Physical Effects of Fiber in the Small Intestine

Type 2 Diabetes/Glycemic Control

Epidemiological evidence consistently shows that high consumption of cereal fibers are associated with a reduced risk of developing type 2 diabetes (relative risk 0.77), but the evidence is less convincing for fruit fibers (relative risk 0.94) and vegetable fibers (relative risk 0.95) [4]. When isolated fibers are assessed in well-controlled intervention studies, only soluble gel-forming fibers significantly improve glycemic control, and the effect is proportional to the viscosity of the gelling fiber [5]. In the fed state, the motor activity of the small bowel comprises segmental (mixing) contractions. Chyme is normally low in viscosity and easily mixed with digestive enzymes for degradation. The large surface area of the mucosa (roughly equivalent to a tennis court) results in efficient absorption of nutrients, which typically occurs early in the proximal small bowel. Introduction of a gel-forming fiber [e.g., P-glucan (oatmeal), psyllium] significantly increases the viscosity of chyme, which slows the mixing of chyme with digestive enzymes, and slows nutrient absorption, which can decrease postprandial blood glucose concentrations [3, 5]. It is well established that this effect occurs only with consumption of a gel-forming fiber, and efficacy is highly correlated with the viscosity of the gel-forming fiber [6]. In a study published in 1978 [6], subjects underwent a glucose (50 g) tolerance test with and without several different fiber supplements. The study showed that high-viscosity gel-forming fibers had a significant effect on postprandial peak blood glucose concentration, but as viscosity declined, so did the effect (r = 0.926; p < 0.01) [6].

Gel-forming fibers also have a significant effect on long-term glycemic control. The delay of nutrient absorption can deliver nutrients to the distal ileum (where nutrients are not normally present), stimulating mucosal L-cells to release glucagon-like peptide-1 (GLP-1), a peptide that has significant metabolic effects: decreased appetite, increased pancreatic p-cell growth, improved insulin production and sensitivity, decreased glucagon-secretion, and stimulation of the “ileal brake” (slowing gastric emptying and small bowel transit) [3, 5]. Note that a viscous, gel-forming fiber can slow the absorption of nutrients, but it does not reduce total nutrient absorption [5]. The ileal brake phenomenon effectively slows gastric emptying and small bowel transit to impede the loss of nutrients to the large bowel [5]. Multi-month clinical studies have demonstrated that consumption of a viscous, gelforming fiber (dosed with meals) can lower fasting blood glucose, insulin, and HbA1c in patients with metabolic syndrome and patients being treated for type 2 diabetes mellitus (T2DM) [3]. The effectiveness of a gel-forming fiber is proportional to baseline glycemic control: no significant effect in eugly- cemia, a modest effect in pre-diabetes [e.g., -19.8 mg/dL (-1.1 mmol/L) for psyllium; -9 mg/dL (-0.5 mmol/L) for guar gum), and the greatest effect in patients being treated for type 2 diabetes [e.g., psyllium, -17.3 mg/dL (-1.0 mmol/L) to -89.7 mg/dL (-5.5 mmol/L)] [3]. A recent meta-analysis of 35 multi-month clinical studies showed that psyllium significantly improved both fasting blood glucose concentration [-37 mg/dL (-2.1 mmol/L); p < 0.001] and HbA1c (-1.0; p = 0.048) in patients being treated for T2DM [7]. The improvements in glycemic control observed with psyllium are additive to the effects of a restricted diet and stable doses of prescription drugs (sulfonylureas and/ or metformin) [4]. Even though a gel-forming fiber will not directly cause hypoglycemia, fasting blood glucose concentrations should be monitored when starting an effective fiber therapy in patients already taking prescription drugs to control blood glucose. The added benefit of a gel-forming fiber may decrease the required dose for prescription drugs that can cause hypoglycemia.

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