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Review: Viscous fiber reduces HbA1c, fasting glucose, and insulin resistance in type 2 diabetes

Question In patients with type 2 diabetes mellitus, what is the effect of viscous fiber supplementation on glycemic control? Review scope Included studies compared viscous fiber (-glucan, guar gum, konjac, psyllium, pectin, xanthan gum, locust bean gum, alginate, or agar) with placebo, background diet, or supplements that were fiber-free or contained insoluble fiber for 3 weeks in patients with type 2 diabetes. Outcomes included hemoglobin (Hb) A1c, fasting glucose, fasting insulin, and homeostatic model assessmentinsulin resistance (HOMA-IR). Review methods MEDLINE, EMBASE/Excerpta Medica, and Cochrane Central Register of Controlled Trials (all to Jun 2018); and reference lists were searched for randomized controlled trials. 28 RCTs (n =1394, median age 60 y, median duration 8 wk) met selection criteria. Viscous fibers were -glucan (6 RCTs), guar gum (15 RCTs), konjac (2 RCTs), and psyllium (5 RCTs). Risk for bias was low for randomization (3 RCTs), blinding (17 RCTs), and incomplete outcome data (16 RCTs); all trials had unclear risk for bias for allocation concealment. Quality of the evidence was assessed with Grading of Recommendations Assessment, Development and Evaluation (GRADE). Main results Median dose of viscous fiber was 13.1 g/d. The main results are in the Table. Evidence for each outcome was moderate based on GRADE (further research will likely affect confidence in the estimate and may change the estimate). Conclusion Moderate-quality evidence shows that viscous fiber supplements reduce hemoglobin A1c, fasting glucose, and homeostatic modelassessment insulin resistance but not fasting insulin in patients with type 2 diabetes. Viscous fiber vs control (placebo, background diet, or fiber-free or insoluble fiber supplements) for type 2 diabetes* Outcomes Number of trials (n) Mean difference between groups (95% CI) at a median 8 wk I 2 Hemoglobin A1c 20 (1353) 0.58% (0.88 to 0.28) 91% Fasting glucose (mmol/L) 28 (1665) 0.82 (1.32 to 0.31) 92% Fasting insulin (pmol/L) 9 (343) 17.56 (37.54 to 2.42) 90% HOMA-IR 11 (767) 1.89 (3.45 to 0.33) 94% *HOMA-IR = homeostatic model assessmentinsulin resistance; CI defined in Glossary. Some patients are counted twice in cross-over trials for each of the outcomes. Commentary A network meta-analysis found that adding adjunctive oral antidiabetes medications to metformin in patients with type 2 diabetes reduced HbA1c between 0.43% and 1.29% (1). Jovanovski and colleagues suggest that viscous fiber supplements reduce HbA1c to a similar degree. 2 caveats include the short duration of the included RCTs (8 wk) and high interstudy heterogeneity (I 2 =91%, P <0.00001). Even with these limitations, the potential benefit, low cost, and low risk for side effects of viscous fiber supplements merit consideration for treatment of type 2 diabetes. Viscous fiber helps gut microbiota produce short-chain fatty acids that seem to lower HbA1c by increasing glucagon-like peptide-1 production and decreasing indole and hydrogen sulfide production (2). Does HbA1c reduction translate into an improvement in quality of life or mortality? At least 1 RCT found that aggressively treating HbA1c, blood pressure, and cholesterol did not differ from usual care for the composite outcome of myocardial infarction, stroke, revascularization, or all-cause mortality (3). In the absence of a well-designed, multiyear RCT comparing viscous fiber with other oral agents and measuring patient-oriented outcomes, I suggest that providers advise patients with type 2 diabeteswho have quit smoking (if applicable) and are using metforminto consider increasing soluble fiber intake by >10 g/d because it seems to be as effective as some medications for controlling HbA1c.