A diverse gut is usually a good sign, though it isn’t a report card

Key takeaways

• The famous “30 plants a week” finding compared 41 people who ate more than 30 plant types with 44 who ate 10 or fewer, out of more than 11,000 volunteers who reported their own diets.
• Karen refugees who moved from Thailand to the U.S. began trading native gut bacteria for Western-type bacteria within six to nine months, and immigrants’ children showed bigger changes.
• In a 2013 study of 292 Danes, the 23% with the fewest gut microbial genes carried more body fat and more signs of insulin resistance and inflammation, a link that doesn’t prove cause.
• Higher diversity can mislead: slow transit through the colon goes with higher microbial richness and more potentially harmful protein breakdown products.
• Across 64 fiber trials, extra fiber raised helpful Bifidobacterium and Lactobacillus but didn’t change overall diversity, so don’t expect one fiber powder to move a diversity score.

For a little over a year, scientists collected 350 stool samples from 188 Hadza people, hunter-gatherers who live in Tanzania¹². Their gut communities changed with the calendar¹.

During the rainy months, honey made up a large part of what people ate, and some bacterial species fell too low for tests to find². Once the dry months brought more fiber-rich tubers to the menu, those species reappeared². Samples taken in the same season a year apart looked nearly identical².

The Stanford-led team then compared the Hadza with 18 populations in 16 countries¹. The microbes that swung most with the Hadza seasons were the same ones that separate traditional peoples from industrialized ones. Some of those lineages have become rarer in modern guts¹.

That study captures the idea behind microbiome diversity. What you eat shapes which microbes stay, and modern life seems to have thinned the crowd. It’s one reason “a more diverse gut is a healthier gut” has become a popular rule of thumb.

The rule has real support. It also has exceptions, and diversity scores depend heavily on how they’re measured. This article covers how to read the science and how to feed your gut’s variety with food.

Diversity counts how many kinds live in your gut and how evenly they share it

Scientists break diversity into a few pieces:

• Richness is the number of different kinds of microbes in a sample.
• Evenness is how equally those kinds share the space.
• Alpha diversity sums up richness and evenness within one sample, such as one person’s stool.
• Beta diversity compares samples: how different your gut is from someone else’s, or from your own a month ago³.

Picture two gardens with 20 plant species each. In one, grass covers almost everything. In the other, all 20 grow in similar amounts. Both have the same richness, but only the second is even.

Common scores, such as the Shannon and Simpson indexes, blend richness and evenness in different proportions, and both tend to miss rare species⁴. So one stool sample can earn different “diversity” numbers depending on the formula and the lab method³⁴. Our article on home microbiome tests explains why kits can disagree about the same sample.

Traditional diets keep microbes that many modern guts have lost

The Hadza eat 100 grams or more of fiber a day on average, while the average modern person gets about 15 grams, according to Justin Sonnenburg, the Stanford study’s senior author².

Sonnenburg’s lab tested what fiber does to diversity in mice. In a 2016 study, cutting fiber shrank the animals’ gut diversity, which came back when fiber returned². But when mice stayed on low fiber across four generations, some species were lost for good².

People show a similar pattern when their diets change. A University of Minnesota team compared Hmong and Karen people, two communities with roots in China and Burma, living in Thailand with those who had resettled in the U.S.⁵. Immigrants’ children and white American adults rounded out the comparison⁵.

Nineteen Karen refugees were tracked through the move itself. Within six to nine months of arriving, they were already losing Prevotella, a group common in non-Western guts, as Bacteroides, a group typical of Western guts, gained ground⁵. The longer people had lived in America, the lower their diversity, and their children’s microbes had shifted even further⁵.

A more Westernized microbiome went with more obesity, though the study couldn’t show cause and effect⁵. Diet explained part of the change, not all of it. Senior author Dan Knights cautioned that the shift might not always be harmful⁵.

These studies echo an idea many natural-health readers already hold: traditional ways of eating carry something modern life tends to strip away. That doesn’t mean you need a hunter-gatherer diet. It does suggest that the food culture you keep, and pass on, matters to the microbes you carry.

The “30 plants a week” rule came from a citizen-science snapshot

The American Gut Project, started in 2012 by researchers including Rob Knight, let anyone mail in a stool sample for $99⁶. By mid-2017 it had 15,096 samples from 11,336 people, mostly in the U.S., Britain and Australia⁶.

In a 2018 paper in the journal mSystems, the team reported that people who ate more than 30 plant types a week had more diverse gut microbes than people who ate 10 or fewer⁶. Their stool also held different mixes of molecules and fewer genes for antibiotic resistance⁶.

That finding is the source of the “30 plants a week” trend⁷. What’s easy to miss: the comparison involved 41 people at the high end and 44 at the low end⁶. The 11,000-plus volunteers in the middle weren’t part of that contrast. The study shows that the two ends differed, not that benefits start at 30.

The diet data were self-reported⁷. The finding is an association, and the researchers said they couldn’t yet tell whether raising diversity would improve health⁶. Even the University of Roehampton nutrition blog that explains the trend frames the goal as roughly 30, a nudge toward variety rather than an exact target⁷.

Variety isn’t right for every gut at every moment, either. Knight noted that fiber, usually helpful, can be harmful for people with inflammatory bowel disease (IBD)⁶.

Researchers still have good reasons to like diversity

The best-known evidence comes from Denmark. In a 2013 study in Nature, researchers counted the microbial genes in the guts of 292 Danish adults, a measure of richness⁸. About 23% had low gene counts⁸.

Compared with the high-richness group, those people carried more body fat and showed more insulin resistance, more unhealthy blood fats and more signs of inflammation⁸. Among people with obesity, those with low richness gained more weight over time⁸. The study showed a link, not proof that low richness causes these problems.

Federal nutrition advice reflects this cautious optimism. The 2025–2030 Dietary Guidelines say vegetables, fruits, fermented foods and high-fiber foods support a diverse microbiome, “which may be beneficial for health”⁹. They also say highly processed foods can disrupt the microbiome’s balance⁹.

That word “may” is the honest summary. Diversity looks like a good sign in many studies. It hasn’t been proven to be the reason people stay well.

Antibiotics can knock diversity down for months

Antibiotics are among the clearest influences on gut diversity. In the American Gut data, 139 people who had taken antibiotics in the past month had less diverse microbes than 117 people who hadn’t taken any in the past year⁶.

A 2018 University of Copenhagen experiment showed the effect directly; our article on what gut bacteria do walks through it. In brief, four days of three powerful antibiotics nearly emptied the guts of 12 healthy men, and nine common helpful species were still absent six months later¹⁰.

Lead researcher Oluf Pedersen pointed out that Western populations carry less gut diversity than some peoples in Africa and the Amazon, and he suggested widespread antibiotic use may be one reason¹⁰. He also worried that repeated courses over a lifetime could cause lasting losses, something a four-day study can’t test¹⁰.

None of this means refusing antibiotics. They save lives when an infection is bacterial. It does mean asking your clinician whether an infection is likely bacterial before you take them. If you get frequent watery diarrhea during or after a course, call your clinician.

When more diversity isn’t a good sign

This is where the rule of thumb breaks down.

Slow transit. Danish researchers measured how long food took to pass through the colon in adults who swallowed small markers that show up on X-rays¹¹. Longer transit went with higher microbial richness¹¹. It also went with a shift from fermenting carbohydrates to breaking down protein, and with more potentially harmful protein byproducts in urine¹¹. The authors concluded that high richness doesn’t necessarily mean a healthy gut¹¹.

Stool form. In a Belgian study of 53 healthy women, firmer stools went with higher species richness¹². The authors said stool consistency should be measured in microbiome studies¹². A high diversity score can partly reflect a slow gut.

Disease. A 2024 perspective article in The ISME Journal noted that higher diversity has been tied to disease in several human conditions, including bacterial vaginosis, HIV and IBS³. Across animal research, the authors found no clear link between alpha diversity and health³.

Infants and mood. Oxford researchers noted in 2016 that formula-fed babies tend to have more diverse but less stable gut communities than breastfed babies⁴. They also pointed to a study that found more diverse gut bacteria in people with major depression than in healthy controls⁴.

The lesson: diversity is a clue, read in context. A score on its own can’t tell you whether your gut is thriving.

What fiber trials say about diversity

If diversity mostly reflects diet, you might expect fiber to raise it quickly. Trials say otherwise.

In a 2018 review in the American Journal of Clinical Nutrition, researchers pooled 64 studies with 2,099 healthy adults¹³. Added fiber raised levels of Bifidobacterium and Lactobacillus, two groups generally considered helpful, and nudged up butyrate, a compound gut microbes make from fiber, though that result was borderline¹³. Fructans and galacto-oligosaccharides, two types of fermentable fiber, drove much of that increase¹³. But fiber didn’t change alpha diversity¹³.

A 2021 Stanford trial found the same stability: diversity held steady over 10 weeks on a high-fiber diet, while it rose on a diet high in fermented foods, a trial our fermented foods article covers in depth¹⁴.

Here’s our reading, as an interpretation rather than a proven fact. Short trials of single fibers feed particular microbes. Broad diversity seems to track long stretches of varied eating, as the Hadza seasons and the multigeneration mouse work suggest². Fiber still matters; our fiber article covers how much to eat and where to find it.

A 30-plants tracker and a diversity plan without supplements

Use this tracker for a week to see where you stand. Our counting rules, which are the library’s own and not from a study:

• Count each plant once per week, however often you eat it.
• Whole or minimally processed forms count, including frozen and canned.
• Juice, refined flour, sugar and oils don’t count.
• Herbs and spices count, up to five points a week, so they don’t crowd out bigger servings of plants.

Category	What counts	Easy ways to add one
Vegetables	Each different vegetable	Toss frozen mixed vegetables into soup or eggs
Fruits	Each whole fruit	Buy one fruit you didn’t buy last week
Whole grains	Oats, brown rice, barley, quinoa, whole wheat, rye	Rotate your breakfast grain
Legumes	Each bean, lentil or pea	A can of mixed beans can add three or four
Nuts and seeds	Each kind	Keep a jar of mixed seeds by the cereal
Herbs and spices	Each kind, up to five points	Use a spice blend you already like
Fermented plant foods	Sauerkraut, kimchi, miso count as their plant	Add a spoonful as a side9

A one-week, food-first plan:

1. Days 1 to 2: Count last week’s plants as accurately as you can. That’s your baseline, not a grade.
2. Day 3: Add one legume and one new vegetable.
3. Day 4: Swap one refined grain for a whole grain you rarely eat.
4. Day 5: Top breakfast or a salad with mixed nuts or seeds.
5. Day 6: Cook a dish from a family or regional tradition that’s heavy on plants.
6. Day 7: Recount. Aim for a few more types than your baseline, not a magic 30.

Go slowly if beans or extra fiber give you gas, and give your gut weeks to adjust. Expect changes in your microbes over months, not days²⁵¹³. If you have IBD or IBS, plan changes with your clinician or a dietitian.

Tracking is meant to add foods, not police them. If counting starts to feel stressful or rigid, stop. If worries about food are taking over, the National Alliance for Eating Disorders helpline is 1-866-662-1235.

How strong is the evidence?

Claim	Evidence level	Why
Broad antibiotics cut gut diversity	Strong evidence	Near-total wipeout in a controlled study; lower diversity in recent users610
Diet and lifestyle shifts reshape which microbes you carry	Strong evidence	Seasonal Hadza changes, immigrant studies, mouse work and fiber trials12513
Higher diversity generally signals better health	Promising but early	Links to metabolic health, with real exceptions3811
Eating 30-plus plant types a week goes with a more diverse gut	Promising but early	One comparison of small groups in a self-reported survey; no trial yet67
Adding one fiber raises overall diversity within weeks	Evidence says no	No alpha-diversity change across 64 trials, though helpful groups rose13
Fermented foods raise diversity	Promising but early	One 10-week trial; see our fermented foods article14
A diversity score grades your gut health	Not enough evidence	Scores shift with method, formula and stool form3412

Questions readers ask

The bottom line

A diverse gut microbiome is usually a good sign. It shows up more often in traditional food cultures, it drops with broad antibiotics and with moves to a Western lifestyle, and it tracks with better metabolic health in research. But it isn’t a report card. Slow transit can inflate richness, some diverse states go with disease, and scores depend on how they’re measured.

The lever you control is your plate. Eat a wider range of whole plants, use antibiotics only when they’re truly needed, and give your microbes months, not days. You don’t need a supplement or a score to start.

Keep exploring: digestive effects of diet change.

Related reading

• Same stool, different verdicts: what home gut microbiome tests can tell you
• Most Americans fall far short on fiber, and one swap a week closes the gap
• What your gut bacteria do, given that they don’t outnumber you 10 to 1
• Six servings of fermented food a day reshaped gut microbes in a Stanford trial, so start with one
• The preventive care that costs you nothing, and the exceptions

Sources

1. Smits SA, Leach J, Sonnenburg ED, et al. “Seasonal Cycling in the Gut Microbiome of the Hadza Hunter-Gatherers of Tanzania.” Science, 2017 (CARTA record). carta.anthropogeny.org
2. Stanford Medicine. “Hunter-Gatherers’ Seasonal Gut-Microbe Diversity Loss Mirrors Permanent Loss in Industrialized Humans.” Stanford Medicine News, August 2017. med.stanford.edu
3. Williams CE, Hammer TJ, Williams CL. “Diversity Alone Does Not Reliably Indicate the Healthiness of an Animal Microbiome.” The ISME Journal (Oxford University Press for the International Society for Microbial Ecology), 2024; doi:10.1093/ismejo/wrae133 (National Library of Korea mirror of the Oxford Academic page). doi.org
4. Johnson KV-A, Burnet PWJ. “Microbiome: Should We Diversify From Diversity?” Gut Microbes, 2016 (Oxford University Research Archive). ora.ox.ac.uk
5. University of Minnesota College of Science and Engineering. “Immigration to the United States Changes a Person’s Microbiome” (on Vangay P, et al., Cell, 2018). November 2018. cse.umn.edu
6. University of California San Diego. “American Gut Project Reports Microbiome Results” (on McDonald D, et al., “American Gut: An Open Platform for Citizen Science Microbiome Research,” mSystems, 2018). ScienceDaily, May 2018. sciencedaily.com
7. Reeves S. “What’s Behind the 30 Plants per Week Trend?” University of Roehampton blog, January 2025. blog.roehampton.ac.uk
8. Le Chatelier E, Nielsen T, Qin J, et al. “Richness of Human Gut Microbiome Correlates With Metabolic Markers.” Nature, 2013 (Technical University of Denmark record). orbit.dtu.dk
9. U.S. Department of Agriculture and U.S. Department of Health and Human Services. “Dietary Guidelines for Americans, 2025–2030.” January 2026. cdn.realfood.gov
10. University of Copenhagen, Novo Nordisk Foundation Center for Basic Metabolic Research. “The Composition of Gut Bacteria Almost Recovers After Antibiotics” (on Palleja A, et al., Nature Microbiology, 2018). October 2018. cbmr.ku.dk
11. Roager HM, Hansen LBS, Bahl MI, et al. “Colonic Transit Time Is Related to Bacterial Metabolism and Mucosal Turnover in the Gut.” Nature Microbiology, 2016 (Technical University of Denmark record). orbit.dtu.dk
12. Vandeputte D, Falony G, Vieira-Silva S, Tito RY, Joossens M, Raes J. “Stool Consistency Is Strongly Associated With Gut Microbiota Richness and Composition, Enterotypes and Bacterial Growth Rates.” Gut, 2016. pubmed.ncbi.nlm.nih.gov
13. So D, Whelan K, Rossi M, et al. “Dietary Fiber Intervention on Gut Microbiota Composition in Healthy Adults: A Systematic Review and Meta-Analysis.” American Journal of Clinical Nutrition, 2018 (Bond University record). research.bond.edu.au
14. Stanford Medicine. “Fermented-Food Diet Increases Microbiome Diversity, Decreases Inflammatory Proteins, Study Finds” (on Wastyk HC, et al., Cell, 2021). Stanford Medicine News, July 2021. med.stanford.edu
15. American College of Gastroenterology. “Irritable Bowel Syndrome: Overview.” ACG patient information, updated 2026. gi.org