Your gut microbiome, explained

You are not entirely you. Living in your gut is an ecosystem of microbes so large that, by current estimates, their cells roughly match your own in number.⁶ For most of medical history, we ignored them. Over the past twenty years, we've started to listen.

What it is

The microbiota is the community itself: bacteria, archaea, fungi and viruses. The microbiome usually means their genes and the environment they live in. Most research samples stool, which tells us a lot, but not everything: the community lining your gut wall can differ.¹

What's genuinely established

• Diet shapes it. What you eat changes which microbes thrive and what they produce.¹⁻³
• Antibiotics disrupt it.
• Microbes ferment fibre into compounds your body uses.
• In at least one disease, changing it is a proven treatment. In recurrent C. difficile infection, a transplant of screened donor microbes resolved the infection far more often than antibiotics alone.⁴

What's still association

You'll read that certain bacteria cause weight gain, low mood or faster ageing. Most of these findings show that two things travel together, not that one causes the other. Illness, medicines and diet can all change both.

More diversity isn't automatically better either. It depends which microbes, doing what, where.

Where "30 plants a week" comes from

The American Gut Project collected samples from more than 10,000 volunteers. People who reported eating more than 30 different plants a week had more diverse gut microbes than people who ate 10 or fewer.¹

It's a valuable finding, with limits:

• Volunteers chose to take part and reported their own diets.
• It was observational: nobody was assigned to eat 30 plants.
• It didn't establish 30 as a threshold. Twenty-nine isn't failure.

Don't confuse it with the NHS fibre target of 30 g a day,⁵ which is a weight of fibre, not a count of plants. Most UK adults fall short of it.

What fibre and fermented food did in a trial

In a 17-week Stanford study, one group ate more fermented foods; another ate more fibre. The fermented-food group's microbial diversity rose and some inflammatory markers fell. The high-fibre group's microbes changed what they could digest, but diversity didn't rise during the study.² A small study, but a nice example of how specific these effects are.

In older Europeans, a year of Mediterranean-style eating shifted gut bacteria in ways linked with lower frailty.³ Again, linked, not proven to be the cause.

Myths to ignore

• "You need a reset or detox." There's no evidence that healthy people need their microbiome "reset".
• "There's one ideal microbiome." Healthy communities vary between people.
• "A stool test can tell you exactly what to eat." Not yet, for most people.

What you can actually do

Eat a wide variety of plants. Build towards 30 g of fibre a day, gradually. Include fermented foods if you enjoy them. Use antibiotics only when you need them. And talk to your GP about persistent gut symptoms rather than self-testing.