ILLUSTRATIVE CASE
A 57-year-old man who was given a diagnosis of T2D a year ago presents for an office visit. His hemoglobin A1C level at diagnosis was 8.3%. He is otherwise healthy and has been adhering well to a plan of metformin 1000 mg twice daily, regular exercise, and a low-carbohydrate diet you recommended. His most recent hemoglobin A1C is 7.3%. He is pleased with his progress, so he is discouraged when you tell him that he is not yet at goal. He asks if there are other things that he can do to further lower his hemoglobin A1C. What can you recommend for him?
According to the National Diabetes Statistics Report, 2020 from the Centers for Disease Control and Prevention, approximately 34.1 million US adults ≥ 18 years of age (13% of the adult population) have diabetes, 50% of whom have a hemoglobin A1C > 7%. The report also states that approximately 88 million US adults—more than one-third of the population—have prediabetes.2
The American Diabetes Association (ADA) estimated that diabetes-related health care costs in the United States for 2017 totaled $237 billion, an increase of 26% from 2012. More than $30 billion of this expense comes directly from diabetes medications; the remainder of these costs are related to lost wages, clinic visits, hospitalizations, and treatment for diabetic complications and comorbidities. After controlling for age and gender, medical expenditures for people with diabetes are 2.3 times higher than for those without diabetes.3
The 2019 ADA Nutrition Therapy for Adults With Diabetes or Prediabetes: A Consensus Report makes general recommendations concerning fiber intake for patients with diabetes or prediabetes, stating that these patients should consume approximately 14 g of fiber for every 1000 kcal consumed, giving preference to whole-food sources rather than supplements.4 The report indicates that some studies have shown hemoglobin A1C reductions of 0.2% to 0.3% with daily fiber intake exceeding 50 g. However, this level of intake can cause unpleasant gastrointestinal adverse effects, including bloating, diarrhea, and flatulence.4,5
STUDY SUMMARY
Effect on A1C exceeded the FDA threshold for new drugs
This systematic review and meta-analysis searched MEDLINE, Embase, and the Cochrane Central Register of Controlled Trials to identify randomized controlled trials that studied the effects of viscous fiber supplementation on glycemic control in patients with T2D. Eligible studies included those that: (1) had a duration ≥ 3 weeks; (2) allowed isolation of the viscous fiber effects; and (3) reported at least 1 of the following glycemic measures: hemoglobin A1C (n = 1148 patients), fasting glucose (n = 1394), fasting insulin (n = 228), homeostatic model assessment of insulin resistance (HOMA-IR; n = 652), and fructosamine (n = 23).
Data were pooled using the generic inverse variance method and expressed as mean difference (MD) with 95% confidence intervals (CIs). Heterogeneity was assessed and quantified (Cochran Q and I2 statistics, respectively). I2 ≥ 50% indicates substantial heterogeneity. The Grading of Recommendations Assessment, Development, and Evaluation (GRADE) approach was used to evaluate the overall strength of evidence.
Twenty-eight eligible studies were compared. The median age of included patients was 60 years. The median dose of viscous fiber was 13.1 g/d (range, 2.55-21). Viscous fiber type varied between the studies and included psyllium, guar gum, β-glucan, and konjac, and was consumed in powder, tablet, capsule, and limited food-based forms (in 1 of the included studies). The median trial duration was 8 weeks, with 11 trials lasting ≥ 12 weeks.
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