The Gut Microbiome: What It Is and How to Feed It Well

Right now, trillions of microbes are living in your gut. Bacteria, mostly, plus fungi and viruses, the vast majority of them in your large intestine. Together they're called your gut microbiome, and they're busy.

They help break down fiber your body can't digest on its own, make certain vitamins and short-chain fatty acids, and interact with your immune system. Scientists are studying links between the microbiome and everything from digestion to mood. A lot of that research is early, and plenty of headlines get ahead of the evidence. But a few things are pretty well established, and they mostly come down to what you eat.

Diversity seems to matter

A healthy gut usually has a wide variety of microbes. And one of the most consistent findings is that a varied diet supports a varied microbiome.

The American Gut Project, a large citizen-science study, found that people who ate 30 or more different types of plants a week had more diverse gut bacteria than people who ate 10 or fewer. "Plants" here means fruits, vegetables, whole grains, beans, nuts, seeds, herbs and spices. A sprinkle of oregano counts.

Fiber is your microbes' favorite food

Gut bacteria ferment fiber and turn it into short-chain fatty acids, which help keep the lining of your gut healthy. Most adults in the US eat far less fiber than recommended.

Easy ways to add more:

• Swap white bread and rice for whole-grain versions.
• Add a can of rinsed beans or lentils to soups, salads and pasta sauce.
• Keep the skin on potatoes, apples and pears.
• Snack on a handful of nuts or a piece of fruit.
• Add oats or chia seeds to breakfast.

Go slowly. Adding a lot of fiber overnight can mean gas and bloating for a few days. Drink plenty of water as you increase it.

Fermented foods may help too

Yogurt with live cultures, kefir, sauerkraut, kimchi, miso and kombucha contain live microbes. A 2021 study from Stanford University found that people who ate several servings of fermented foods a day for 10 weeks ended up with a more diverse microbiome and lower levels of several markers of inflammation.

It was a small study, so it isn't the final word. But fermented foods are an easy, tasty addition for most people. Watch the sugar in flavored yogurts and kombucha, and the salt in sauerkraut and kimchi.

What about probiotic supplements?

Probiotics have good evidence for a few specific situations, such as some kinds of diarrhea linked to antibiotics. For general "gut health" in healthy people, the evidence is much weaker, and products vary a lot in what they actually contain. If you're thinking about one for a specific problem, ask your doctor or pharmacist which strain, if any, makes sense.

Things that can work against a healthy gut

• Diets low in fiber and high in ultra-processed food. They give your microbes less to work with.
• Unnecessary antibiotics. Antibiotics save lives, but they also hit helpful bacteria. Take them when they're prescribed and never for a virus like a cold.
• Poor sleep and chronic stress. Both are linked to changes in the gut, and both are worth working on for plenty of other reasons.

When to see a doctor

Food changes are a great place to start, but they aren't a fix for every gut problem. See a doctor if you have blood in your stool, unexplained weight loss, ongoing diarrhea or constipation, severe pain, or symptoms that wake you at night.