Gut Health and Blood Sugar: What the Evidence Shows

Gut health and blood sugar are linked, but the evidence is less settled than many headlines suggest. People with type 2 diabetes tend to carry different gut bacteria, and trials that try to change those bacteria show small and uncertain effects on glucose.^[1][2]

This guide covers what studies in people show on gut bacteria and diabetes, probiotics, fermented foods, fiber, metformin and sweeteners. It keeps human and animal evidence apart. For the wider picture, see our guide to smart nutrition choices. This guide is general education. If you use insulin or other glucose medicines, follow the plan your diabetes team gave you.

How are gut health and blood sugar linked?

Mostly by association so far. A 2015 study in Nature looked again at 784 gut samples from Denmark, Sweden and China. It showed that diabetes medicines distort the picture. Once the authors accounted for metformin, people with type 2 diabetes still had fewer bacteria that make butyrate, a short-chain fatty acid.^[1] The work was funded by EU research programmes, and some of the groups belonged to a centre backed by the Novo Nordisk Foundation.

Why would butyrate matter? Gut bacteria ferment fiber into short-chain fatty acids. In mouse and cell studies, these fatty acids triggered the release of GLP-1, a gut hormone that helps the body release insulin after a meal. Mice that lacked the receptors for these fatty acids released less GLP-1 and handled glucose less well.^[7] Part of that 2012 work ran at a lab of the drug firm Takeda. GLP-1 is also the hormone that drugs such as semaglutide copy, as our Ozempic guide explains.

So the chain from gut bacteria to blood sugar is plausible. In people it still rests mostly on association. Keep that in mind when you read bold claims about gut health and diabetes.

Do probiotics help blood sugar?

The effect in trials is small and uncertain. A 2025 meta-analysis in the Journal of Clinical Endocrinology and Metabolism pooled 54 randomised trials with 3,390 adults with type 2 diabetes. It covered probiotics, prebiotics, synbiotics and faecal transplants.^[2]

For probiotics alone, HbA1c differed from placebo by 0.19 percentage points on average (about 2 mmol/mol) across 22 comparisons. Fasting glucose differed by 7.88 mg/dL (about 0.44 mmol/L) across 29 comparisons.^[2] Our HbA1c guide explains what those units mean.

Look at the limits before reading much into probiotics and blood sugar. The trials ran for a median of 8 weeks and used many different strains. In the six European comparisons there was no measurable difference in HbA1c. The authors also found signs of publication bias for HbA1c. That means trials with null results may be missing, so the pooled figure may be too high.^[2] That matters. The review had public funding in China, and the authors declared no conflicts.

No major guideline recommends probiotics for blood sugar. The 2026 ADA Standards of Care make no recommendation on them, and the NICE guideline for type 2 diabetes in adults does not mention them.^[3][4]

Are fermented foods good for type 2 diabetes?

Most of the evidence is about the risk of getting type 2 diabetes, not about managing it once you have it. In 2024 the FDA announced a qualified health claim for yogurt. It found some credible evidence for a link between yogurt and a reduced risk of type 2 diabetes, but said this evidence is limited. Its minimum is 2 cups (about 475 mL, or 3 servings) a week.^[5] The petition came from Danone North America, a yogurt maker.

Much of that rests on cohort studies. In three large US cohorts, each daily serving of yogurt was associated with a 17% smaller risk of type 2 diabetes (hazard ratio 0.83). The people studied were mostly health professionals of European ancestry.^[8] One author sat on a Unilever scientific advisory board. Studies like these show association, not cause.

What happens when people eat a lot of fermented food? A 2021 trial in Cell put 36 healthy adults on a high-fiber or a high-fermented-food diet in a 17-week study. The fermented food group went from about 0.4 to 6.3 servings a day. Their gut bacteria became more diverse and some inflammatory markers fell. Blood glucose and insulin did not differ between the groups or over time.^[9] Some authors are founders or shareholders of microbiome and glucose technology companies.

So fermented foods and type 2 diabetes are linked in cohort data, but these trials showed no glucose change. Plain yogurt can still fit a balanced plate. Our guide to milk and blood sugar covers other dairy choices.

Fermented foods and probiotics at a glance

Does fiber feed gut bacteria?

Yes. Gut bacteria ferment fiber. The ADA says fiber changes the make-up of gut bacteria and increases their diversity. It adds that there is still much to learn about the gut microbiome, but that higher-fiber eating patterns are advantageous.^[3]

A 2018 trial in Science gave adults with type 2 diabetes in China a specially designed high-fiber diet. People whose fiber-fed bacteria were more diverse and more abundant had a bigger change in HbA1c, partly through more GLP-1.^[12] It was a single-site trial in China, and it had public funding in China.

How much fiber? The ADA advises people with diabetes to eat at least 14 g (about 0.5 oz) per 1,000 kcal, with at least half of grains whole and intact.^[3] For someone eating 2,000 kcal a day, that is 28 g (1.0 oz). The NHS says adults should aim for 30 g (about 1.1 oz) a day, while most eat about 20 g (0.7 oz). It counts a food as high in fiber when it has 6 g (0.21 oz) or more per 100 g (3.5 oz).^[13] NICE advises high-fiber, low glycaemic index carbohydrates such as fruit, vegetables, wholegrains and pulses.^[4] Our fiber guide has easy ways to add more.

Can people with diabetes take probiotics safely?

For most people with a healthy immune system probiotics appear to be safe, but they are not tested like medicines. The NHS says probiotics are generally classed as food rather than medicine, so they do not go through the same testing.^[11] It advises anyone with an existing health condition or a weakened immune system to talk to a doctor before taking them. If you have diabetes, start with your GP or primary care team.

The NHS page does not deal with diabetes as such. The 2025 meta-analysis also found that trials reported side effects in an incomplete and inconsistent way, so risks may be underestimated.^[2] If you want to try a probiotic, keep taking your prescribed medicines and tell your GP or primary care team.

Changing what you eat can change your glucose. The NHS treats a reading below 4 mmol/L (72 mg/dL) as low blood sugar.^[14] In the US a low is a reading below 70 mg/dL (3.9 mmol/L).^[15] Lows are more likely with insulin or a sulfonylurea such as gliclazide or glimepiride.^[14] In the UK, the NHS says to call 999 if someone may have very low blood sugar and is not responding normally or is unconscious, and a glucagon injection is not available or you do not know how to use it, or they have not recovered within 10 minutes of a glucagon injection, or they have been drinking alcohol.^[14] In the US, the ADA says to call 911 right away if someone is unconscious and glucagon is not available or no one knows how to use it.^[15]

Does metformin affect the gut?

Yes, in two ways. First, digestive side effects are common. In the placebo-controlled trial on the US label, diarrhea affected 53.2% of people on metformin against 11.7% on placebo. Nausea or vomiting affected 25.5% against 8.3%. Diarrhea led 6% of people on metformin to stop taking it.^[6] Our metformin side effects guide has more. Talk to your clinician before changing a dose. Severe vomiting or diarrhea can cause dehydration, and the US metformin label and the NHS warn about a rare, serious side effect called lactic acidosis.^[6][16] Call 999 or NHS 111 in the UK, or 911 in the US, if you take metformin and get severe nausea, vomiting or stomach pain, fast or shallow breathing, unusual muscle pain, or feel very weak, cold or dizzy.^[6][16]

Second, metformin changes the gut bacteria themselves. In a 2017 randomised trial in Nature Medicine, adults with type 2 diabetes who had not yet taken diabetes medicine took metformin or placebo for 4 months. Metformin had strong effects on their gut bacteria.^[17] When the team moved stool from treated people into germ-free mice, the mice handled glucose better. That part is mouse evidence. The Novo Nordisk Foundation, which controls a diabetes drug maker, helped fund the work.

This is also why gut studies in diabetes need to account for metformin.^[1]

Do artificial sweeteners affect gut bacteria?

They may, but the data come from healthy adults. In a 2022 trial in Cell, 120 healthy adults took saccharin, sucralose, aspartame or stevia sachets for 2 weeks, at doses below the acceptable daily intake. All four changed gut and mouth bacteria. As groups, saccharin and sucralose impaired the glucose response to a test drink. Aspartame and stevia did not.^[18] Effects differed from person to person. The trial did not include anyone with diabetes, and two authors are scientific co-founders of DayTwo, a personalised nutrition company.

The ADA still says people with diabetes can use non-nutritive sweeteners in place of sugar-sweetened products, in moderation and for the short term.^[3] Our sweeteners guide compares the options.

What can you do for gut health and blood sugar?

Start with food. Four steps:

1. Build meals around vegetables, pulses and whole grains, as NICE and the ADA advise.^[4][3]
2. Add plain yogurt or other fermented foods if you enjoy them, and count their carbohydrates.^[5][10]
3. Talk to your GP or primary care team before trying a probiotic supplement.^[11]
4. Keep taking your prescribed medicines. If metformin upsets your stomach, tell your GP or primary care team rather than stopping it on your own.^[6]

Questions to ask your diabetes team

• How much fiber should I aim for each day, and should I raise it gradually?^[3][13]
• Is a probiotic supplement safe for me given my health conditions and medicines?^[11]
• What should I do if metformin causes ongoing diarrhea or stomach upset?^[6][16]
• Do I need to adjust my mealtime insulin or carb count when adding yogurt or high-fiber foods?^[3]

Frequently asked questions

Can probiotics replace diabetes medicine?

No. No major guideline recommends probiotics for blood sugar, and the effects in trials were small and uncertain. Do not stop or change a prescribed medicine without your clinician.^[3][2]

Is yogurt good for type 2 diabetes?

It can fit. The FDA allows a qualified claim that eating yogurt regularly may reduce the risk of type 2 diabetes, but it calls the evidence limited. The claim covers risk, not people who already have diabetes.^[5]

Does metformin cause diarrhea?

It can. In the trial on the US label, diarrhea affected 53.2% of people on metformin and 11.7% on placebo. Talk to your clinician before changing a dose.^[6]

How much fiber should a person with diabetes eat?

At least 14 g (about 0.5 oz) per 1,000 kcal, according to the ADA. The NHS advises 30 g (about 1.1 oz) a day for adults, while most eat about 20 g (0.7 oz).^[3][13]

Where to go next

Looking after your gut when you have diabetes comes down to everyday food choices, steady fiber and talking to your care team before adding supplements.

• Fiber and blood sugar, for easy ways to eat more of it.
• Metformin side effects, if your stomach struggles with it.
• Is Diabec legit?, for what is in our supplement and what it does not do.

Note: If you are pregnant, breastfeeding, or considering giving any supplement or herbal product to a child, consult your healthcare provider before making any changes to your diet, supplement routine, or prescribed medications.