How long does it take to change your gut microbiome?

Composition shifts within 24 to 48 hours of a major dietary change. Stable changes take longer. A sustained dietary pattern is needed to hold those changes, because composition reverts toward baseline within days of stopping.

Do probiotics change your microbiome?

Most generic probiotic supplements pass through the gut without colonising. Effects, where they exist, tend to be temporary and stop when supplementation stops. Specific strains have evidence for specific situations, like Saccharomyces boulardii for antibiotic-associated diarrhoea, but a daily generic capsule is unlikely to produce a lasting microbiome shift.

Can you test your gut microbiome at home?

Yes. Stool tests from companies like ZOE, Atlas Biomed and Viome sequence the microbes in a sample. They are useful for a one-off snapshot and education. They are less useful for tracking change over time because the test is expensive to repeat. Results can also vary day to day depending on what you ate.

Does the microbiome cause weight gain?

Studies suggest the microbiome influences how much energy is extracted from food and how strongly satiety signals fire. Lower microbial diversity is associated with higher body weight and metabolic risk. The relationship is bidirectional: weight changes affect the microbiome, and microbiome composition affects weight gain or loss patterns.

Microbiome science

Gut microbiome

Also called: gut flora, intestinal microbiota

The gut microbiome is the community of trillions of bacteria, archaea, fungi and viruses living in your gut, mostly in the colon. Roughly half the cells in the human body belong to it. The mix you carry helps digest fibre, makes vitamins like K and B12, trains the immune system, and influences mood, sleep and weight.

What lives there

Most of the gut microbiome is in the colon, where conditions are dense, anaerobic and slow-moving. The small intestine has far fewer microbes because food and bile move through quickly. A typical adult colon hosts somewhere between 500 and 1,000 different species, though the exact number varies depending on the sequencing method used.

Bacteria are the dominant residents but not the only ones. There are also archaea (like the methane-producers Methanobrevibacter smithii), yeasts and other fungi, and a vast collection of viruses that infect the bacteria. Recent research has shifted from just counting species to looking at the functions encoded in their genes, which gives a more practical picture of what the community can actually do.

Why microbial diversity matters

A more diverse microbiome is more resilient. It can handle a missed meal, a course of antibiotics, or a sudden change in diet without losing function. Lower diversity is associated with most chronic gut and metabolic conditions, including IBS, IBD, type 2 diabetes, and obesity. American Gut Project data show that people who eat more than 30 plant types a week have more diverse microbiomes than those who eat 10 or fewer.

What the microbiome actually does

Ferments fibre into short-chain fatty acids (butyrate, acetate, propionate) that fuel the colon lining and reduce inflammation.
Synthesises some B vitamins, vitamin K, and small amounts of certain amino acids.
Trains the immune system, especially in the first three years of life when the system is calibrating.
Produces neurotransmitters and neurotransmitter precursors. Around 90 percent of the body's serotonin is made in the gut, although most of that supply does not cross into the brain.
Influences how much energy you extract from food. Twin studies suggest the microbiome affects body weight independent of calorie intake.

How to feed it well

The single most-cited piece of advice in the field is to eat a wide variety of plant species. The 30 plants a week target, popularised by the American Gut Project and adopted by ZOE, comes from American Gut Project data showing that people who ate more than 30 plant types a week had more diverse microbiomes than those who ate 10 or fewer. Plants here include vegetables, fruits, nuts, seeds, whole grains, herbs, spices, beans and pulses.

Aim for 30 different plant species across a week, not just five favourites.
Include fermented foods daily where tolerated: kefir, live yoghurt, sauerkraut, kimchi.
Hit at least 30 grams of fibre a day. UK average is around 19 grams.
Limit ultra-processed foods. Emulsifiers and certain artificial sweeteners reduce diversity in studies.
Sleep enough. Short-term sleep restriction has been linked to shifts in microbiome composition in small studies.

Common questions

How long does it take to change your gut microbiome?: Composition shifts within 24 to 48 hours of a major dietary change. Stable changes take longer. A sustained dietary pattern is needed to hold those changes, because composition reverts toward baseline within days of stopping.
Do probiotics change your microbiome?: Most generic probiotic supplements pass through the gut without colonising. Effects, where they exist, tend to be temporary and stop when supplementation stops. Specific strains have evidence for specific situations, like Saccharomyces boulardii for antibiotic-associated diarrhoea, but a daily generic capsule is unlikely to produce a lasting microbiome shift.
Can you test your gut microbiome at home?: Yes. Stool tests from companies like ZOE, Atlas Biomed and Viome sequence the microbes in a sample. They are useful for a one-off snapshot and education. They are less useful for tracking change over time because the test is expensive to repeat. Results can also vary day to day depending on what you ate.
Does the microbiome cause weight gain?: Studies suggest the microbiome influences how much energy is extracted from food and how strongly satiety signals fire. Lower microbial diversity is associated with higher body weight and metabolic risk. The relationship is bidirectional: weight changes affect the microbiome, and microbiome composition affects weight gain or loss patterns.

Sources

McDonald et al. 2018, American Gut: an Open Platform for Citizen Science Microbiome Research (>30 vs <=10 plant types) (mSystems (PMC5954204))
Goodrich et al. 2014, Human Genetics Shape the Gut Microbiome (TwinsUK, Dept of Twin Research KCL) (Cell (PMID 25417156))
HMP Consortium 2012, Structure, function and diversity of the healthy human microbiome (Nature (PMID 22699609))
Sender, Fuchs, Milo 2016, Revised Estimates for the Number of Human and Bacteria Cells in the Body (PLOS Biology (PMID 27541692))
Donaldson, Lee, Mazmanian 2016, Gut biogeography of the bacterial microbiota (colon density, small-intestine bile/transit) (Nature Reviews Microbiology (PMC4837114))
Gut microbiota-derived short chain fatty acids are potential mediators in gut inflammation (butyrate fuels colonocytes, anti-inflammatory) (Animal Nutrition / PMC9040132)
Exploring the vitamin biosynthesis landscape of the human gut microbiota (B vitamins, vitamin K, B12) (PMC11494892)
Development of Gut Microbiota in the First 1000 Days after Birth (immune maturation window) (Nutrients (PMC10457741))
Bakshi et al., Serotonin - Its Synthesis and Roles (95% gut serotonin, does not cross BBB) (PMC8124334)
Involvement of Reduced Microbial Diversity in Inflammatory Bowel Disease (low diversity, reduced stability) (Journal of Immunology Research (PMC5198157))
Exploring the Gut Microbiota: obesity, metabolic syndrome, type 2 diabetes (low diversity, energy harvest) (J Clin Endocrinol Metab (PMC11479700))
David et al. 2014, Diet rapidly and reproducibly alters the human gut microbiome (24h composition shift) (Nature (PMC3957428))
NHS: How to get more fibre into your diet (30 g/day target, ~20 g UK average) (NHS)
Chassaing et al. 2022, Randomized Controlled-Feeding Study of Dietary Emulsifier Carboxymethylcellulose (reduced diversity) (Gastroenterology (PMC9639366))
Benedict et al. 2016, Gut microbiota response to recurrent partial sleep deprivation (two nights, no species-diversity change) (Molecular Metabolism (PMC5123208))
Zmora et al. 2018, Personalized Gut Mucosal Colonization Resistance to Empiric Probiotics (transient, person-specific) (Cell (PMID 30193112))
Szajewska & Kolodziej 2015, Systematic review with meta-analysis: Saccharomyces boulardii in prevention of antibiotic-associated diarrhoea (Aliment Pharmacol Ther (PMID 26216624))