Can Gut Bacteria Cause Type 2 Diabetes? What the Research Shows

By · Metabolic & Lifestyle Disorders · · 7 min read

Do gut bacteria cause diabetes, or does diabetes change your gut?

No study has shown that gut bacteria cause type 2 diabetes on their own. They can however influence how you digest sugar.

A 2019 study in Nature Genetics looked at 952 adults with normal blood sugar. Those whose genes led their gut bacteria to make more butyrate, a fatty acid made from fiber, had a better insulin response.[1] Your genes were set at birth, so diabetes can’t have caused that difference.

It works the other way too. Metformin, the most widely used diabetes medicine, affects gut bacteria. That shift may be part of how it lowers blood sugar.[2] Food changes them as well.

Does gut bacteria affect blood sugar?

Yes, and the clearest link is fiber. Your body can’t digest most fiber, so it reaches your colon as it is. Bacteria there ferment it into short-chain fatty acids such as butyrate. These prompt your gut to release GLP-1, a hormone that tells your pancreas to release insulin after a meal. Drugs like Ozempic copy the same hormone.

In a trial of adults with type 2 diabetes, a high-fiber diet grew these bacteria. People who grew more of them saw bigger drops in A1C, a two-to-three-month blood sugar average.[3]

Your gut lining matters too. When it lets bacterial fragments called endotoxin into your blood, they can feed low-grade inflammation. In 7,169 Finnish adults followed for 10 years, those with the most endotoxin had a 52% higher risk of diabetes than those with the least. [4] Our guide to leaky gut symptoms covers what can loosen the lining.

Can gut bacteria predict type 2 diabetes?

A recent study in September 2026 presented at the European Association for the Study of Diabetes meeting in Milan said gut changes “could provide early warning of type 2 diabetes.” [5]

What the study looked at

What it showed

Who took part

229,025 UK and US adults, 73.8% of them women, average age 50, including 3,627 with type 2 diabetes and 14,022 with prediabetes

What they found

789 species linked to type 2 diabetes, after accounting for age, gender, and BMI. 587 of them were also linked to prediabetes in people not on diabetes medicines

The early signal

In 135,093 people with normal blood sugar, those whose blood sugar took longer to come down after a meal were more likely to carry the diabetes pattern

What it can’t tell you

Whether the bacteria cause diabetes or follow it. It didn’t follow people over time. It’s also a conference presentation, not yet a full published paper

In another study Finnish researchers collected stool samples from 5,572 adults and followed them for about 16 years. The 432 people who developed diabetes were more likely to carry four particular species. Each one raised the risk only modestly.[6]

So, can a stool kit tell you your own risk? It can’t yet. An international panel of microbiome experts found that consumer gut tests have no proven value in clinical care.[7] For your own risk today, a blood test tells you more.

What each blood sugar test measures, and what it misses

In 2023, 115.2 million US adults had prediabetes. Only about 1 in 5 of them knew it.[8] The standard tests don’t all catch the same people, because each one looks at a different moment.

Test

What it measures

Prediabetes range

What it can miss

A1C

Your average blood sugar over the past 2 to 3 months[9]

5.7% to 6.4%

Some people whose sugar already runs high after meals[10]

Fasting glucose

Your blood sugar after at least 8 hours without food[9]

100 to 125 mg/dL

How your body handles a meal[10]

Two-hour glucose tolerance test

Your blood sugar two hours after a 75-gram glucose drink[9]

140 to 199 mg/dL

A rise at one hour that has settled by two hours[10]

One-hour reading in the same test

Your blood sugar one hour after the drink

155 mg/dL or higher flags risk, per the International Diabetes Federation[10]

It isn’t in the ADA’s US criteria, so ask whether your lab will report it

Stool microbiome test

Which bacteria are in one sample

None

Your diabetes risk. No test is validated for that

In the 2026 Milan study, the gut pattern tracked blood sugar after meals, not the fasting number. That’s the stage a glucose tolerance test sees.

If your A1C and fasting glucose are normal but diabetes runs in your family, ask your doctor about a glucose tolerance test. Ask for the one-hour reading too. Eat your usual carbs for three days before it. Cutting them back can push the result falsely high.

Catching this stage early matters, because lifestyle changes work here. The US Diabetes Prevention Program followed 3,234 adults with raised blood sugar for about three years. Those who changed their diet and activity had 58% fewer new cases of diabetes than those on placebo.[11]

Do probiotics help with blood sugar?

Some have promising trials, but the ones we found are small and short. In a 2020 US trial of 76 adults with type 2 diabetes, a five-strain probiotic lowered A1C by 0.6 points more than placebo over 12 weeks. The researchers work for the company that developed it.[12]

Before you buy a probiotic, feed the bacteria you already have. Only about 5% of Americans eat the recommended amount of fiber.[13]

Even fiber doesn’t work the same for everyone. In 802 Chinese adults with prediabetes, six months of added fiber did no better than usual care overall. The researchers then sorted people into four groups by age, weight, A1C and insulin measures. Two of the groups benefited. Gut bacteria predicted who would respond.[14]

Why digestion belongs in the diabetes conversation

All of this research comes back to digestion. Ayurvedic Science starts there too. It describes madhumeha, a condition marked by high blood sugar and excess urination. It traces madhumeha back to weakened agni, your body’s digestive fire. [15] A gut pattern that shows up before routine blood tests turn abnormal fits that view. Our guide to agni explains how it works.

How LYBL supports your blood sugar

Diabetes care often centers on the glucose number and the medicine that lowers it. That’s important work, and it should continue. However, this research tracked how your blood sugar behaves after meals. That depends on how you eat, digest, sleep, and handle stress.

Because people respond to the same food differently, our Diabetes Program starts with you. First comes the health assessment then a free consultation of up to 60 minutes if needed with a Holistic Health Physician. Your plan includes nutrition guidance tailored to your body type, habits, season, and location. It also includes targeted formulations, yoga therapy and lifestyle support along with a 30-minute follow-up every month.

In a 90-day LYBL pilot, five adults with type 2 diabetes added the program’s protocol to the medicines they already took. Their average A1C fell from 7.07% to 6.02%. Their fasting glucose went from 124 to 119 mg/dL. All five changed their food, movement, and formulations at once. Read more about it here.

Frequently asked questions

Can a stool test tell me if I’ll get type 2 diabetes?

No stool test has been validated to predict type 2 diabetes for one person. Large studies link certain gut bacteria to diabetes only across groups of people. An A1C, fasting glucose or glucose tolerance test is still how to check your risk.

Does metformin change your gut bacteria?

Yes, studies comparing people on and off metformin show it shifts gut bacteria. That may be part of how it lowers blood sugar, and why it can upset the stomach. Don’t stop or change your dose without talking to your doctor.

What are the signs your gut bacteria are unhealthy?

For blood sugar, there usually aren’t any you can feel. In a 2026 study, the gut pattern linked to diabetes showed up in people whose routine blood tests were normal but whose blood sugar took longer to come down after meals. A glucose tolerance test can detect that.

Can a leaky gut cause high blood sugar?

It may play a part. When bacterial fragments called endotoxin get into the blood, they can feed low-grade inflammation. In 7,169 Finnish adults followed for 10 years, those with the most endotoxin had a 52% higher risk of diabetes than those with the least.

Does this research apply to women?

It applies better than most microbiome research does. Nearly three in four of the 229,025 people in the 2026 study were women, with an average age of 50.

References

  1. Sanna S, van Zuydam NR, Mahajan A, et al. Causal relationships among the gut microbiome, short-chain fatty acids and metabolic diseases. Nat Genet. 2019;51(4):600-605. View original source ↩
  2. Forslund K, Hildebrand F, Nielsen T, et al. Disentangling type 2 diabetes and metformin treatment signatures in the human gut microbiota. Nature. 2015;528(7581):262-266. View original source ↩
  3. Zhao L, Zhang F, Ding X, et al. Gut bacteria selectively promoted by dietary fibers alleviate type 2 diabetes. Science. 2018;359(6380):1151-1156. View original source ↩
  4. Pussinen PJ, Havulinna AS, Lehto M, Sundvall J, Salomaa V. Endotoxemia is associated with an increased risk of incident diabetes. Diabetes Care. 2011;34(2):392-397. View original source ↩
  5. Baldanzi G, Bermingham KM, Spadazzi R, et al. Human gut microbiome species linked to type 2 diabetes, insulin resistance, and glucose metabolism in >200,000 individuals. Abstract 291. European Association for the Study of Diabetes Annual Meeting; 28 September to 2 October 2026; Milan, Italy. Press release: View original source ↩
  6. Ruuskanen MO, Erawijantari PP, Havulinna AS, et al. Gut microbiome composition is predictive of incident type 2 diabetes in a population cohort of 5,572 Finnish adults. Diabetes Care. 2022;45(4):811-818. View original source ↩
  7. Porcari S, Mullish BH, Asnicar F, et al. International consensus statement on microbiome testing in clinical practice. Lancet Gastroenterol Hepatol. 2025;10(2):154-167. View original source ↩
  8. Centers for Disease Control and Prevention. National Diabetes Statistics Report. View original source ↩
  9. American Diabetes Association Professional Practice Committee for Diabetes. 2. Diagnosis and classification of diabetes: Standards of Care in Diabetes-2026. Diabetes Care. 2026;49(Suppl 1):S27-S49. View original source 123
  10. Bergman M, Manco M, Satman I, et al. International Diabetes Federation position statement on the 1-hour post-load plasma glucose for the diagnosis of intermediate hyperglycaemia and type 2 diabetes. Diabetes Res Clin Pract. 2024;209:111589. View original source 1234
  11. Knowler WC, Barrett-Connor E, Fowler SE, et al. Reduction in the incidence of type 2 diabetes with lifestyle intervention or metformin. N Engl J Med. 2002;346(6):393-403. View original source ↩
  12. Perraudeau F, McMurdie P, Bullard J, et al. Improvements to postprandial glucose control in subjects with type 2 diabetes: a multicenter, double blind, randomized placebo-controlled trial of a novel probiotic formulation. BMJ Open Diabetes Res Care. 2020;8(1):e001319. View original source ↩
  13. Quagliani D, Felt-Gunderson P. Closing America’s fiber intake gap: communication strategies from a food and fiber summit. Am J Lifestyle Med. 2017;11(1):80-85. View original source ↩
  14. Song D, Feng G, Ma Y, et al. Gut microbiome predicts personalized responses to dietary fiber in prediabetes: a randomized, open-label trial. Nat Commun. 2025;16(1):11506. View original source ↩
  15. Xi, Y., & Xu, P. F. (2021). Diabetes and gut microbiota. World journal of diabetes, 12(10), 1693–1703. View original source ↩