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

An ACP Journal Club commentary recommends advising patients with type 2 diabetes who don't smoke and who take metformin to also consider increasing their soluble fiber intake by more than 10 g/d.

Viscous fiber supplementation, at a median dose of about 13.1 g/d, significantly improved markers of glycemic control in patients with type 2 diabetes, according to a systematic review and meta-analysis of 28 randomized controlled trials (RCTs). The review found statistically significant reductions in HbA1c, fasting blood glucose levels, and insulin resistance in patients who received fiber supplements compared to those who got usual care.

The review was published in the January Diabetes Care. The following commentary by Anupam Goel, MD, FACP, was published in the ACP Journal Club section of the May 21 Annals of Internal Medicine.

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%. 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 (I2 = 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. 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.

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 diabetes—who have quit smoking (if applicable) and are using metformin—to consider increasing soluble fiber intake by >10 g/d because it seems to be as effective as some medications for controlling HbA1c.