Viscous Fiber Supplements in Diabetes Control: Results from a Systematic Review and Meta-analysis of Randomized Controlled Trials
By Heather Nelson Cortes, PhD and Kevin C Maki, PhD
According to the recent 2019 American Diabetes Association (ADA) Standards of Medical Care in Diabetes, people with diabetes should increase their intake of viscous fiber from sources such as oats, legumes, and citrus to help regulate blood glucose levels and lower risk of cardiovascular disease1. Viscous fibers are described as such because they increase viscosity in the human gut, thereby reducing the rate at which carbohydrates are digested and the glucose molecules from them absorbed. These effects occur because the viscous solution impedes the ability of digestive enzymes to reach starch molecules and slows the rate at which glucose molecules reach the brush border in the intestinal lumen, where absorption can occur. The result is flattening of the postprandial glycemic and insulinemic responses. While these acute effects are well established, the longer-term impacts of viscous fibers on glycemic control markers are not well known2.
In a systematic review and meta-analysis, Jovanovski2 investigated the effect of viscous dietary fiber supplementation on markers of glycemic control in people with type 2 diabetes (T2D). A comprehensive literature search on MEDLINE, Embase, and Cochrane Central Register of Controlled Trials through June 15, 2018 identified 2,716 potential RCTs. After reviewing the studies based on the inclusion/exclusion criteria, 27 studies (n = 1,394) were identified for the review and meta-analysis. Inclusion criteria were: ≥3 weeks in duration, studied viscous fiber supplementation (β-glucan, guar gum, konjac, psyllium, pectin, xanthan gum, locust bean gum, alginate, agar) compared to an appropriate control (i.e., fiber-free supplement or one containing insoluble fiber, background diet, or placebo), and included at least one glycemic measurement (glycated hemoglobin [HbA1c], fasting glucose, fasting insulin, homeostatic model assessment of insulin resistance [HOMA-IR], or fructosamine).
The median age of subjects was 60 years (range 48-67) and they had a median body mass index of 27 kg/m2 (range 26-32 kg/m2). The median dose of viscous fibers in the studies was 13.1 g/day (range 2.55-21.0 g/day) and the median study duration was 8 weeks (range 3-52 weeks).
Compared to control groups, inclusion of viscous fiber in the diet was associated with significant reductions in HbA1c, fasting blood glucose and HOMA- IR.
- HbA1c: mean difference (MD) -0.58% 95% confidence interval (CI) -0.88%, -0.28%; p = 0.0002;
- Fasting blood glucose: MD -14.8 mg/dL 95% CI -23.8, -5.59; p = 0.001
- HOMA-IR: MD -1.89 95% CI -3.45, -0.33; p = 0.02.
There were no differences between viscous fiber groups and controls for fasting insulin (MD -2.53 µU/mL 95% CI -5.41, 0.35; p = 0.08) or fructosamine (MD -0.12 mmol/L 95% CI -0.39, 0.14; p = 0.37). Only 2 studies reported fructosamine, so this finding should be interpreted with caution. There was no evidence of a significant dose-response effect. Results for HbA1c, fasting glucose, fasting insulin, and HOMA-IR were graded moderate for certainty of evidence, while fructosamine was graded low.
Comment. Viscous fiber intake, through consumption of food sources such as legumes, whole fruits (e.g., apples and pears) and whole grain oats and barley, as well as dietary supplementation with products such as psyllium (e.g., Metamucil®), methylcellulose (e.g., Citrucel®) or konjac (e.g., Lipozene®), appears to have several benefits regarding cardiometabolic health. For those with T2D, this meta-analysis shows evidence to support favorable effects on glycemic control and insulin sensitivity. More research is needed to establish more clearly whether all viscous fibers enhance insulin sensitivity, or whether this property is limited to those with certain characteristics, such colonic fermentability or content of specific bioactive compounds3,4. Evidence from other sources also shows that viscous fiber lowers the circulating cholesterol level, likely by trapping cholesterol and bile acids, thus preventing their absorption/reabsorption3. In addition, viscous fibers appear to play a role in appetite regulation, enhancing satiety after meal5.
The meta-analysis by Jovanovski and colleagues shows that inclusion of viscous fiber in the diet produces clinically meaningful improvements in glycemic control for patients with T2D. Based on this, as well as evidence for other benefits (cholesterol lowering and enhanced satiety), inclusion of viscous fiber from foods and/or supplements should be considered an important component of the management plan for patients with T2D.
- American Diabetes Association. 10. Cardiovascular Disease and Risk Management: Standards of Medical Care in Diabetes—2019. Diabetes Care. 2019;42(Supplement 1): S103-S123.
- Jovanovski E, Khayyat R, Zurbau A, et al. Should viscous fiber supplements be considered in diabetes control? Results from a systematic review and meta-analysis of randomized controlled trials. Diabetes Care. 2019 Jan; doi:10.2337 [Epub ahead of print].
- Weickert MO, Pfeiffer AFH. Impact of dietary fiber consumption on insulin resistance and the prevention of type 2 diabetes. J Nutr. 2018;148:7-12.
- Kärkkäninen O, Lankinen MA, Vitale M, et al. Diets rich in whole grains increase betainized compounds associated with glucose metabolism. Am J Clin Nutr. 2018;108:971-979.
- Rebello CJ, Chu YF, Johnson WD, et al. The role of meal viscosity and oat ß–glucan characteristics in human appetite control: a randomized crossover trial. Nutr J. 2014;13:49.