Cereal Fiber Improves Whole-Body Insulin Sensitivity in Overweight and Obese Women
Cereal fiber intake is linked to reduced risk of type 2 diabetes in epidemiological observations. The pathogenic background of this phenomenon is unknown. Based on recent findings, we hypothesized that intake of purified insoluble oat fiber may improve whole-body insulin sensitivity.
Cereal fiber consumption is linked to a reduced risk of type 2 diabetes and cardiovascular disease in prospective cohort studies. Notably, associations between cereal fiber intake and reduced diabetes risk remain significant after correction for confounding factors, e.g., changes in body weight, age, exercise, intake of fat, smoking, alcohol intake, or a family history of diabetes. To date, no obvious mechanisms for the beneficial effects of cereal fibers have been described. Based on recent findings by our group, we hypothesized that consumption of purified insoluble fiber, which is the predominant fraction of cereal fiber, might improve whole-body insulin sensitivity. Here we show that an intake of insoluble dietary fiber within the recommended daily range for a time period as short as 3 days significantly improved whole-body insulin sensitivity in overweight and obese women, as assessed by the euglycemic-hyperinsulinemic clamp. Insulin sensitivity was improved by 13% in those subjects, who were likely to have ingested the test meals, and this effect was attenuated, but still significant, when results for all subjects were analyzed. Notably, the magnitude of pharmacological improvement in insulin-stimulated glucose metabolism after a 3-month treatment with the insulin-sensitizing drug rosiglitazone was between 20 and 68%, depending on the administered doses of insulin during the clamp and with the most pronounced effect using relatively low insulin doses (120 vs. 20 mU/m2 per min). Potential molecular mechanisms leading to improved insulin sensitivity remain unexplained by the current study. However, the presented data clearly indicate that insoluble fibers, containing mainly cellulose and hemicellulose, are unlikely to be physiologically inert and may be interesting candidates for future research. We have previously shown that the insoluble fiber used in our experiments is unlikely to influence macronutrient absorption or gastric emptying, which is in accordance with the literature. An increased magnesium intake has been shown to improve insulin sensitivity. However, magnesium contents of the test meals in the current study were virtually identical, and serum magnesium concentrations were unchanged by fiber intake. No effect of fiber intake on blood lipids, serum ghrelin, or serum adiponectin concentrations could be detected as other possible drivers that might have influenced insulin sensitivity.
Dietary intervention studies in humans commonly face the problem of diet control, and dietary nonadherence is likely to increase variance and therefore to weaken the results. This may at least partly explain controversial results of former studies, with some but not all indicating metabolic improvement after longer-term interventions with insoluble dietary fibers. We have previously shown that the insoluble fiber used in the current study caused slight, but significantly enhanced, colonic fermentation compared with control, as assessed by postabsorptive hydrogen breath concentrations. Therefore, in the current study, we used this test as a tool for controlling dietary adherence. When performing a subanalysis, excluding four subjects with unchanged or lower hydrogen breath concentrations after the intake of the fiber-enriched meals compared with control, improvement of insulin sensitivity was highly significant, indicating that dietary nonadherence is likely to have attenuated our findings. Fermentation processes in the colon resulting in increased production of short-chain fatty acids have been proposed as being involved in improvement of hepatic insulin sensitivity, possibly by upregulating glucagon-like peptide 1 and thus suppressing glucagon secretion. In the current study, basal plasma glucose concentrations were not altered by fiber consumption. Moreover, we have recently shown that improved glucose handling after a 24-h intake of purified insoluble wheat or oat fiber was completely independent of the rate of colonic fermentation, and glucagon-like peptide 1 concentrations remained unchanged. In addition, other authors did not detect altered hepatic insulin sensitivity after intake of high-fiber diets. Therefore, it is reasonable to assume that only subjects that indeed consumed the fiber-enriched test meals showed improved insulin sensitivity, but a lack of an insulin-sensitizing effect in subjects with low capacity of colonic fermentation upon fiber ingestion cannot be excluded.
In a randomized, controlled, cross-over study, Robertson et al. recently described increased insulin sensitivity after a 4-week intake of resistant starch, as assessed by euglycemic-hyperinsulinemic clamp. The authors performed muscle and fat biopsies and could not detect any differences in expression of a number of skeletal muscle genes, including insulin-receptor substrate 1, phosphatidylinositol-3-kinase, or GLUT-4. However, a complex network of downstream transcription factors and coactivators is likely to be affected by dietary interventions and might interfere with insulin signaling in diverse tissues, including the liver. Moreover, the main components of cereal fibers are cellulose and hemicellulose, which are likely to have distinct biological effects and physicochemical properties compared with resistant starch.
The use of fiber supplements in dietary recommendations has been criticized. However, in light of the discrepancy between recommended fiber intake of 20–35 g total fiber/day and actual consumption of <15 g/day, it seems to be important to identify active substances of dietary fibers that might have favorable health effects.
The current study may be of public health relevance, providing a potential link between cereal fiber consumption and reduced risk of type 2 diabetes. By favorably influencing whole-body insulin sensitivity, intake of insoluble fiber by quantity could be underrepresented in current dietary recommendations. An emphasis on cereal, fruit, and vegetable consumption containing a particularly high proportion of insoluble dietary fiber might be a safe, effective, and low-cost approach to reduce insulin resistance. This could be of specific advantage for overweight and obese subjects at risk of developing type 2 diabetes. Further studies are needed to assess whether the current findings can be generalized to other cohorts, including males and subjects with marked insulin resistance, such as those with type 2 diabetes.