Does fiber help control blood sugar?

Yes, if you have diabetes, and by a modest amount. A 2020 meta-analysis of 42 trials in 1,789 adults with prediabetes or diabetes found that eating more fiber lowered HbA1c by 2.00 mmol/mol and fasting glucose by 10 mg/dL (0.56 mmol/L) compared with lower fiber diets. The fasting glucose evidence was graded high, though the trials varied a lot. In adults with overweight but no diabetes, 51 trials of fiber supplements found fasting glucose fell by only 1.3 mg/dL (0.07 mmol/L).

Established. In people with type 2 diabetes, soluble and viscous fiber lower long-term blood sugar. A 2021 meta-analysis of 29 trials in 1,517 adults found soluble fiber supplements lowered HbA1c by 0.63 percentage points, with a 95 percent confidence interval of 0.37 to 0.90. That is a real and modest change, found across trials that varied substantially.

What the trials found

Two more meta-analyses limited to type 2 diabetes land in the same place. Viscous fiber at a median of about 13 g a day, given in 28 trial comparisons to 1,394 people on top of standard care, lowered HbA1c by 0.58 percentage points and fasting glucose by 15 mg/dL (0.82 mmol/L), on moderate certainty evidence. Soluble fiber supplements in 29 trials lowered fasting glucose by 16 mg/dL (0.89 mmol/L), and the dose-response analysis pointed to 7.6 to 8.3 g a day. The reviewers noted substantial differences between trials and asked for longer, better ones.

The size of the effect depends on where you start. A meta-analysis of 35 psyllium trials found the glucose-lowering effect was proportional to fasting glucose at the start. There was no significant lowering in people with normal glucose, a modest improvement in prediabetes, and the largest in people being treated for type 2 diabetes. In that last group psyllium taken before meals lowered fasting glucose by 37.0 mg/dL and HbA1c by 0.97 percentage points. Part of that evidence comes from internal reports of Procter and Gamble, which makes the psyllium supplement Metamucil, and the abstract does not give the number of participants.

Pregnancy is the other group where fiber has trials behind it. A 2025 meta-analysis of 20 trials in 1,061 pregnant women with diabetes, mostly gestational, found more fiber lowered fasting glucose by 6.3 mg/dL (0.35 mmol/L) and glucose two hours after a meal by 16 mg/dL (0.90 mmol/L). The odds of needing insulin were 76 percent lower (odds ratio 0.24), and the abstract does not give the share of women who needed it in each group, so we cannot turn that into an absolute number. Most of those trials were unblinded and at high risk of bias, and they disagreed with each other a great deal.

Not all fibers are equal. A 2023 network meta-analysis of 46 trials in 2,685 people with type 2 diabetes compared 16 soluble fibers and ranked galactomannans, a class of plant gums, first for lowering HbA1c and fasting glucose. A ranking says which fiber came out ahead most often. It does not say by how much, and most of the comparisons were low or moderate certainty.

The single trials are smaller and weaker. In a randomized trial of 40 people with type 2 diabetes, 10.5 g of psyllium a day for 8 weeks took HbA1c in the psyllium group from 8.5 to 7.5 percent. Those are before and after numbers in one group, and the control group ate their usual diet with no placebo.

Unsettled. Nobody knows whether the effect lasts. The 2020 review found no controlled trial of more fiber in adults with diabetes that ran for 12 months or longer. Every number above comes from trials of weeks or months.

Observational data point the same way for preventing diabetes, which is a different question from controlling it. A 2019 Lancet series of meta-analyses of prospective studies linked the highest fiber intakes with a 15 to 30 percent lower incidence of type 2 diabetes than the lowest. That is an association, and the reports give it in relative terms only.

Who should be careful

Not for everyone. If you take levothyroxine for your thyroid, the approved US label says dietary fiber may bind the drug and lower how much you absorb. A big change in how much fiber you eat is worth mentioning to whoever checks your thyroid levels.

Fiber supplements can cause wind. In a 2022 meta-analysis of 16 trials in 1,251 adults with chronic constipation, flatulence was higher on fiber than on control, with a standardized difference of 0.80. Those were people with constipation and not diabetes, but the supplements are the same kind. The 2023 WHO carbohydrate guideline found no undesirable effects of eating at least 25 g of fiber a day, with one exception: very low certainty evidence of a higher risk of endometrial cancer with higher intakes. If you take insulin or tablets that lower blood sugar, we found no trial that measured whether adding fiber changes the risk of glucose running too low, so a big change is worth discussing with whoever manages your diabetes.

What expert bodies say

The World Health Organization's 2023 guideline recommends at least 25 g of dietary fiber a day for adults, as a strong recommendation for the general population and not specifically for blood sugar. The European Union is narrower. It authorizes a blood sugar claim only for beta-glucans from oats or barley eaten as part of a meal, which may say they reduce the rise in blood glucose after that meal. It allows no blood sugar claim for fiber in general.

How we searched

What would change this answer

• A controlled trial of more fiber in people with diabetes lasting a year or longer. Without one, we do not know whether the drop in HbA1c holds.
• A trial that measures low blood sugar episodes when fiber supplements are added to insulin or glucose-lowering tablets.
• Head-to-head trials of the fibers themselves at matched doses. The ranking that put galactomannans first comes from indirect comparisons.
• Independent psyllium trials reported in full, since part of the current psyllium evidence comes from the manufacturer.

Related: fiber and cholesterol, fiber and weight loss, and the rest of the nutrient in the fiber guide.

Sources

1. ev-fibbs-01 · Meta-analysis or systematic review · systematic review and meta-analyses of controlled trials and prospective cohorts (GRADE) · n = 1789
   Increased fibre intakes reduced glycated haemoglobin (HbA1c; mean difference [MD] -2.00 mmol/mol, 95% CI -3.30 to -0.71 from 33 trials), fasting plasma glucose (MD -0.56 mmol/L, 95% CI -0.73 to -0.38 from 34 trials), insulin (standardised mean difference [SMD] -2.03, 95% CI -2.92 to -1.13 from 19 trials), homeostatic model assessment of insulin resistance (HOMA IR; MD -1.24 mg/dL, 95% CI -1.72 to -0.76 from 9 trials), total cholesterol (MD -0.34 mmol/L, 95% CI -0.46 to -0.22 from 27 trials), low-density lipoprotein (LDL) cholesterol (MD -0.17 mmol/L, 95% CI -0.27 to -0.08 from 21 trials), triglycerides (MD -0.16 mmol/L, 95% CI -0.23 to -0.09 from 28 trials), body weight (MD -0.56 kg, 95% CI -0.98 to -0.13 from 18 trials), Body Mass Index (BMI; MD -0.36, 95% CI -0·55 to -0·16 from 14 trials), and C-reactive protein (SMD -2.80, 95% CI -4.52 to -1.09 from 7 trials) when compared with lower fibre diets.
   
   Who: adults with prediabetes, gestational diabetes, type 1 or type 2 diabetes in controlled trials of higher fiber intake without other lifestyle change
   Effect: HbA1c MD -2.00 mmol/mol (95% CI -3.30 to -0.71, 33 trials); fasting plasma glucose MD -10 mg/dL (-0.56 mmol/L) (-0.73 to -0.38, 34 trials); fasting glucose evidence graded high
   Certainty: High heterogeneity in all RCT analyses; the effect did not depend on fiber type or diabetes type. Full text PMC7059907 read (GRADE: fasting glucose high because of dose dependence).
   PLoS Med, 2020 · checked 2026-10-07 · we read the abstract
2. ev-fibbs-02 · Meta-analysis or systematic review · systematic review and meta-analysis of RCTs (GRADE) · n = 1394
   Viscous fiber at a median dose of ∼13.1 g/day significantly reduced HbA1c (MD -0.58% [95% CI -0.88, -0.28]; P = 0.0002), fasting blood glucose (MD -0.82 mmol/L [95% CI -1.32, -0.31]; P = 0.001), and HOMA-insulin resistance (IR) (MD -1.89 [95% CI -3.45, -0.33]; P = 0.02) compared with control and in addition to standard of care.
   
   Who: people with type 2 diabetes in RCTs of 3 weeks or longer, search to June 2018
   Effect: HbA1c MD -0.58% (95% CI -0.88 to -0.28); fasting glucose MD -15 mg/dL (-24 to -5.6; metric: -0.82 mmol/L, -1.32 to -0.31); HOMA-IR MD -1.89; moderate certainty
   Certainty: Viscous fiber supplements (psyllium, beta-glucan, guar, konjac); no full text in PMC (no_pmc).
   Diabetes Care, 2019 · checked 2026-10-07 · we read the abstract
3. ev-fibbs-03 · Meta-analysis or systematic review · systematic review and dose-response meta-analysis of RCTs (GRADE) · n = 1517
   Results showed that supplemental soluble dietary fiber significantly reduced glycosylated hemoglobin (HbA1c, MD -0.63%, 95% CI [-0.90, -0.37]; P < 0.00001), fasting plasma glucose (FPG, MD -0.89 mmol/L, 95% CI [-1.28, -0.51]; P < 0.00001), fasting insulin (SMD -0.48, 95% CI [-0.80, -0.17]; P = 0.003), homeostatic model assessment of insulin resistance (HOMA-IR, SMD -0.58, 95% CI [-0.86, -0.29], P < 0.0001), fructosamine (SMD -1.03, 95% CI [-1.51, -0.55]; P < 0.0001), 2-h postprandial plasma glucose (SMD -0.74, 95% CI [-1.00, -0.48]; P < 0.00001), and BMI (SMD -0.31, 95% CI [-0.61, -0.00], P = 0.05) compared with control diets in patients with type 2 diabetes.
   
   Who: adults with type 2 diabetes in RCTs of soluble fiber supplements, search to February 2020
   Effect: HbA1c MD -0.63% (95% CI -0.90 to -0.37); fasting plasma glucose MD -16 mg/dL (-23 to -9.2; metric: -0.89 mmol/L, -1.28 to -0.51); dose-response pointed to 7.6 to 8.3 g/day
   Certainty: Substantial heterogeneity; the authors call for longer high-quality RCTs.
   Clin Nutr, 2021 · checked 2026-10-07 · we read the abstract
4. ev-fibbs-04 · Meta-analysis or systematic review · meta-analyses of randomized controlled studies (8 meta-analyses) · n = —
   Glycemic effects were proportional to baseline FBG; no significant glucose lowering was observed in euglycemic subjects, a modest improvement was observed in subjects with pre-T2DM, and the greatest improvement was observed in subjects who were being treated for T2DM.
   
   Who: non-diabetic, prediabetic and type 2 diabetic participants in 35 randomized controlled studies
   Effect: in type 2 diabetes, psyllium before meals: fasting glucose -37.0 mg/dL, HbA1c -0.97%; modest in prediabetes; none significant in euglycemic people
   Certainty: Part of the data are internal reports of Procter & Gamble (maker of Metamucil); no N of participants in the abstract.
   Am J Clin Nutr, 2015 · checked 2026-10-07 · we read the abstract
5. ev-fibbs-05 · Meta-analysis or systematic review · systematic review and meta-analysis of RCTs · n = 3420
   Dietary fiber supplementation improved fasting glucose concentrations -0.07 mmol/L (95 % CI: -0.12, -0.02; P = 0.0005; I2 = 54 %), reduced fasting insulin levels -5.89 pmol/L (95 % CI -9.18, -2.60, P = 0.0004; I2 = 89 %), and decreased HOMA-IR -0.38 (95 % CI: -0.68, -0.08; P < 0.00001; I2 = 94 %).
   
   Who: adults with overweight or obesity without type 2 diabetes, RCTs of 1 to 12 months versus placebo
   Effect: fasting glucose -1.3 mg/dL (95% CI -2.2 to -0.36; metric: -0.07 mmol/L, -0.12 to -0.02); fasting insulin -5.89 pmol/L; HOMA-IR -0.38
   Certainty: Clinically small effect on glucose in people without diabetes; more noticeable on insulin resistance.
   Clin Nutr, 2025 · checked 2026-10-07 · we read the abstract
6. ev-fibbs-06 · Meta-analysis or systematic review · rapid review with meta-analyses of RCTs (GRADE) · n = 499
   Pooled findings indicate that higher fiber higher carbohydrate diets reduced HbA1c (mean difference [MD] -0.50% [95% confidence interval -0.99 to -0.02]), fasting insulin (MD -0.99 μIU/mL [-1.83 to -0.15]), total cholesterol (MD -0.16 mmol/L [-0.27 to -0.05]) and low-density lipoprotein cholesterol (MD -0.16 mmol/L (-0.31 to -0.01) when compared with lower carbohydrate lower fiber diets.
   
   Who: people with type 1 or type 2 diabetes (98% type 2) in RCTs that changed both fiber and carbohydrate amount
   Effect: HbA1c MD -0.50% (95% CI -0.99 to -0.02); fasting insulin MD -0.99 uIU/mL; moderate or high certainty
   Certainty: Diets are compared, not supplements; RCTs with calorie restriction excluded.
   Obes Rev, 2025 · checked 2026-10-07 · we read the abstract
7. ev-fibbs-07 · Meta-analysis or systematic review · systematic review and meta-analysis of RCTs · n = 1061
   On meta-analysis, a higher fibre intake was associated with reduced FBG (MD: -0.35 mmol/L, 95% CI: -0.53, -0.18, p < 0.01), PBG (MD: -0.90 mmol/L, 95% CI: -1.39, -0.40, p < 0.01) and requirement for insulin (OR: 0.24, 95% CI: 0.13, 0.46, p < 0.01).
   
   Who: pregnant women with diabetes in pregnancy (mostly gestational) in RCTs of high-fiber diets or fiber supplements
   Effect: fasting glucose MD -6.3 mg/dL (95% CI -9.5 to -3.2; metric: -0.35 mmol/L, -0.53 to -0.18); 2-h postprandial glucose MD -16 mg/dL (-25 to -7.2; metric: -0.90 mmol/L, -1.39 to -0.40); need for insulin OR 0.24 (0.13 to 0.46)
   Certainty: Most RCTs have a high risk of bias (no blinding), heterogeneity >90%.
   Diabet Med, 2025 · checked 2026-10-07 · we read the abstract
8. ev-fibbs-08 · Meta-analysis or systematic review · systematic review and Bayesian network meta-analysis of RCTs (GRADE) · n = 2685
   Galactomannans had the highest effect on reducing the levels of HbA1c (SUCRA: 92.33%) and fasting blood glucose (SUCRA: 85.92%).
   
   Who: adults with type 2 diabetes in RCTs of 16 soluble fiber types
   Effect: galactomannans ranked first for HbA1c (SUCRA 92.33%) and fasting glucose (SUCRA 85.92%); psyllium first for HOMA-IR; most comparisons low or moderate certainty
   Certainty: SUCRA ranking, not absolute effect size; examples of galactomannans (guar, fenugreek) come from the class name, not listed in the abstract.
   Am J Clin Nutr, 2023 · checked 2026-10-07 · we read the abstract
9. ev-fibbs-09 · Randomized controlled trial(s) · randomized controlled trial, 8 weeks, usual-diet control · n = 40
   Moreover, water soluble fiber supplementation proven to improve FBS (163 to 119 mg/dl), HbA1c (8.5 to 7.5 %), insulin level (27.9 to 19.7 μIU/mL), C-peptide (5.8 to 3.8 ng/ml), HOMA.IR (11.3 to 5.8) and HOMA-β % (103 to 141 %).
   
   Who: type 2 diabetes patients, non-smokers, aged over 35
   Effect: fasting glucose 163 to 119 mg/dL; HbA1c 8.5% to 7.5% in the psyllium group over 8 weeks versus usual diet
   Certainty: Small RCT without placebo; the numbers are before/after in the intervention group.
   Nutr J, 2016 · checked 2026-10-07 · we read the abstract
10. ev-fibbs-10 · Observational data · series of systematic reviews and meta-analyses of prospective cohort studies and trials · n = —
    Observational data suggest a 15-30% decrease in all-cause and cardiovascular related mortality, and incidence of coronary heart disease, stroke incidence and mortality, type 2 diabetes, and colorectal cancer when comparing the highest dietary fibre consumers with the lowest consumers Clinical trials show significantly lower bodyweight, systolic blood pressure, and total cholesterol when comparing higher with lower intakes of dietary fibre.
    
    Who: adults without chronic disease in 185 prospective studies, about 135 million person-years
    Effect: 15-30% lower incidence of type 2 diabetes and other outcomes, highest vs lowest fiber intake; greatest benefit at 25-29 g/day; moderate certainty
    Certainty: This is about the onset of diabetes in cohorts, not about glucose control; level C. No full text in PMC (no_pmc).
    Lancet, 2019 · checked 2026-10-07 · we read the abstract
11. ev-fibbs-11 · Meta-analysis or systematic review · systematic review and meta-analysis of RCTs · n = 1251
    Flatulence was higher in fiber groups compared to control groups(SMD: 0.80; 95% CI: 0.47, 1.13; P < 0.00001).
    
    Who: adults with chronic constipation in RCTs of fiber supplements
    Effect: flatulence SMD 0.80 (95% CI 0.47 to 1.13) fiber vs control
    Certainty: Population with constipation, not diabetes; tolerability of fiber supplements in general. Practice: increase the dose gradually.
    Am J Clin Nutr, 2022 · checked 2026-10-07 · we read the abstract
12. ev-fibbs-12 · Position of an expert body · FDA-approved drug label, drug interactions section · n = —
    Soybean flour, cottonseed meal, walnuts, and dietary fiber may bind and decrease the absorption of levothyroxine sodium from the gastrointestinal tract.
    
    Who: people taking levothyroxine
    Effect: decreased levothyroxine absorption with dietary fiber
    Certainty: Label text; when the amount of fiber changes, steady intake and TSH monitoring.
    Levothyroxine sodium, FDA-approved label (DailyMed), effective 2026-03-24 · checked 2026-10-07 · we read the section
13. ev-fibbs-13 · Position of an expert body · regulatory register entry · n = —
    POL-HC-6331 Beta-glucans from oats and barley Consumption of beta-glucans from oats or barley as part of a meal contributes to the reduction of the blood glucose rise after that meal Authorised
    
    Who: EU Register of nutrition and health claims, claim POL-HC-6331
    Effect: authorized
    Certainty: The conditions of use (4 g beta-glucan per 30 g of available carbohydrates) are not given in the register snapshot.
    EU Register on nutrition and health claims, claim POL-HC-6331, snapshot 2026-09-21 · checked 2026-10-07 · we read the dataset
14. ev-fibbs-14 · Position of an expert body · WHO guideline (NUGAG, GRADE) · n = —
    With the exception of increased risk of endometrial cancer with higher intakes of dietary fibre (very low certainty evidence), no undesirable effects were identified with dietary fibre intakes of at least 25 g per day, and no mitigating factors were identified that would argue against dietary fibre intake at this level.
    
    Who: adults, WHO guideline recommendation 4
    Effect: at least 25 g/day dietary fiber for adults (strong recommendation); 25-29 g/day showed greatest benefit across outcomes
    Certainty: Recommendation for the general population, not specific to glucose.
    WHO, 2023, Carbohydrate intake for adults and children: WHO guideline summary · checked 2026-10-07 · we read the section
15. ev-fibbs-15 · Meta-analysis or systematic review · systematic review and meta-analyses of controlled trials and prospective cohorts (GRADE) · n = 1789
    Potential limitations were the lack of prospective cohort data in non-European countries and the lack of long-term (12 months or greater) controlled trials of increasing fibre intakes in adults with diabetes.
    
    Who: adults with prediabetes or diabetes, 42 trials and 2 cohorts
    Effect: no long-term (12 months or more) controlled trials; cohort data only from European countries
    Certainty: Knowledge boundary: there are no long-term RCTs.
    PLoS Med, 2020 · checked 2026-10-07 · we read the abstract