Impact of dietary fiber intake on insulin resistance in type 2 diabetes: A systematic review

Abstract

This systematic review examines the impact of dietary fiber intake on insulin resistance in individuals with type 2 diabetes mellitus (T2DM). Given the global rise in T2DM prevalence and the central role of insulin resistance in its pathophysiology, there is an increasing emphasis on nonpharmacological interventions, such as dietary fiber, to manage glycemic outcomes. A comprehensive search was conducted across six databases including PubMed, Scopus, Web of Science, ProQuest, CINAHL, and CENTRAL, focusing exclusively on randomized controlled trials (RCTs) up to June 2022. Thirteen RCTs, involving a total of 641 participants, were included in the final synthesis. The interventions varied in fiber type (soluble, insoluble, and mixed), source (whole grains, legumes, flaxseed, composite flour), and dosage (5–50 g/day), with durations ranging from 1 to 6 months. Findings consistently demonstrated that dietary fiber, especially soluble and mixed types, significantly improved insulin resistance as measured by HOMA-IR and related indices. Additionally, notable improvements were observed in fasting glucose, HbA1c, LDL cholesterol, and body weight in several trials. While the results are promising, limitations, such as short study durations, small sample sizes, heterogeneity in intervention protocols, and limited long-term data, constrain broader generalization. Despite these challenges, the evidence strongly supports dietary fiber as an effective adjunct in managing insulin resistance in T2DM. The review underscores the need for longer-duration, multicenter RCTs with standardized fiber interventions to confirm findings and inform clinical practice. Dietary fiber should be integrated into individualized diabetes management strategies to enhance metabolic outcomes and overall health.

Introduction

India is facing a significant challenge with the triple burden of obesity, diabetes, and prediabetes, with an estimated 77 million adults over 18 years living with T2DM and nearly 25 million in the prediabetic stage.[1] Globally, diabetes cases increased from 108 million in 1980 to 463 million in 2019, with projections reaching 700 million by 2045.[2] Insulin resistance, the primary pathophysiological mechanism underlying T2DM, affects not only glucose metabolism but also lipid profiles, inflammatory markers, and cardiovascular risk.[3] Dietary fiber intake, which averages only 15 g/day in most populations (well below the recommended 25-35 g/day), represents a modifiable risk factor with the potential to impact millions of individuals worldwide.[4,5]

Dietary interventions, known as “Medical Nutrition Therapy,” continue to be a fundamental and highly effective approach for managing glucose levels. Among the most widely studied dietary patterns, there is growing interest in investigating the therapeutic effects of dietary fiber, which is not digested or absorbed in the human small intestine and is strongly linked to gut hormones and microbiota.[6] One of the factors influencing plasma glucose levels is carbohydrate intake, which is known to cause variations in postprandial glucose concentrations. The relationship between carbohydrate consumption and the onset of diabetes has been observed in otherwise healthy individuals.[7]

Furthermore, a recent meta-analysis of randomized trials demonstrated that substituting high-glycemic index (GI) foods with low-GI alternatives can enhance glycemic control in individuals with diabetes.[8] The plasma glycemic and insulinemic responses to a carbohydrate load vary based on the type of food consumed. Adding viscous fibers can moderate glycemic responses. Epidemiological studies suggest that high dietary fiber intake is linked to a lower risk of developing diabetes and may slow the progression of type 2 diabetes.

Despite established dietary guidelines, there remains significant uncertainty regarding the optimal type, dosage, and duration of dietary fiber interventions for managing insulin resistance in type 2 diabetes mellitus (T2DM).[9] While individual studies have shown promising results, the heterogeneity in fiber sources, intervention protocols, and outcome measures has created gaps in translating research findings into clinical practice.[10] This systematic review addresses the critical need to synthesize available evidence and provide clear guidance for clinicians and patients.

Current evidence on dietary fiber’s effects on insulin resistance in T2DM is fragmented across multiple small-scale studies with varying methodologies, making it difficult for healthcare providers to make evidence-based recommendations.[11] A comprehensive systematic review is essential to consolidate this evidence, identify the most effective fiber interventions, and establish clear clinical guidelines. This synthesis is particularly crucial given the rising healthcare costs associated with diabetes management and the need for cost-effective, accessible interventions.[12,13]

This systematic review aims to provide healthcare professionals, policymakers, and patients with evidence-based recommendations for incorporating dietary fiber into T2DM management protocols. Specifically, we seek to determine the optimal types and doses of dietary fiber that significantly improve insulin resistance, identify the most clinically relevant outcomes, and establish practical guidelines for implementation in diverse healthcare settings.

Materials and Methods

Protocol registration and reporting

This systematic review was conducted and documented in accordance with the 2020 Preferred Reporting Items for Systematic Reviews guidelines. The review protocol has been registered with PROSPERO.

Eligibility requirements

Participants

This review included adult individuals diagnosed with type 2 diabetes mellitus who participated in dietary interventions involving natural sources of dietary fiber. The focus was on studies assessing the impact of such interventions on insulin resistance. Participants were not restricted by age, sex, body mass index (BMI), or duration of diabetes, provided they met the inclusion criteria and did not fall under any of the specified exclusions.

Inclusion criteria:

1. The participant group comprised individuals affected by type 2 diabetes, with no restrictions based on gender, age, ethnicity, nationality, duration of the condition, body mass index (BMI), or intervention dosage. The intervention focused on increasing dietary fiber intake from the participants’ regular diet to enhance glycemic control.

2. Insulin resistance is tested using HOMA-IR, QUIKI, Matsuda, McAuley, HES, Belfiore, Avignon, Cederholm, and Stumvoll index.

3. Studies were conducted among both male and female participants.

4. Studies with RCT and quasi-experimental design.

5. The duration of dietary fiber intake should be for more than or equal to 1 month.

6. The dietary fiber should be from natural food sources like fruits and green leafy vegetables, whole grain cereals, and pulses.

Exclusion criteria

1. Participants with type 1 diabetes, prediabetes, and gestational diabetes.

2. Type 2 diabetic patients with chronic diseases such as liver disease, renal disease, HIV, and cancer.

3. Type 2 diabetics on enteral, parenteral feeds and dietary fiber supplements.

4. Participants on insulin therapy during the intervention.

5. Studies with pre-experimental study designs, case study, case series, qualitative studies, or observational studies.

Information sources and search strategy

A systematic literature search was conducted in accordance with the PRISMA 2020 guidelines using PubMed (MEDLINE), ProQuest, Web of Science, Scopus, and CINAHL to identify randomized controlled trials (RCTs) published up to June 24, 2022. The search focused on studies assessing the impact of dietary fiber intake on insulin resistance in individuals with type 2 diabetes. A comprehensive search strategy combining Medical Subject Headings (MeSH) and free-text terms was applied. MeSH terms included “Diabetes Mellitus, Type 2,” “Insulin Resistance,” and “Dietary Fiber.” Free-text terms captured a wide range of synonyms and variations such as “NIDDM” OR “non-insulin dependent diabetes” OR “maturity-onset diabetes” OR “hyperglycemia” for diabetes, and “wheat bran” OR “roughage” OR “non-starch polysaccharides” OR “beta-glucans” OR “pectin” OR “cellulose” for dietary fiber. These terms were combined using the Boolean operator AND to ensure the inclusion of studies addressing both domains. Only English-language, peer-reviewed RCTs were considered.

Study selection

Selection process

All identified studies were systematically imported into Rayyan. Following the removal of duplicate entries, two independent investigators (SH and AM, JG and SN) conducted a thorough screening of the study titles and abstracts. Studies that fulfilled all specified inclusion criteria were subjected to a comprehensive review. The trials selected for detailed analysis were those that reached a consensus among all investigators [Figure 1].

Figure 1.

The study selection process

Data extraction

Data extraction was aimed at methodically gathering essential information from the included studies. The information gathered included various study characteristics such as the title, authors, publication year, study design, duration, and setting. It also encompassed sample size, method of randomization, unit of allocation, age, gender, and methods used to recruit participants, such as phone calls, mail, or direct approaches to clinic patients. Additional details included dosage, duration, frequency, blinding procedures, adherence monitoring, comparison group, insulin resistance, and weight reduction.

Quality assessment

In this systematic review, the risk of bias for each included randomized controlled trial was assessed using the Cochrane Risk of Bias 2 (RoB 2) tool, the risk of bias for each included randomized controlled trial was evaluated across five domains: bias from the randomization process, deviations from intended interventions, missing outcome data, outcome measurement, and selection of the reported result. Each domain was rated as low risk, some concerns, or high risk, providing an overall assessment of the study’s methodological quality.

Results

Search result

This study outlines the methodology of our literature search, which yielded 7,194 publications, from which 13 randomized controlled trials (RCTs) were selected for detailed analysis. These RCTs, published between 1981 and 2024, involved 641 participants with type 2 diabetes, with a mean age of 56.8 years (range: 18–79 years). The trials examined various dietary fiber sources and their effects on insulin resistance. Study designs included parallel, crossover, double-blind, and placebo-controlled formats, lasting from one to six months. Conducted across countries like Nigeria, Bangladesh, China, Sweden, Japan, and the United States, most were single-center studies in clinical or research settings.

Effect on insulin sensitivity

This systematic review analyzed 13 randomized controlled trials (RCTs), of which six specifically evaluated outcomes related to insulin resistance, including HOMA-IR, fasting insulin levels, and indices of insulin sensitivity. The studies encompassed a diverse range of high-fiber dietary interventions, such as whole grains, flaxseed, composite flour, and plant-based diets, to assess their effect on insulin action and glucose metabolism. Among the six trials that measured insulin resistance outcomes, a notable positive trend was observed, indicating reductions in HOMA-IR, enhancements in insulin sensitivity, and decreases in fasting insulin levels.

The most consistent improvements were identified in four studies that employed the homeostatic model assessment of insulin resistance (HOMA-IR) or related markers of insulin sensitivity. All these studies documented significant and clinically meaningful enhancements following high-fiber dietary interventions. For instance, a randomized controlled trial involving 130 participants demonstrated that adherence to a high-fiber, low-glycemic index diet resulted in significant reductions in fasting insulin, fasting plasma glucose, and HOMA-IR scores after a 6-month intervention period (control: 4.42 ± 0.78; intervention: 3.43 ± 0.61). Similarly, Chen et al.[14] reported a dose-dependent improvement in insulin resistance, with both 10 g/day and 20 g/day soluble fiber interventions significantly decreasing HOMA-IR values compared to the control group. Specifically, in the 20 g/day fiber group, HOMA-IR values decreased from 3.92 ± 2.77 to 2.12 ± 1.29, while the 10 g/day fiber group saw a reduction from 3.40 ± 2.59 to 2.30 ± 1.58; the control group exhibited a decline from 3.34 ± 2.64 to 2.56 ± 2.17, despite the relatively brief one-month intervention period.

Anisha et al.[15] conducted a crossover study with tightly controlled dietary intake, revealing that a high-fiber diet (50 g/day) significantly reduced the 24-hour plasma insulin area under the curve by 12% and decreased pre-prandial glucose concentrations, indicating enhanced insulin action, although HOMA-IR was not explicitly assessed. Furthermore, in a community-based randomized trial, Ramal, Champlin, and Bahjri observed a significant reduction in HbA1c over 6 months among participants adhering to a high-fiber, plant-based diet, thereby supporting improved insulin sensitivity, although direct HOMA-IR values were not reported.[16]

In summary, these findings underscore the advantageous effects of increased dietary fiber intake, particularly soluble fiber and low-glycemic index foods, on mitigating insulin resistance and enhancing metabolic control in individuals diagnosed with type 2 diabetes mellitus. Interventions prioritizing soluble fiber and whole grains exhibited the most pronounced efficacy in achieving these beneficial health outcomes.

Effect on glycemic control

All 13 randomized controlled trials included in this systematic review reported significant improvements in glycemic control following high-fiber or plant-based dietary interventions. These studies consistently demonstrated favorable reductions in both fasting plasma glucose (FPG) and postprandial glucose (PPG) levels, independent of the study’s duration, design, or source of fiber. In the research conducted by Cai et al.,[17] the FPG levels were significantly reduced from 6.22 ± 0.21 mg/dL in the control group to 5.38 ± 0.14 mg/dL in the intervention group. Similarly, Chen et al.[14] observed significant reductions in glycosylated hemoglobin (HbA1c), with control levels at −0.45 ± 2.36 and intervention levels indicating improvements of 1.00 ± 2.36 for the consumption of 10 g of fiber and 2.58 ± 2.74 for 20 g of fiber. These findings suggest a dose-dependent relationship, wherein higher soluble fiber intake correlates with greater glycemic benefits.

Anisha et al.[15] reported a reduction of 13 mg/dL in mean preprandial glucose levels and a 10% decrease in the 24-hour glucose area under the curve. Furthermore, Islam et al.[18] noted a reduction in 2-hour postprandial blood glucose levels from 15.918 ± 1.866 mg/dL in the control group to 11.607 ± 0.606 mg/dL in the intervention group. Ikem et al.[19] documented modest but clinically relevant improvements, with a decrease in 2-hour postprandial blood glucose levels from − 2.3 ± 3.5 to − 5.6 ± 2.9 mg/dL following interventions. Incorporating mixed cereal bread and high-fiber traditional diets. Evidence of glycemic benefits was further corroborated by Thakur et al.[20] reported that the intervention group experienced a reduction in glucose levels from 1548 mg/dL to 1367 mg/dL after the introduction of flaxseed gum. Guévin et al.[21] evaluated total glucose and insulin responses and demonstrated a 31% decrease in glucose and a 13% decrease in insulin levels associated with meals containing 20 g of dietary fiber compared to those containing 10 g. Additionally, Tappy, Gugolz, and Wursch employed controlled crossover designs to reveal a 50% reduction in postprandial glucose responses following the consumption of high-fiber breakfast cereals.[22]

Ramal, Champlin, and Bahjri did not specify direct glucose markers, yet notable reductions in HbA1c indicated improved long-term glycemic regulation, evidenced by a reduction in mean A1C levels compared to the control group (F1, 30 = 10.90).[16] Other trials, including Kondo et al.[23] and Simpson et al.,[24] provided further support for glycemic benefits through observed improvements in both fasting and postprandial glucose dynamics, despite variability in fiber types (e.g. legumes, cereals, brown rice, soluble fiber) and study contexts.

In conclusion, the collective findings from these trials strongly support the hypothesis that high-fiber dietary patterns confer significant benefits for glycemic control in individuals diagnosed with type 2 diabetes mellitus.

Effect on lipid profile

Some of the studies included here reported beneficial lipid parameter modifications after high-fiber diet interventions, affirming the cardiometabolic advantage of fiber consumption in type 2 diabetes or metabolic syndrome. In the randomized controlled trial of Goutam Thakur et al.[20] (total cholesterol (TC) intervention 163 ± 9, control 180 ± 11 and triglyceride intervention 133 ± 9, control 135 ± 9), three months’ supplementation with flaxseed gum led to significant decreases in total cholesterol, low-density lipoprotein cholesterol (LDL-C), and triglycerides with concomitant glycemic benefit, implicating a dual role of fiber for lipid and glucose metabolism. Chen et al.[14] also showed that supplementation with soluble dietary fiber (10–20 g/day) for 1 month reduced serum triglycerides and LDL-C in type 2 diabetic patients by a significant margin (triglyceride (mmol/l)—control 0.37 ± 2.19, 10g fiber 0.91 ± 1.98, 20 g fiber 0.84 ± 1.6). Of note, the 20 g/day group had the highest lipid-lowering effects, with a significant improvement in the insulin resistance index and decreased waist circumference, attesting to the dose-dependent metabolic efficacy of soluble fiber. In a double-blind, placebo-controlled study, Bañuls et al.[25] assessed a bread product enriched with 15.08 g of dietary fiber and L-carnitine in metabolic syndrome patients. Although no significant change was observed in standard lipid profile parameters (total cholesterol, LDL-C, HDL-C, and triglycerides), the study showed a substantial reduction in the proportion of small dense LDL particles known to be an atherogenic subfraction reflecting an improvement in lipid particle quality but not quantity (LDL-C (mg/dl) placebo 12 week, 143 ± 22, enriched product 12 week 148 ± 23). This qualitative shift in lipoprotein composition, along with reductions in insulin, C-peptide, and HOMA-IR, aligns with the hypothesis that fiber-based interventions may exert cardioprotective effects through modulation of glucose metabolism and lipid particle characteristics.[25]

Effect on weight and BMI

Of the trials included in this review, only two trials explicitly measured the effect of high-fiber or plant-based diet interventions on anthropometric measures like weight, BMI, and waist circumference. Ramal, Champlin, and Bahjri compared the effects of a culturally adapted, plant-based diet and behavioral support on Latino type 2 diabetic patients for 6 months in a community-based randomized controlled trial. While glycemic control was the primary outcome, the intervention group also demonstrated modest but consistent reductions in BMI and waist hip circumference (F1, 29 = 5.34) and fat intake (F1, 30 = 5.35), demonstrating enhanced energy regulation and food compliance as secondary benefits of the high-fiber, whole-food diet. These anthropometric improvements were paralleled by significant decreases in HbA1c, confirming the interdependence of glycemic and weight control.[16] Ikem et al.[19] also performed a randomized study in Nigeria where type 2 diabetic patients were given a structured high-fiber diet consisting of indigenous foods. During the 3-month duration of the intervention, the participants demonstrated a statistically significant reduction in BMI (intervention −0.3 ± 0.9, control −0.32 ± 0.6) and reductions in fasting blood glucose, demonstrating that fiber in the diet improves metabolic factors and leads to normalization of weight, especially in resource-constrained settings where pharmacological treatments may be in short supply. While weight loss was modest in magnitude in both trials, the findings confirm the potential for fiber-rich diets to play a role in overall diabetes management through incremental reductions in adiposity.

Characteristics of the studies

The studies included in this review (n = 13) primarily consisted of randomized controlled trials conducted between 1981 and 2017. Sample sizes for these trials varied from eight to 130 participants, with intervention durations ranging from one to six months. The interventions involved daily supplementation of 5 to 50 g of dietary fiber derived from diverse sources, such as flaxseed gum, β-glucan, legumes, and fiber-rich breads or cereals. Except for one trial, all studies utilized standard or low-fiber diets as control conditions and enrolled adult participants diagnosed with type 2 diabetes. Randomization techniques employed across the studies were heterogeneous, including simple randomization, stratification, and crossover designs [Table 1].

Table 1.

Study Characteristics of Included Randomized Controlled Trials Evaluating Dietary Fiber Interventions in Type 2 Diabetes

Title		Authors		Publication Year		Study Design		Study Duration		Sample Size		Randomization Method		Aim of the study		Intervention Description		Dose		Duration
Effect of Flaxseed Gum on Reduction of Blood Glucose & Cholesterol in Type 2 Diabetic Patients		Goutam Thakur et al.		2009		Randomize d controlled trial		3 month		120		NR		To evaluate the effect of flaxseed gum on blood glucose and cholesterol levels in T2DM patients		Consumption of chapattis enriched with flaxseed gum		5g flaxseed gum in 6 chapattis per day		3 month
Impact of Bread Made from Mixed Cereals and Pulses on the Glycemic Profile in Type 2 Diabetic Patients		Md. Mominul Islam et al.		2015		Randomize d controlled trial		1 month 26 days		30		NR		To assess whether composite flour bread improves glycemic control		High-fiber composite flour bread (wheat, maize, Bengal gram, bean)		Bread with 40g fiber/day		1 month 26 days
A Controlled Comparison of the Effect of a High Fiber Diet on the Glycaemic and Lipid Profile of Nigerian Clinic Patients with Type 2 Diabetes		Babatope Kolawole et al		2007		Randomize d controlled trial		3 month		60		Simple randomization		To evaluate the impact of a high-fiber diet on glycemic and lipid control in Nigerian T2DM patients		High-fiber diet with increased intake of vegetables, legumes, and whole grains		40g fiber/day		3 month
Effects of Breakfast Cereals Containing Various Amounts of β- Glucan Fibers on Plasma Glucose and Insulin Responses in NIDDM Subjects		L. Tappy et al.		1996		Randomize d crossover trial		1 month		8		Latin square design		To determine the relationship between β-glucan fiber intake and plasma glycemic and insulinemic responses in Type 2 Diabetes (NIDDM) patients.		Breakfast cereals with different amounts of β-glucan (4.0g, 6.0g, 8.4g) vs. a continental breakfast (0g β-glucan)		35g carbohydrate per meal		1 month
Therapeutic Effects of Soluble Dietary Fiber Consumption on Type 2 Diabetes Mellitus		Chunye Chen et al.		2016		Randomize d, double-blind trial		1 month		117		Random number table		To investigate the effect of soluble dietary fiber (DF) intake on metabolic control in patients with Type 2 Diabetes Mellitus (DM2).		Patients followed the ADA diet, with the control group receiving 0g/day soluble fiber, while the 10DF and 20DF groups received 10g/day and 20g/day supplements, respectively.		10g/day or 20g/day of soluble fiber		1 month
The Consumption of a Bread Enriched with Dietary Fiber and L-Carnitine Improves Glucose Homeostasis and Insulin Sensitivity in Patients with Metabolic Syndrome		Celia Bañulset al.		2015		Randomize d placebo-controlled trial		4 month		54		Alternation method		To assess whether fiber-enriched bread with L-carnitine improves glucose homeostasis and insulin sensitivity in metabolic syndrome (MetS) patients		Daily consumption of enriched bread (15.08g fiber + 2325 mg L-carnitine) vs. placebo bread		15.08 g of dietary fiber (9.49 g insoluble fiber and 5.59 g soluble fiber) + 2325 mg of L-carnitine per day in every 130 g of enriched bread		4 month
A High Carbohydrate Leguminous Fiber Diet Improves All Aspects of Diabetic Control		H. C. R. Simpson et al.		1981		Randomize d Cross- over Study		1 and a half month		27		NR		To assess the impact of a high carbohydrate leguminous fiber diet on diabetic control		High carbohydrate leguminous fiber diet (HL)		NR		1 and a half month
Impact of a Plant-Based Diet and Support on Mitigating Type 2 Diabetes Mellitus in Latinos Living in Medically Underserved Areas		Edelweiss Ramal et al.		2017		Experiment al randomized controlled community pilot study		6 month		32		Randomly assigned		To determine the impact of a high-fiber, low-fat diet with support on self-management of T2DM in Latinos from MUAs		5-week education program + follow-up support at 1, 3, and 6 months		NR		6 month
Postprandial Glucose, Insulin, and Lipid Responses to Four Meals Containing Unpurified Dietary Fiber		Nathalie Guevin et al.		1996		Randomize d, controlled, crossover trial		2 month		8		Random sequence for meal allocation		Evaluate effects of different levels of dietary fiber on postprandial glucose, insulin, and lipid metabolism		Four test meals with 10g or 20g fiber; Soluble: Insoluble fiber ratios (1:4 vs 2:3)		10g or 20g dietary fiber per meal		2 month
Beneficial Effects of High Dietary Fiber Intake in Patients with Type 2 Diabetes Mellitus		Manisha Chandalia et al.		2000		Randomize d, crossover study		3 month		13		NR		To determine the effects of a high-fiber diet on glycemic control and lipid levels in type 2 diabetes patients		Comparison between a moderate-fiber diet (24g/day) and a high-fiber diet (50g/day)		50g of fiber daily in the high-fiber diet		3 month
Fiber-Rich Diet with Brown Rice Improves Endothelial Function in Type 2 Diabetes Mellitus		Keiko Kondo et al.		2017		Randomize d controlled trial (parallel design)		2 month		28		Stratified		To assess whether a fiber-rich diet with brown rice improves endothelial function in type 2 diabetes patients		Brown rice diet vs. white rice diet		Brown rice provided for 10 out of 21 meals per week		2 month
Dietary Fiber Decreases Fasting Blood Glucose Levels and Plasma LDL Concentration in Noninsulin-Dependent Diabetes Mellitus Patients		Barbro Hagander et al.		1988		Randomize d crossover study		4 month		14		NR		To assess metabolic effects of dietary fiber without altering macronutrient intake		High-fiber diet (beet-fiber enriched bread and fiber-rich foods) vs. low-fiber diet		25.5g fiber/day (low-fiber) vs. 45.9g fiber/day (high-fiber)		4 month
Effect of High Dietary Fiber Low Glycemic Index Diet on Intestinal Flora, Blood Glucose, and Inflammatory Response in T2DM Patients		Xiaojun Cai et al.		2017		Randomize d controlled trial		6 month		130		Random number table		To assess effects of high-fiber, low-GI diet on intestinal flora, blood glucose, and inflammation		High-fiber, low-GI diet (soluble fiber, whole grains)		10g fruit/vegetable fiber + 50g buckwheat daily		6 month
Title						Comparison Group		Duration		Primary Outcome (s)		Insulin resistance		Weight reduction		Glycemic Control		Lipid Profile
Effect of Flaxseed Gum on Reduction of Blood Glucose & Cholesterol in Type 2 Diabetic Patients		Daily		NR		Participants on standard diet (without flaxseed gum)		3 month		reduction in fasting blood sugar (FBS)		The experimental group showed a decrease in values from 2.2±0.2 to 2.0±0.2, whereas the control group remained unchanged, shifting slightly from 2.2±0.3 to 2.3±0.2.		NR		Experimental group: FBS decreased from 154±8 mg/dL to 136±7 mg/dL (P=0.03), Control group: No significant change (152±8 mg/dL to 154±6 mg/dL)		LDL-↓ Cholesterol: 110±8 mg/dL to 92±9 mg/dL (P=0.02) TC: ↓182±11 mg/dL to 163±9 mg/dL (p=0.03), Serum Insulin: ↓106±14 pmol/L to 98±12 pmol/L in the experimental group (unchanged in control) HOMA-β-cell function: ↑ 54.6±0.4 to 64.,4±0.5 HOMA-insulin sensitivity: ↑ 45.4±0.7 to 50.3±0.6
Impact of Bread Made from Mixed Cereals and Pulses on the Glycemic Profile in Type 2 Diabetic Patients		Daily		NR		Standard wheat bread		1 month 26 days		reduction in fasting blood sugar (FBS) and postprandial blood glucose		NR		NR		Intervention group - Postprandial blood glucose significantly ↓ (14.892±1.790 ↓ 11.607±0.606 mmol/L, P<0.001), while fasting blood glucose reduction was significant only after 28 days (8.715±1.949 ↓ 7.746±1.789 mmol/L, P<0.05)		NR
A Controlled Comparison of the Effect of a High Fiber Diet on the Glycaemic and Lipid Profile of Nigerian Clinic Patients with Type 2 Diabetes		Daily		NR		standard diabetes diet		3 month		reduction in fasting blood sugar (FBS) and HbA1c		NR		NR		Fasting Blood Glucose: Reduced from 8.2±1.5 mmol/L to ±1.3 mmol/L (P=0.01), HbA1c: Decreased from 7.9±0.8% to 6.8±0.7% (P=0.02)		LDL-Cholesterol: Reduced from 3.8±0.9 mmol/L to 3.2±0.8 mmol/L (P=0.03), HDL-Cholesterol: Increased from 1.1±0.3 mmol/L to 1.3±0.4 mmol/L (P=0.04)
Effects of Breakfast Cereals Containing Various Amounts of β- Glucan Fibers on Plasma Glucose and Insulin Responses in NIDDM Subjects		Four separate test occasions		NR		Continental breakfast (bread, milk, cheese, ham)		1 month		Glycemic response (plasma glucose levels) to different doses of β-glucan.		NR		NR		4 g β-glucan: 33% reduction in glycemic response (P<0.05), 6 g β-glucan: 60% reduction in glycemic response (P<0.001), 8.4 g β-glucan: Similar 60% reduction in glycemic response (P<0.001)		4 − 8.4 g β-glucan reduced postprandial insulin levels by ~35% (P<0.05)
Therapeutic Effects of Soluble Dietary Fiber Consumption on Type 2 Diabetes Mellitus		Daily		Double-blind		ADA diet alone		1 month		fasting blood glucose, 2-hour postprandial blood glucose, insulin resistance index, and glycated albumin—showed significant improvements in the dietary fiber groups, with the greatest effects observed in the high-dose group		In the 20DF group, values decreased from 3.92±2.77 to 2.12±1.29. In the 10DF group, values dropped from 3.40±2.59 to 2.30±1.58. In the control group, values declined from 3.34±2.64 to 2.56±2.17.		NR		Significant improvement in fasting blood glucose, 2-hour blood glucose, glycated albumin, and insulin resistance index in dietary fiber groups compared to control		Significant reduction in waist/hip ratio, triglycerides, and LDL in high-dose dietary fiber group
The Consumption of a Bread Enriched with Dietary Fiber and L-Carnitine Improves Glucose Homeostasis and Insulin Sensitivity in Patients with Metabolic Syndrome		twice per day daily		double blind		Placebo bread (same macronutrient composition but without dietary fiber or L-carnitine). Calorie-restricted diet (same as the intervention group)		4 month		insulin sensitivity and glucose homeostasis, reduced small dense LDL particles		HOMA-IR decreased by 14.8%, insulin levels dropped by 12.4%, and C- peptide levels were reduced by 8.9%.		NR		HOMA-IR ↓14.8%, Insulin ↓12.4%, C-peptide ↓ 8.9%, BMI ↓ 0.7 kg/m², Waist ↓~3 cm in the enriched group.		Small dense LDL ↓ (enriched group).No change in lipid profile & inflammation (hsCRP, IL-6, TNF-α).
A High Carbohydrate Leguminous Fiber Diet Improves All Aspects of Diabetic Control		NR		NR		Standard low carbohydrate diet (LC)		1 and a half month		Blood glucose control (preprandial and postprandial), Glycosuria, Total cholesterol, HDL/LDL ratio		NR		NR		Significant reduction in blood glucose levels, improved HDL/LDL ratio		NR
Impact of a Plant-Based Diet and Support on Mitigating Type 2 Diabetes Mellitus in Latinos Living in Medically Underserved Areas		NR		NR		Standard care		6 month		A1C levels		NR		NR		Significant decrease in experimental group (8.53% → 7.31%, P=0.002)		Control (9.57 → 9.49); Experimental (8.53 → 7.31); P=0.002
Postprandial Glucose, Insulin, and Lipid Responses to Four Meals Containing Unpurified Dietary Fiber		one-week interval		NR		Each participant served as their own control		2 month		Postprandial glucose, insulin, and triglyceride response		Evaluated through insulin levels postmeal		NR		Higher fiber intake significantly reduced glucose and insulin levels (P<0.05); No effect on lipid response		NR
Beneficial Effects of High Dietary Fiber Intake in Patients with Type 2 Diabetes Mellitus		Daily		NR		The comparison group is the pretraining responses of the same participants (pretest vs. post-test comparison).		3 month		The primary outcome of the study is the improvement in glycemic control, measured by reductions in plasma glucose and insulin levels.		Reduced insulin and glucose levels with high-fiber diet		No significant difference in weight		Glycemic Control: ↓ Plasma glucose (13 mg/dL, P=0.04), ↓ Urinary glucose (1.3 g/day, P=0.008), ↓ Insulin (12%, P=0.05). Lipid Profile: ↓ Total cholesterol (6.7%, P=0.02), ↓ Triglycerides (10.2%, P=0.02), ↓ VLDL cholesterol (12.5%, P=0.01).		A high-fiber diet significantly improved glycemic control, reduced hyperinsulinemia, and lowered plasma lipid concentrations in patients with type 2 diabetes.
Fiber-Rich Diet with Brown Rice Improves Endothelial Function in Type 2 Diabetes Mellitus		Daily		Open label		The brown rice diet group showed greater improvement in endothelial function (FDR +20.4% vs. −5.8%, peak FBF +118.8% vs. −43.3%, RH duration + 9.7s vs. −1.1s) compared to the white rice diet group.		2 month		Improvement in fasting flow debt repayment (20.4% vs. −5.8%, P=0.004)		no significant changes in insulin resistance (HOMA-IR) between the brown rice and white rice diet groups after the 8-week intervention		No significant difference in weight		The brown rice group showed improved endothelial function (FDR +20.4% vs. −5.8%, peak FBF +118.8% vs. −43.3%, RH duration + 9.7s vs. −1.1s) and lower postprandial glucose excursions, with no significant changes in HbA1c, insulin resistance, lipid profiles, or body weight		The brown rice group showed improved endothelial function (FDR + 20.4% vs. −5.8%, peak FBF +118.8% vs. −43.3%, RH duration + 9.7s vs. −1.1s) and lower postprandial glucose excursions, with no significant changes in HbA1c, insulin resistance, lipid profiles, or body weight
Dietary Fiber Decreases Fasting Blood Glucose Levels and Plasma LDL Concentration in Noninsulin-Dependent Diabetes Mellitus Patients		Daily		NR		The high-fiber diet improved fasting blood glucose and lipid profile in NIDDM patients without affecting insulin levels, while postprandial glucose response and HbA1c remained unchanged.		4 month		reduction in fasting blood glucose levels and improvement in the lipid profile (lower LDL cholesterol and improved LDL: HDL ratio) in NIDDM patients following a high-fiber diet.		No significant difference in fasting insulin, C-peptide		No significant weight change		The primary outcome results showed a significant reduction in fasting blood glucose levels (6.3 mmol/L vs. 6.7 mmol/L, P<0.01) and improved lipid profile with lower LDL cholesterol (P<0.025) and a better LDL: HDL ratio (P< 0.025) in the high-fiber diet group.		The secondary outcomes showed no significant changes in postprandial glucose, HbA1c, or hormone levels, while fasting triglycerides were lower in the high-fiber group but not statistically significant.
Effect of High Dietary Fiber Low Glycemic Index Diet on Intestinal Flora, Blood Glucose, and Inflammatory Response in T2DM Patients		Daily		NR		the low-fiber diet period		6 month		Significant decrease in fasting glucose (6.13 vs. 6.52 mmol/L, P<0.05)		insulin resistance was not significantly affected.		No significant weight change		Fasting blood glucose was significantly lower after the high-fiber diet (6.3 mmol/L) than the low-fiber diet (6.7 mmol/L, P<0.01), indicating improved metabolic control.		HbA1c showed no significant difference between diets, LDL cholesterol decreased (P<0.025), HDL remained unchanged, and fasting insulin, C-peptide, glucagon, and somatostatin levels showed no significant changes.

Notably, several trials reported improvements in insulin resistance. For instance, Chen et al.[14] observed a substantial decrease in HOMA-IR, reducing from 3.92 ± 2.77 to 2.12 ± 1.29 within the high-fiber group. Similarly, Bañuls et al.[25] documented a 14.8% reduction in HOMA-IR following the intake of fiber- and L-carnitine-supplemented bread. These findings provide compelling evidence for the potential role of dietary fiber in enhancing insulin sensitivity, alongside its established effects on glycemic control and lipid profiles.[25]

Quality assessment

Risk of bias assessment

The risk of bias evaluations of the studies incorporated in this systematic review are depicted in Figure 2. The evaluations were carried out across numerous domains using the ROB framework criteria. Five studies[8,11,12,15,22] were assessed as having low risk of bias due to confounding. Nine studies were identified to have some methodological confounding concerns.[14,16,20,21,24]

Figure 2.

Quality assessment of the articles included in the review

Regarding bias in participant selection, most studies were at low risk or posed some concerns, suggesting that participants were generally representative of the target population. However, two studies[21,22] were rated as having a high risk of selection bias.

The classification bias remained unclear for almost all of the studies, with only one[8] furnishing adequate detail to make an assured rating. Therefore, in this domain, “no information” was largely defaulted.

With respect to bias resulting from departures from intended interventions, loss of outcome data, outcome measurement, and choice of reported result, most studies were assessed to be at low risk or with some concerns, implying that investigators tended to follow good methodological practices in these areas.

When viewing the overall risk of bias, most of the studies were rated as having some concerns. Incidentally, two studies[11,12] were rated as having a low overall risk, but two studies[21,22] were rated as having an increased risk of bias. Such judgments of study quality, depicted in Figure 2, must be meticulously examined when drawing conclusions from study findings and establishing conclusions from the present review.

Discussion

This systematic review highlights compelling evidence supporting the role of dietary fiber in improving insulin resistance and glycemic control in individuals with type 2 diabetes mellitus (T2DM). Interventions involving high-fiber diets, legume-enriched meals, composite flour-based breads, and isolated soluble fibers, such as β-glucan and flaxseed gum, demonstrated significant improvements in glycemic and lipid profiles. Soluble fibers, particularly those with high viscosity, were consistently associated with enhanced insulin sensitivity and better glycemic outcomes. Several randomized controlled trials reported reductions in HOMA-IR, fasting insulin, and postprandial glucose levels, underscoring the therapeutic potential of dietary fiber as a metabolic regulator in T2DM management.

Our findings align with recent meta-analyses in this field. Xie et al.[26] demonstrated that soluble fiber supplementation resulted in HbA1c reductions of 0.63% and HOMA-IR improvements of 0.58, which closely matches our narrative findings of 0.4-0.9% HbA1c reductions and substantial HOMA-IR improvements (ranging from 0.8 to 1.5 point reductions). Similarly, Mao et al.[2] reported consistent glycemic benefits with fiber interventions, supporting our conclusions about dose-dependent relationships.

However, our review provides additional insights by focusing specifically on natural food sources of fiber rather than isolated supplements, which may have greater translational relevance for clinical practice. Unlike previous reviews that primarily examined purified fiber supplements, our inclusion of whole food interventions (legumes, whole grains, composite flours) offers more practical dietary guidance for patients and healthcare providers.

Moreover,[27] provided mechanistic insights, showing that a prebiotic fiber-enriched nutritional formula improved HbA1c and increased the abundance of butyrate-producing gut bacteria, suggesting that modulation of immunometabolism could be a crucial mechanism. In terms of dose-response relationships, a meta-analysis by[28] revealed that for every 5 g/day increase in soluble fiber, LDL-C was reduced by 5.57 mg/dL and total cholesterol by 6.11 mg/dL in adults, further supporting the therapeutic benefits of fiber intake when consumed in appropriate amounts.

In addition to improving glycemic control, soluble fiber also demonstrated cardiometabolic benefits. A meta-analysis by[29] found that soluble fiber supplementation in individuals with T2DM significantly reduced triglycerides by 16.97 mg/dL, LDL-C by 11.14 mg/dL, and total cholesterol by 13.87 mg/dL. These lipid-lowering effects are likely due to mechanisms such as bile acid sequestration and enhanced LDL receptor expression in the liver. Furthermore, anthropometric improvements were observed. For instance, a randomized controlled trial[30] showed that psyllium-based soluble fiber supplementation resulted in reductions in BMI, fasting blood glucose, HbA1c, insulin levels, and HOMA-IR in patients with T2DM.

While the overall evidence is encouraging, several limitations must be considered. A majority of the included studies were of short duration (typically <3 months) and involved small sample sizes, with some trials enrolling fewer than 10 participants, thereby limiting statistical power and the ability to assess long-term outcomes. Substantial heterogeneity was observed in fiber type, dosage, formulation (e.g., whole foods vs. supplements), and intervention protocols, which precluded direct comparisons and limited the feasibility of conducting a meta-analysis. The predominance of single-center studies further restricts external validity. Additionally, inadequate adherence tracking and inconsistent reporting of critical statistical measures (e.g., effect sizes, confidence intervals) reduced the interpretability and comparability of outcomes.

Despite these constraints, the review encompassed studies with rigorous methodologies, including randomized controlled, crossover, and blinded designs conducted in clinical settings. Notably, the integration of culturally adapted dietary strategies, such as composite flours and traditional fiber-rich foods, enhances the translational relevance of findings.

Strengths and limitations

Strengths

This systematic review demonstrates several methodological strengths that enhance the validity and reliability of its findings. The comprehensive search strategy employed multiple high-quality databases with well-defined inclusion and exclusion criteria, ensuring thorough coverage of relevant literature. The review adhered to established PRISMA 2020 guidelines, demonstrating transparency and reducing the risk of selective reporting bias. The inclusion of diverse study designs (parallel, crossover, double-blind, and placebo-controlled trials) from multiple countries enhances the external validity and generalizability of findings across different populations and healthcare settings. The rigorous quality assessment using the RoB 2 tool provided a systematic evaluation of methodological quality across all included studies. Additionally, the review encompassed a broad range of fiber sources and interventions, from whole foods to isolated compounds, providing comprehensive insights into various therapeutic approaches. The consistent findings across different fiber types and populations strengthen the evidence base for dietary fiber’s beneficial effects on glycemic control and insulin resistance in type 2 diabetes management.

Limitations

Despite these strengths, several limitations must be acknowledged. The predominance of short-duration studies (typically <3 months) limits the assessment of long-term efficacy and sustainability of fiber interventions, which is crucial for chronic disease management. Substantial heterogeneity in fiber types, dosages, formulations, and intervention protocols precluded the conduct of meta-analyses, reducing the ability to provide quantitative estimates of treatment effects. Small sample sizes in many included studies (some with fewer than 10 participants) compromised statistical power and limited the generalizability of findings to broader populations. The single-center nature of most studies further restricts external validity across different healthcare systems and populations. Inadequate reporting of adherence monitoring and compliance rates in several studies raises concerns about the true exposure to interventions and may have influenced outcome measurements. Additionally, inconsistent reporting of critical statistical measures, including effect sizes and confidence intervals, reduced the interpretability and comparability of results across studies. The limited assessment of potential adverse effects and the lack of standardized outcome measures for insulin resistance indices further constrain the clinical applicability of findings.

Future recommendations

Future research should prioritize large-scale, multicenter randomized controlled trials with extended follow-up periods (≥12 months) to establish the long-term efficacy and sustainability of fiber-based interventions in type 2 diabetes management. Standardized protocols for fiber classification, dosage optimization, and outcome measurement are essential to enhance comparability across studies and facilitate robust meta-analyses. The integration of advanced methodologies, including continuous glucose monitoring, metabolomic profiling, and comprehensive gut microbiome analysis, will provide deeper mechanistic insights into fiber’s therapeutic effects and support the development of precision nutrition approaches tailored to individual metabolic profiles. Additionally, investigating the synergistic effects of dietary fiber interventions combined with lifestyle modifications, such as yoga and physical activity breaks, as demonstrated in recent studies showing improved vascular parameters and cognitive performance in sedentary populations,[31,32] may offer comprehensive therapeutic strategies for diabetes management. Furthermore, research should explore the neurometabolic connections between dietary interventions and brain health, considering findings that suggest age-dependent relationships between metabolic factors and hippocampal volume in populations with lower educational attainment.[33] Future studies should also emphasize culturally adapted, community-based interventions that incorporate traditional fiber-rich foods and address socioeconomic barriers to dietary adherence, particularly in resource-constrained settings where pharmacological treatments may be limited. The development of standardized reporting guidelines for dietary fiber research, including detailed documentation of fiber sources, processing methods, and bioavailability, will enhance the translation of research findings into clinical practice and public health policy.

Conclusion

This systematic review provides compelling evidence supporting the therapeutic role of dietary fiber, particularly soluble and mixed types, in improving insulin resistance and glycemic control in individuals with T2DM. The consistent benefits observed across diverse populations and intervention types suggest that dietary fiber should be considered a cornerstone of comprehensive diabetes management.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

This work was supported by Grant-in-Aid, ICMR-DHR (Grant no R.11014/12/2023-GIA/HR).