Your gut microbiome, explained.

What is the gut microbiome?

Your gut microbiome is the community of bacteria, fungi, viruses and other microbes that live in your digestive tract, most of them in the large intestine (the colon). There are a lot of them: a careful 2016 estimate put the number of bacteria in an adult body at around 38 trillion, roughly the same as the number of human cells, and almost all of them live in the colon.¹

Each person's microbiome is different, shaped by genes, where and how you were born, medicines such as antibiotics, age, and above all by what you eat day to day.^2,4

What do gut bacteria do?

Your own digestive enzymes can't break down most fibre. Your gut bacteria can. In the colon they ferment fibre and resistant starch, producing short-chain fatty acids such as butyrate, acetate and propionate. Butyrate is the main fuel for the cells lining the colon, and these compounds are involved in keeping the gut lining healthy, in the immune system and in how the body handles energy.³

Gut microbes also help make some vitamins, compete with unwelcome microbes for space and food, and send signals that reach well beyond the gut. Researchers are actively studying the links between the gut and the brain, often called the gut-brain axis, although much of that work is still in animals or at an early stage in people.⁴

Why diversity matters

A more diverse microbiome, with a wider range of species, is generally linked with better health in large studies.² In one study of more than 1,000 people, eating more healthy, plant-based foods was associated with gut microbes that were in turn linked with better markers of blood sugar, blood fat and inflammation.⁵ These are associations, so they don't prove that one causes the other, but the pattern is consistent across many studies.

What you can do

The habits linked with a healthier, more diverse gut are well established, and they are the backbone of every recipe in the book.

• Eat more fibre. UK guidance is 30g a day for adults, and most of us fall well short.⁶ Wholegrains, beans, lentils, vegetables, fruit, nuts and seeds all count.
• Eat a wide variety of plants. Different plants feed different microbes. Many people aim for 30 different plants a week; see 30 plants a week.
• Add fermented foods. Live yoghurt, kefir, sauerkraut and kimchi bring live microbes of their own; see Fabulous ferments.
• Cut back on ultra-processed food. Higher intakes are linked with a range of poorer health outcomes, though evidence for the gut specifically is still developing.⁷

Sources

1. Sender R, Fuchs S, Milo R. Revised estimates for the number of human and bacteria cells in the body. PLoS Biology, 2016. doi.org/10.1371/journal.pbio.1002533
2. Valdes AM, Walter J, Segal E, Spector TD. Role of the gut microbiota in nutrition and health. BMJ, 2018. doi.org/10.1136/bmj.k2179
3. Koh A, De Vadder F, Kovatcheva-Datchary P, Bäckhed F. From dietary fiber to host physiology: short-chain fatty acids as key bacterial metabolites. Cell, 2016. doi.org/10.1016/j.cell.2016.05.041
4. Cryan JF et al. The microbiota-gut-brain axis. Physiological Reviews, 2019. doi.org/10.1152/physrev.00018.2018
5. Asnicar F et al. Microbiome connections with host metabolism and habitual diet from 1,098 deeply phenotyped individuals. Nature Medicine, 2021. doi.org/10.1038/s41591-020-01183-8
6. Scientific Advisory Committee on Nutrition. Carbohydrates and Health. Public Health England, 2015. www.gov.uk/government/publications/sacn-carbohydrates-and-health-report
7. Lane MM et al. Ultra-processed food exposure and adverse health outcomes: umbrella review of epidemiological meta-analyses. BMJ, 2024. doi.org/10.1136/bmj-2023-077310