What actually helps your gut?

How to Improve Gut Health: What the Studies Show

Reviewed 2026-11-03 · 6 sources · By Anthony Falcone

The short answer: Eat more fibre from a wide mix of plants, and add fermented foods that still contain live cultures, such as yoghurt, kefir and unheated sauerkraut or kimchi. In a 10-week trial, a diet rich in fermented foods raised the diversity of gut microbes and lowered 19 of 93 inflammatory markers in the blood. Probiotic capsules did not change the gut bacteria of healthy adults in 7 trials, but they did lower the risk of diarrhoea during antibiotics.^2,4,5,6

Before you start

This guide is for healthy adults. See a doctor first, rather than changing your diet, if you have blood in your stool, a change in bowel habits that lasts, weight loss you can't explain, or belly pain or bloating that keeps coming back.
If you have a bowel condition such as irritable bowel syndrome, coeliac disease or inflammatory bowel disease, ask your doctor or a dietitian before adding a lot of fibre or fermented foods. If your immune system is weakened (for example during chemotherapy or after a transplant), or you are seriously ill, ask your doctor before taking probiotics.
Prescribed antibiotics? Ask your doctor or pharmacist whether a probiotic makes sense for you. In trials it lowered the risk of diarrhoea mainly where that risk was moderate or high.⁶
Some fermented foods are salty (soy sauce, kimchi) or contain alcohol, and some contain histamine and similar amines that can bring on migraines in sensitive people. If you have high blood pressure or migraines, ask your doctor which ones suit you.⁴

25–29 g fibre a day at which the lower disease risk was greatest, across 185 cohort studies and 58 trials¹
15–30% lower rates of death, coronary heart disease, stroke, type 2 diabetes and bowel cancer in the highest vs lowest fibre eaters (observational)¹
19 of 93 inflammatory markers in the blood that fell over 10 weeks of a diet rich in fermented foods (18 adults)²
30+ types of plants a week: people eating more than 30 had more fibre-fermenting gut bacteria than people eating 10 or fewer (observational)³
0 of 7 trials in healthy adults in which probiotics changed gut-bacteria diversity, richness or evenness compared with placebo⁵
37% lower risk of antibiotic-associated diarrhoea in adults who took probiotics with their antibiotics (42 trials, 11,305 people)⁶

How to improve gut health naturally: what can be measured

There is no agreed definition of a healthy gut microbiome. The authors of a systematic review of probiotic trials point out that the make-up of a healthy community of gut bacteria has not been defined, and that a change in gut bacteria is only a stepping stone, not proof of better health.⁵

What researchers can measure is diversity: how many different kinds of microbes live in the gut. Low diversity has been linked with conditions such as obesity and diabetes, which is why trials use it as a marker. The diet changes with human studies behind them are the ones below: more fibre, a wider variety of plants and fermented foods.^2,3

How much fibre per day for gut health

The largest analysis we found pooled 185 prospective studies (just under 135 million person-years) and 58 trials with 4,635 adults. In the observational data, people who ate the most fibre had 15–30% lower rates of death from any cause and from cardiovascular disease, and of coronary heart disease, stroke, type 2 diabetes and bowel cancer, than people who ate the least. In the trials, more fibre went with lower body weight, systolic blood pressure and total cholesterol.¹

The lower risk was greatest at 25–29 g of fibre a day, and the dose-response curves suggested that more might help further. The same review found similar results for whole grains, and its authors expect replacing refined grains with whole grains to benefit health. It rated the certainty of the fibre evidence as moderate.¹

Fibre feeds gut bacteria, but don't expect a quick change in diversity. In a Stanford trial, people raised their fibre from 21.5 to 45.1 g a day, building up over 4 weeks and then keeping it high for 6. Their gut bacteria made more of the enzymes that break down plant fibre and their stools got softer, but microbe diversity did not change. More undigested carbohydrate turned up in the stool as intake rose, and the authors think the gut bacteria may need longer than the trial lasted to catch up.²

Eat more types of plants, not just more plants

The American Gut Project, a citizen-science study with samples from 11,336 people by 2017, found that the number of different plants people ate was linked with the diversity of their gut microbes, while diet labels such as vegan or omnivore were not. People who ate more than 30 types of plants a week had more of the bacteria thought to ferment fibre, and fewer of some antibiotic-resistance genes, than people who ate 10 or fewer.³

The study counted every different plant: a soup with carrots, potatoes and onion counted as 3, each grain in a multigrain bread counted, and fruit counted too. This is observational: people reported their own diets and volunteered (and mostly paid) to take part, so it shows a link, not cause and effect.³

Fermented foods for gut health: the trial

The same Stanford trial gave 18 healthy adults a diet rich in fermented foods for 10 weeks. They went from 0.4 to 6.3 servings a day, choosing among yoghurt, kefir, fermented cottage cheese, kombucha, vegetable brine drinks and fermented vegetables such as kimchi. Their gut microbe diversity rose steadily and was highest in the 4 weeks after the main diet phase, when they ate less of these foods but still more than before.²

In their blood, 19 of 93 inflammatory markers fell over the fermented-food diet, including IL-6, a key signal of long-term inflammation. Most of the new microbes found in their guts did not come from the foods themselves, so the foods seem to have changed the gut's own community. Yoghurt and vegetable brine drinks were the foods most strongly linked with the rise in diversity, possibly because people ate the most of them.²

Two cautions. The trial's main outcome, a measure of how immune cells respond to signals, did not change on either diet. And people on the fermented-food diet reported more bloating while they built up their intake, which was no longer significant by the end.²

Which fermented foods actually contain live microbes?

An expert panel of the International Scientific Association for Probiotics and Prebiotics (ISAPP) defines fermented foods as foods made through the wanted growth of microbes. Many of them no longer contain anything alive when you eat them: bread is baked, most beers and wines have their microbes removed, and heat-treated or pasteurised sauerkraut, soy sauce, vinegar and some kombuchas have none. Vegetables pickled in vinegar are not fermented at all.⁴

Foods that usually still have live microbes include yoghurt and kefir, unheated fermented vegetables such as kimchi and sauerkraut, tempeh and most kombuchas. The panel suggests labels say "contains live and active cultures" only when the microbes are still present, and "made by fermentation" when they are not.⁴

Fermented does not mean probiotic. Fermented foods often contain microbes that have not been identified down to the strain, in variable amounts, and their health benefits have generally not been shown. Yoghurt and cultured milk have been tested in more than 20 trials; kefir, kimchi, sauerkraut, natto, vinegar and sourdough bread in at least one each; the evidence for kombucha comes mostly from lab and animal work.⁴

Probiotics: benefits, and what they don't do

Probiotics are live microorganisms that give a health benefit when taken in adequate amounts. A systematic review found 7 trials that tested probiotics against placebo in healthy adults (21 to 81 people each, for 21 to 42 days, mostly Lactobacillus strains). None found a change in the diversity, richness or evenness of gut bacteria, and only 1 found a change in their overall make-up.⁵

Where probiotics do help is alongside antibiotics. A 2021 meta-analysis of 42 trials with 11,305 adults found that taking probiotics with antibiotics lowered the risk of antibiotic-associated diarrhoea by 37%: about 20 people need to take them for one to avoid it. Higher doses worked better than lower doses of the same probiotic, the species that worked were mainly Lactobacillus and Bifidobacterium, and in trials where the risk of diarrhoea was low to begin with there was no difference. None of the 32 trials that reported side effects found a serious one.⁶

Gut-healthy foods: what to put on the plate

For fibre and plant variety: vegetables, fruit, legumes, nuts and seeds, and whole grains in place of refined ones. These are the plant food groups the Stanford trial counted fibre from, and the fibre review found benefits for whole grains as well as fibre.^2,1

For fermented foods: yoghurt, kefir, unheated sauerkraut and kimchi, and other fermented vegetables labelled with live cultures. Keep salty ones such as soy sauce and kimchi, and alcoholic ones, in moderation, as the ISAPP panel advises.^2,4

What to do with this

Work up to at least 25–29 g of fibre a day, the range linked with the largest drop in risk. Build up over a few weeks, as the trial did over 4.^1,2
Count different plants rather than portions: people eating more than 30 types a week had more fibre-fermenting gut bacteria than people eating 10 or fewer.³
Add fermented foods with live cultures most days, building up slowly if they make you bloated. Bread, wine, most beers and vinegar pickles don't count.^2,4
Skip probiotic capsules just to change a healthy gut; the trials found no change. When you're prescribed antibiotics, ask your doctor or pharmacist whether one suits you.^5,6

How sure is this?

The fermented-food trial was small (18 people per diet), had no control group, could not be blinded and lasted 10 weeks. Most participants were women (73%), white (81%) and college-educated (89%). Its main outcome did not change; the rise in diversity and fall in inflammation markers were secondary results. Three authors declare that they founded or hold shares in companies (Interface Biosciences, January AI, Novome Biotechnologies).²
The 15–30% lower disease rates with fibre come from observational studies, which can't prove cause and effect; the trials measured risk factors such as weight and blood pressure, not disease. We could read only the summary of this review.¹
The American Gut Project is self-selected and self-reported. Its participants smoked less, were less often obese and had higher education and income than the population, so its results may not apply to everyone.³
The fermented-foods definitions come from an expert consensus, not a trial. ISAPP is funded by member companies, and most of the panel declare paid work or grants from food, dairy or probiotic companies.⁴
The probiotic evidence in healthy adults is 7 small trials lasting 21 to 42 days that measured gut bacteria, not symptoms. The antibiotic meta-analysis covered adults only, and its trials varied widely in design, strain, dose and timing.^5,6

Common questions

How can I improve my gut health naturally?

Eat more fibre, towards 25–29 g a day, from a wide variety of plants, and add fermented foods with live cultures. These are the changes with human studies behind them.^1,3,2

Do probiotics really work?

For some uses. Taken with antibiotics, they lowered adults' risk of diarrhoea by 37% across 42 trials, but in 7 trials of healthy adults they did not change the make-up of gut bacteria compared with placebo.^6,5

What are the best foods for gut health?

Fibre-rich plants (vegetables, fruit, legumes, nuts, seeds and whole grains) and fermented foods that still contain live cultures, such as yoghurt, kefir, kimchi and unheated sauerkraut.^2,1,4

How long does it take to improve gut health?

Weeks to months. In a trial, gut microbe diversity rose slowly over 10 weeks of fermented foods and was highest in the 4 weeks after; doubling fibre did not change diversity within the trial, and the authors think it may take longer.²

Is sourdough bread a probiotic?

No. Baking kills the fermentation microbes, so sourdough is a fermented food without live microorganisms.⁴

Is kombucha good for your gut?

We don't know yet. Most kombuchas contain live microbes but some are pasteurised, and the evidence on its health effects comes mostly from lab and animal studies.⁴

Sources

Reynolds A, Mann J, Cummings J, Winter N, Mete E, Te Morenga L. Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. The Lancet, 2019. Meta-analysis. doi.org/10.1016/S0140-6736(18)31809-9
Wastyk HC, Fragiadakis GK, Perelman D, Dahan D, Merrill BD, Yu FB, Topf M, Gonzalez CG, Van Treuren W, Han S, Robinson JL, Elias JE, Sonnenburg ED, Gardner CD, Sonnenburg JL. Gut-microbiota-targeted diets modulate human immune status. Cell, 2021. Randomised trial. doi.org/10.1016/j.cell.2021.06.019
McDonald D, Hyde E, Debelius JW, Morton JT, Gonzalez A, Ackermann G, et al. American Gut: an open platform for citizen science microbiome research. mSystems, 2018. Survey / forum study. doi.org/10.1128/mSystems.00031-18
Marco ML, Sanders ME, Gänzle M, Arrieta MC, Cotter PD, De Vuyst L, Hill C, Holzapfel W, Lebeer S, Merenstein D, Reid G, Wolfe BE, Hutkins R. The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods. Nature Reviews Gastroenterology & Hepatology, 2021. Position stand. doi.org/10.1038/s41575-020-00390-5
Kristensen NB, Bryrup T, Allin KH, Nielsen T, Hansen TH, Pedersen O. Alterations in fecal microbiota composition by probiotic supplementation in healthy adults: a systematic review of randomized controlled trials. Genome Medicine, 2016. Review. doi.org/10.1186/s13073-016-0300-5
Goodman C, Keating G, Georgousopoulou E, Hespe C, Levett K. Probiotics for the prevention of antibiotic-associated diarrhoea: a systematic review and meta-analysis. BMJ Open, 2021. Meta-analysis. doi.org/10.1136/bmjopen-2020-043054

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