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How to improve gut health: what the evidence supports, ranked

Six gut habits have some human data, of very different strength. Here they are ranked by evidence, with what to skip and a first week.

Feed Your Gut Editorial

Updated 7 min read 15 sources

Short answer

Six habits have some human data behind them, of very different strength: more fiber (most adults eat well under the 25 to 30 g targets), a wider range of plants, live fermented foods, fewer ultra-processed foods, antibiotics only when you need them, and regular exercise. The exercise effect came from one small trial, depended on body weight and faded afterward. Sleep, stress, cleanses and gut tests have much weaker support for rebuilding the microbiome. Start with fiber.

Key takeaways

  • Fiber has the strongest evidence. A Lancet analysis of 185 cohort studies and 58 trials found the largest risk reductions at 25 to 29 g a day.
  • UK adults average 16.4 g a day and German adults 18 to 19 g, so the usual gap to the 30 g target is 10 g or more.
  • In a trial of 36 adults with a 10-week diet phase (17 weeks in total), a high-fermented-food diet raised microbiome diversity and lowered 19 inflammatory proteins. The high-fiber diet did not change diversity in that time.
  • Ultra-processed food is linked to worse health in an umbrella review, but the studies are observational and most were rated low or very low certainty.
  • Cleanses and probiotics for everyone are not supported. The AGA guideline backs specific strains for specific situations only.
  • Blood in your stool, unexplained weight loss or bloating that does not go away needs a doctor, not a new diet.

Ten volunteers ate an animal-based diet for five days, and their gut microbes shifted within days. Two days after the diet ended, the community had returned to its starting structure.8 Moving microbes is easy. Knowing whether the move matters is harder. This page ranks six habits by how much human evidence stands behind each one, from large reviews to a study of three people, and marks where the evidence is thin.

What “improve” can and cannot mean

Nobody can hand you a gut score from a lab. In research, a healthy gut usually means two things: digestion that works comfortably, and a microbial community that is rich and varied.15 The habits below are mostly tested against the second. Whether a richer microbiome makes you feel better is a separate question, and for most of these habits it has not been answered directly.

The ranking

Habit Best evidence How far to trust it
More fiber Lancet analysis: 185 cohort studies and 58 trials Moderate certainty, per the authors
More plant variety American Gut Project: more than 10,000 people, extremes compared Suggestive. Observational
Live fermented foods Stanford trial: 36 adults, 10-week diet phase Promising. Small
Fewer ultra-processed foods BMJ umbrella review of 45 pooled analyses Consistent signal. Mostly low certainty
Antibiotics only when needed Ciprofloxacin study in 3 people Plausible and well described. Tiny, three people
Regular exercise 32 adults, 6 weeks Early. Effect differed by body weight and faded
Sleep and stress Mechanism reviews Real pathway. Few gut-specific trials

Fiber: the gap is wider than people think

The Lancet analysis pooled just under 135 million person-years of data. Compared with the lowest intakes, the highest were linked to 15 to 30% lower all-cause and cardiovascular mortality, and to lower rates of heart disease, stroke, type 2 diabetes and colorectal cancer. The clinical trials in it showed lower body weight, blood pressure and cholesterol with more fiber. The authors found the biggest gains between 25 and 29 g a day.1

Adults in the UK average 16.4 g a day and only 4% reach 30 g.11 In Germany the averages are 18 g for women and 19 g for men.12 So the typical job is adding 10 g or more, not fine-tuning. One 150 g portion of cooked lentils delivers about 11.9 g by USDA values.13 The fiber chart has more than 40 foods with grams, and if the first week of extra fiber makes you gassy, this ramp-up plan is built for that.

Variety: the cheap upgrade

In the American Gut Project, which collected samples from more than 10,000 people, researchers compared people who ate more than 30 types of plants a week with people who ate 10 or fewer. The stool-chemistry comparison involved 42 people in the high-variety group and 43 in the low-variety group. The high-variety group had higher microbial diversity and more bacteria that ferment fiber into short-chain fatty acids.2 It is a correlation, not a trial, so treat 30 as a useful target and not a threshold. The full story, with a list of plants, has the counting rules.

Ferments: the one change that moved microbiome diversity in a trial

Stanford researchers randomly assigned 36 healthy adults to a high-fiber or a high-fermented-food diet for 10 weeks, inside a 17-week protocol with a lead-in and a follow-up. The fermented-food group built up to an average of 6.3 servings a day. Their microbiome diversity rose steadily and 19 of 93 inflammatory proteins fell, including interleukin-6. The high-fiber group did not show a cohort-wide diversity increase, though their microbes carried more fiber-digesting enzymes.3

Six servings a day is a lot, and 18 people per arm is small. The trial shows direction, not a prescription. Fiber has its own trials, covered above, so this is not a ranking of ferments over fiber. The practical read is to add one live ferment a day and buy the right kind, since heat treatment kills the cultures.

Ultra-processed food: cut back, no need to panic

An umbrella review of 45 pooled analyses covering almost 9.9 million people found that higher ultra-processed food intake was associated with 32 of the health outcomes studied. The evidence classed as convincing covered cardiovascular mortality, type 2 diabetes, anxiety and common mental disorders, but most of the underlying studies were rated low or very low certainty and none can prove cause.4 In the PREDICT 1 study of 1,098 people, microbes tied to favorable health markers were associated with diets rich in healthy plant-based foods.5 Swapping one packaged snack a day for fruit, nuts or yogurt is a reasonable start. Nobody needs to eat perfectly.

Exercise, sleep and stress

In a six-week supervised program, 32 previously inactive adults trained three times a week. Their microbiome composition shifted, and fecal short-chain fatty acids rose in the lean participants but not in the participants with obesity. Most changes faded after the training stopped.6 Exercise is good for you on many fronts. Count the gut as a possible bonus.

The gut and brain talk through nerves, hormones and immune signals.10 That pathway is real. Strong trials showing that changing sleep or stress alone rebuilds the microbiome are lacking, so treat both as worth doing for their own sake, not as gut hacks.

After antibiotics

Antibiotics matter when you need them, and they do not work on colds or flu, so do not ask for them for one. In a study of three people given two courses of ciprofloxacin, diversity fell within 3 to 4 days. Communities began returning a week after each course but were still different from baseline at the end of ten months.7 If you are prescribed antibiotics, the AGA guideline suggests that certain probiotic strains can reduce the risk of C. difficile infection. Ask your prescriber whether that applies to you.9

What to skip

  • Cleanses and detoxes. No trial shows that one improves the microbiome.
  • Probiotics for everyone. The AGA recommends probiotics for IBS, Crohn’s disease and ulcerative colitis only within clinical trials.9
  • Chasing a perfect result. The five-day diet swap in the opening reverted within two days of ending. Expect the same from any quick fix.8

A first week

Day Do this The number behind it
1 Write down what you eat on a normal day. You cannot close a gap you have not measured.
2 Swap one refined grain for a whole one. 2 slices of whole wheat bread: 4.2 g fiber. White: 1.6 g.
3 Add a legume to one meal. 150 g cooked chickpeas: 11.4 g.
4 Pick one live ferment from the chilled aisle. Heat-treated versions have no live cultures.
5 Eat a plant you have not had this month. Variety is the cheapest part of the score.
6 Walk somewhere you would normally drive. Exercise effects on the microbiome fade without repetition.
7 Look back. What changed in how you feel? Judge by weeks, not days.

All fiber figures come from USDA FoodData Central.13

Bloating that needs a doctor

You feel bloated regularly, stay bloated after changing your diet, or are bloated and also losing weight without trying or seeing blood in your stool. These need a medical check, not a diet experiment.14

Frequently asked questions

How long does it take to improve gut health?

Gut microbes can shift fast. In a small US study, ten volunteers ate an animal-based diet for five days. Their microbial community shifted within days and reverted two days after the diet ended. Fast change does not mean lasting change. The trials on fiber and fermented foods that showed benefits ran for 4 to 10 weeks, so judge a new habit after a month, not a weekend.

What are the signs of an unhealthy gut?

There is no single sign. Bloating that happens regularly, constipation or diarrhea that lasts, and pain after eating can have many causes, from a food that does not suit you to celiac disease or IBS. The NHS advises seeing a doctor if you are bloated regularly or still bloated after changing your diet, and sooner if it comes with weight loss or blood in your stool.

Do I need a gut microbiome test?

Consumer tests report which bacteria are in one stool sample. There is no accepted threshold that says which bacteria you should have, so the advice is the same whatever the report says: more fiber, more plant variety, some live ferments.

Do probiotic supplements improve gut health?

For healthy people there is little evidence that they do. The American Gastroenterological Association suggests specific strains to prevent C. difficile infection in people taking antibiotics, and advises using probiotics for IBS, Crohn’s disease and ulcerative colitis only inside clinical trials. Food-based ferments are a separate question, covered below.

Sources

Figures, study results and official recommendations in this article come from these sources. Everyday cooking and shopping tips are our own judgment.

  1. Reynolds A, et al. Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. The Lancet. 2019;393(10170):434-445. doi.org
  2. McDonald D, et al. American Gut: an open platform for citizen science microbiome research. mSystems. 2018;3(3):e00031-18. doi.org
  3. Wastyk HC, et al. Gut-microbiota-targeted diets modulate human immune status. Cell. 2021;184(16):4137-4153. doi.org
  4. Lane MM, et al. Ultra-processed food exposure and adverse health outcomes: umbrella review of epidemiological meta-analyses. BMJ. 2024;384:e077310. doi.org
  5. Asnicar F, et al. Microbiome connections with host metabolism and habitual diet from 1,098 deeply phenotyped individuals. Nature Medicine. 2021;27(2):321-332. doi.org
  6. Allen JM, et al. Exercise alters gut microbiota composition and function in lean and obese humans. Medicine & Science in Sports & Exercise. 2018;50(4):747-757. doi.org
  7. Dethlefsen L, Relman DA. Incomplete recovery and individualized responses of the human distal gut microbiota to repeated antibiotic perturbation. PNAS. 2011;108(Suppl 1):4554-4561. doi.org
  8. David LA, et al. Diet rapidly and reproducibly alters the human gut microbiome. Nature. 2014;505(7484):559-563. doi.org
  9. Su GL, et al. AGA Clinical Practice Guidelines on the Role of Probiotics in the Management of Gastrointestinal Disorders. Gastroenterology. 2020;159(2):697-705. doi.org
  10. Cryan JF, et al. The microbiota-gut-brain axis. Physiological Reviews. 2019;99(4):1877-2013. doi.org
  11. Office for Health Improvement and Disparities. National Diet and Nutrition Survey 2019 to 2023 report. GOV.UK, 2025 (adults: mean fibre intake 16.4 g/day, 4% meet the 30 g recommendation). gov.uk
  12. Max Rubner-Institut. Scientific assessment: dietary fibre (data: German National Nutrition Survey II; women 18 g, men 19 g per day; DGE recommends at least 30 g). mri.bund.de
  13. U.S. Department of Agriculture, Agricultural Research Service. FoodData Central, SR Legacy: lentils, mature seeds, cooked, boiled, without salt (FDC ID 172421): 7.9 g fiber per 100 g. fdc.nal.usda.gov
  14. NHS. Bloating: causes, self-help and when to see a GP. nhs.uk
  15. Heiman ML, Greenway FL. A healthy gastrointestinal microbiome is dependent on dietary diversity. Molecular Metabolism. 2016;5(5):317-320. doi.org

This article is general information, not medical advice. If you have ongoing digestive symptoms, a diagnosed condition like IBS or IBD, or you are pregnant, talk to a doctor or registered dietitian before changing your diet.

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