2016PLOS Biology
Sender, Fuchs & Milo
Reference body estimate
An average adult carries about 38 trillion bacteria, roughly equal to the number of human cells, overturning the old '10 to 1' claim.
Find on PubMed ↗Research library · Hormones & inner health
Your gut microbiome: the inner ecosystem
About 38 trillion bacteria live in your body, most in your large intestine. They digest fibre, train your immune system and send signals to your brain. What you eat changes them within days.
Good evidence38 trillionbacteria in an average body, roughly one for every human cellSender 2016, PLOS Biology
30+different plants a week linked to a more diverse gutMcDonald 2018, mSystems, American Gut Project
15'good' gut species linked to healthier blood sugar, fats and inflammationAsnicar 2021, Nat Med, 1,098 people
01
The basics
A garden you feed every day
Your gut microbiome is the community of bacteria, fungi and viruses in your intestines. A diverse community is generally a healthier one, and diet is the main thing that shapes it.
Microbes ferment the fibre you can't digest into short-chain fatty acids like butyrate. These feed the cells lining your gut, keep the barrier tight, calm inflammation and help regulate blood sugar and appetite.
The gut and brain talk constantly through the vagus nerve, hormones and immune signals; this is the gut-brain axis. Some gut bacteria make or influence brain chemicals such as serotonin and GABA. People with depression tend to have fewer of certain helpful species.
The gut-brain axis
Gut microbes turn fibre into short-chain fatty acids and chemical messengers that reach the brain through the vagus nerve, hormones and the immune system.
02
How it works
How your microbes help you
1
Eat fibre and plants
Vegetables, beans, whole grains, nuts and seeds feed your microbes.
2
Microbes ferment
Bacteria in the large intestine break down the fibre.
3
Helpful compounds made
Short-chain fatty acids like butyrate are released.
4
Signals spread
Calmer immune system, steadier blood sugar, messages to the brain.
03
Measure it
How to read your gut
- Bristol Stool Scale: types 3–4 (smooth, sausage-shaped) are ideal. Hard lumps suggest too little fibre or water.
- Count plants: fruit, vegetables, grains, beans, nuts, seeds, herbs and spices each count once a week.
- Home microbiome tests: fun but not yet reliable for personal advice; results differ between companies.
- See a doctor: for blood in stool, a lasting change in bowel habit, unexplained weight loss or night-time symptoms.
Try your number
ConstipatedIdealLoose
17
Bristol Stool Form Scale (Lewis & Heaton 1997).
04
What the research says
The key studies
2021Nature Medicine
Asnicar et al. (PREDICT 1)
1,098 adults
Identified 15 gut species linked to healthy diets and better blood sugar, fat and inflammation markers, and 15 linked to worse.
Find on PubMed ↗2021Cell
Wastyk et al.
36 adults, 10 weeks
Eating 6 servings of fermented foods a day raised microbe diversity and lowered 19 inflammation markers.
Find on PubMed ↗2019Nature Microbiology
Valles-Colomer et al.
1,054 + 1,070 adults
Two types of bacteria (Coprococcus and Dialister) were consistently lower in people with depression.
Find on PubMed ↗2018Nature Microbiology
Palleja et al.
12 healthy men
After a 4-day antibiotic course, most gut bacteria recovered within 6 months, but some species stayed missing.
Find on PubMed ↗2023Nature
Valles-Colomer et al.
9,715 samples, 20 countries
Gut and mouth microbes are shared between people who live together, so your microbiome is partly social.
Find on PubMed ↗05
Take action
Feeding a healthy gut
Variety and fibre first. Supplements are rarely needed.
- 1
30 plants a week
Mix your veg, use mixed beans, seeds and herbs. A spice blend counts several times.
- 2
25–30 g fibre a day
Increase slowly over a few weeks and drink water to avoid bloating.
- 3
A fermented food daily
Yoghurt, kefir, kimchi, sauerkraut, miso or kombucha.
- 4
Fewer ultra-processed foods
Some emulsifiers and sweeteners disturb gut microbes in early studies.
- 5
Antibiotics only when needed
They save lives, but don't push for them for colds or flu.
- 6
Move daily
Regular exercise is linked to more butyrate-making bacteria.
06
Myth or fact
Common myths, checked
✕ MythProbiotic pills fix everything.
✓ FactThey help in a few situations, like diarrhoea from antibiotics. For healthy people evidence is weak, and one study found they slowed gut recovery after antibiotics.
✕ MythHome tests tell you exactly what to eat.
✓ FactThe science isn't there yet. Eating more plants and fibre is still the best advice.
✕ MythYou need a cleanse or detox.
✓ FactYour liver, kidneys and gut already do this. Cleanses can upset your microbes.
✕ MythBloating means a bad gut.
✓ FactSome gas is normal when you eat more fibre. Persistent pain or change in habit should be checked.
07
Questions
Frequently asked
Probiotics vs prebiotics?
Probiotics are live microbes (yoghurt, kefir). Prebiotics are the fibres that feed them (onions, garlic, oats, bananas, beans).
How fast can my microbiome change?
Studies show shifts within 1–3 days of a big diet change, but lasting change takes weeks of new habits.
Should I try a low-FODMAP diet?
Only for diagnosed IBS and ideally with a dietitian. It's meant to be short-term, then foods are added back.
08
Sources
References and related topics
- Sender, Fuchs & Milo (2016). PLOS Biology. Reference body estimate. PubMed ↗
- Asnicar et al. (PREDICT 1) (2021). Nature Medicine. 1,098 adults. PubMed ↗
- Wastyk et al. (2021). Cell. 36 adults, 10 weeks. PubMed ↗
- Valles-Colomer et al. (2019). Nature Microbiology. 1,054 + 1,070 adults. PubMed ↗
- Palleja et al. (2018). Nature Microbiology. 12 healthy men. PubMed ↗
- Valles-Colomer et al. (2023). Nature. 9,715 samples, 20 countries. PubMed ↗