended amount of fruits and vegetables, the primary source of fiber and polyphenols. Fiber intake looks similar: the large majority of Americans fall well short of recommended levels. That means most people’s microbes are working with a lot less than they could use.
The good news is that the counting approach makes improvement easy. In addition to focusing on quantity of macronutrients, you need to add range.
The one-week plant challenge
For the next seven days, keep a running list of every distinct plant you eat. A note on your phone works fine. The rules:
- Each plant counts once, no matter how often you eat it.
- Vegetables, fruits, whole grains, beans, lentils, nuts, seeds, herbs and spices all count. So do coffee, tea and cocoa.
- Different varieties count separately. Red and green lentils are two; black beans and chickpeas are two.
- Don’t change anything for the first couple of days. Just see where you are.
Most people are surprised by how low their first number is, and then by how fast it climbs once they start paying attention and modify their diet. A few easy adds:
- Replace plain rice with a mix of rice, quinoa and lentils
- Finish dishes with a handful of fresh herbs
- Buy a mixed bag of nuts and seeds instead of one kind
- Stir a spoonful of beans into salads, soups and bowls
- Pick a fruit you haven’t bought in a while
- Use a spice blend instead of salt and pepper alone
A single meal can knock out a surprising number. A grain bowl with farro, chickpeas, roasted squash, kale, red onion, pumpkin seeds, parsley and a dressing of olive oil, lemon and garlic comes out with eleven plants.
You don’t need to hit 30 in the first week. The goal is to find your baseline and push it up a little each week. Your microbes will respond to what you give them.
Go deeper with The Mayer Model
Food is where most people start, but it isn’t the whole picture. Dr. Mayer’s new book, The Mayer Model, lays out seven connected pillars for optimal health: food, exercise, mindfulness, resilience, community, purpose and sleep. It explains how each one affects the brain-gut-microbiome system, and how improving one makes the others easier.
If this article changed how you think about what’s on your plate, and you want to get more practical guidance, get the book which goes much further.
Some content in this article may have used AI-assisted tools for research, grammar, or outlining. All articles are reviewed and approved by our human editorial team.
E. Dylan Mayer, MS holds a Master’s Degree in Nutrition from Columbia University. He is a graduate from the University of Colorado at Boulder, with a major in Neuroscience and minor in Business.
✓ This article was reviewed and approved by Emeran Mayer, MD