The microbiome, a user's manual

Around a hundred trillion microbes live in your gut. Here is who they are, what they do, and how to keep them on your side.

Who lives down there

Your gut is home to roughly a hundred trillion bacteria, from more than a thousand species, weighing something close to two kilograms in total. Most of them live in the large intestine. Everyone's mix is different, shaped by how you were born, what you eat, where you live, and every course of antibiotics you have ever taken.

What they do for you

These microbes earn their keep. They break down fibres your own body cannot digest, make certain vitamins including K and some B vitamins, help train your immune system, and produce short-chain fatty acids that feed the lining of your gut. They also talk to your brain, but that is a story for another article.

Diversity is the thing

If there is one word that matters here, it is diversity. A varied microbiome is a resilient one, better able to handle a rough week or a round of antibiotics. The single biggest lever you have is a varied, plant-rich, fibre-rich diet. Fermented foods help too, because they add live microbes along with the compounds that feed them.

What knocks it back

A few things work against a healthy microbiome: a diet built on ultra-processed food and low on fibre, antibiotics taken when they are not needed, ongoing stress, and too little sleep. You cannot control all of it, but food and routine are squarely in your hands.

How to feed it well

The advice is refreshingly unglamorous.

• Eat many different plants across the week, not the same three on repeat.

• Include fibre at most meals, since that is the microbes' actual food.

• Add a little fermented food most days, a forkful of kimchi, a glass of kombucha.

You cannot see your microbiome, but you feed it three times a day. Small, steady choices matter far more than any single so-called superfood.