Effects of Soy-Soluble Fiber and Flaxseed Gum on the Glycemic and Insulinemic Responses to Glucose Solutions and Dairy Products in Healthy Adult Males

Soy soluble polysaccharides and flaxseed gum are underutilized dietary fibers of interest to the food industry. However, because the ability of soluble fibers to modulate postprandial glucose and insulin metabolism has been related to their viscous effects, the utility of these and other low-viscosity soluble fibers remains unproven.  The objective of this study was to examine the associations between soy-soluble polysaccharides and flaxseed gum concentration, product viscosity, and the postprandial glycemic and insulinemic responses in the context of glucose solutions as well as fluid and gelled dairy products. Twelve healthy males participated in a randomized crossover postprandial study in which they visited the laboratory following overnight fasts on 11 occasions to consume one of 11 study treatments, each consisting of 50 g available carbohydrates. The study treatments included a glucose reference (in duplicate), glucose solutions containing soy-soluble polysaccharides (6 percent), flaxseed gum (0.7 percent), or guar gum (0.23 percent), all matched for an apparent viscosity of 61 mPa•s at 50 s1, as well as dairy-based beverages and puddings with 0 percent or 1 percent soluble fiber added. Blood samples were collected at fasting and up to 2 hours postprandially for determination of glucose and insulin concentrations. Area under the curve (AUC), peak concentration, and time-to-peak values as well as glycemic index (GI) and insulinemic index (II) were calculated. Fiber fortification of a 50 g glucose solution had no effect on postprandial blood glucose or insulin levels, even at a high concentration (i.e., 6 percent soy soluble polysaccharides). Glucose AUC and GI values for the dairy-based beverage  and pudding controls were significantly lower than the glucose reference. Glucose AUC and GI values for the soy soluble polysaccharide-fortified dairy products and flaxseed gum fortified dairy products were significantly lower than the glucose reference. No significant differences were observed between the fiber fortified fluid and gelled dairy based study treatments and no significant differences were observed in terms of the insulin AUC, II, and peak insulin concentration between any of the dairy products. All dairy products had lower glycemic responses relative to the reference, with no effect of beverage versus pudding matrix observed and minimal impact of 1 percent flaxseed gum or soy soluble polysaccharides.  Product apparent viscosity, but not fiber concentration, was significantly and inversely correlated with glucose AUC and GI. (Authors abstract)

The dietary reference intake for dietary fiber is 14 g per 1000 kcal of total energy consumption per day. This translates into roughly 25 and 38 g for the average North American adult woman and man, respectively. However, North Americans, on average, only consume about 14 g of fiber per day.  One strategy to increase fiber consumption is through fortification of foods. However, it can be challenging to incorporate significant amounts of fiber without inducing deleterious changes in product quality. Though there is significant interest in identifying and incorporating novel fibers into food products, this requires careful consideration of functionality. For example, the ability of soluble fibers to modulate postprandial metabolism has been related to their viscosity. Highly significant negative correlations between postprandial glucose and insulin responses and viscosity have been observed in fiber-fortified glucose solutions. Moreover, the ability of soluble fibers to modulate postprandial glycemic response can be lost when their viscosities are reduced through hydrolyzation.  Flaxseed gum and soy soluble polysaccharides are examples of relatively low-viscosity fibers under consideration for use in fiber-fortified products and for which further nutritional research is required. Flaxseed gum is extracted by soaking flaxseed hulls in water and contains 3 main polysaccharides; that is, a neutral one identified as arabinoxylan and two acidic pectin-like ones characterized as rhamnogalacturonan type I and type II. In a human study, substitution of wheat flour for flaxseed gum in white bread, at approximately 2.16 percent by weight and in the presence of insoluble flax fiber, was correlated with reductions in postprandial blood glucose. In a more recent intervention study, participants with T2DM who consumed 5 g flaxseed gum daily for 3 months saw significant improvements in fasting glucose levels.

This study was undertaken to examine the associations between low viscosity soluble fiber concentration, product rheology, and postprandial glucose and insulin with fiber fortified glucose solutions and dairy products. Two sets of dairy products, thickened beverages and gelled puddings were developed to investigate potential differences between a fluid versus gelled food matrix in terms of postprandial response. It was hypothesized that the viscosity-matched fiber-fortified glucose solutions would elicit similar glycemic and insulinemic responses and that the higher viscosity dairy products would have lower postprandial responses compared to those with lower viscosities. The maximum level of soy soluble polysaccharide addition to a 50 glucose solution was 6.0percent, achieving an apparent viscosity (at 50 s1) of about 61 mPa•s. However, modulations in postprandial glucose or insulin were not observed for the SG sample or the viscosity-matched samples containing flaxseed gum (0.7percent) or guar gum (0.23 percent). The possibility that sample viscosity was too low to exert an effect was the present study was higher than the minimum effective viscosity reported (i.e., in the range of 70–75 mPa•s at the same shear rate, 30 s1), but no effects on postprandial glycemia were observed. The observation of no difference in insulin response between the fiber fortified glucose solutions and reference supports the conclusion that addition of 0.7percent flaxseed gum, 6.0percent soy soluble polysaccharide, or 0.23percent guar gum, achieving a viscosity of about 61 mPa•s at 50 s1, did not influence postprandial carbohydrate metabolism. Although the ability of a soluble fiber to reduce postprandial glucose and insulin is typically attributed to the viscous effects it brings about in the gastrointestinal tract, the viscosity of ingested foods is subject to substantial changes, including through significant dilution, throughout the gastrointestinal tract. Therefore, viscosity of samples prior to gastrointestinal exposure may not be reflective or predictive of physiological effects. Aside, from impacting bulk diffusion of glucose within the gastrointestinal tract, dietary fiber may also modulate glucose absorption by impacting gastric emptying, forming a barrier along the mucosal layer or impacting hormonal feedback mechanisms.

The fiber fortified dairy products in the present study were standardized to contain 1percent fiber and had different viscosities.  Addition of 1percent flaxseed gum induced a higher viscosity in both the beverages and puddings compared to the same level of soy soluble polysaccharide addition. Within the dairy beverage and pudding categories, glucose AUC and GI tended to decrease in the order of control, soy soluble polysaccharides, and flaxseed gum; that is, corresponding inversely with product viscosity. Within each product type, the control products without any fiber present had both the lowest viscosity and the highest glucose AUC and GI values, whereas the flaxseed gum fortified study treatments were the most viscous and had the lowest glucose AUC and GI values. These observations suggest that soy soluble polysaccharides and flaxseed gum induced greater changes in postprandial glucose metabolism relative to the control dairy products of similar composition without added fiber.

Significant reductions in peak glucose after consumption of the SB, FB, and FP were also observed, suggesting that the rate of carbohydrate digestion and/or absorption were decreased by the presence of soy soluble polysaccharides and flaxseed gum.  Regression analysis indicated significant inverse correlations between glucose AUC, GI and glucose time-to-peak with study treatment viscosity. Soy soluble polysaccharides and flaxseed gum concentration were not correlated with modulations in postprandial response, although negative correlations between fiber concentration and postprandial response have previously been reported. This discrepancy is likely explained by the fact that fiber concentration often has a direct effect on solution viscosity, and this was not the case with
the low-viscosity fibers used in this study.

The addition of 0.23 percent guar gum, 0.7percent flaxseed gum, or 6.0percent soy soluble polysaccharides to a 50 g glucose solution, achieving a matched viscosity of 61 mPa•s at a shear rate of 50 s1, had no effect on postprandial glucose AUC, peak concentration, or time to peak. Therefore, the addition of low viscosity soluble fibers, even at relatively high concentrations, may not be biologically functional in terms of attenuating postprandial blood glucose or insulin response.  Overall, the addition of soy soluble polysaccharides and flaxseed gum to the dairy products had modulating effects on postprandial glycemia. Downward trends in glucose AUC and GI with increasing viscosity were observed for the fiber fortified dairy study treatments and significantly lower peak glucose concentrations were observed relative to the reference in all but one case (i.e., SP). Overall, study treatment viscosity was significantly correlated with glucose AUC, GI, and glucose time-to-peak, whereas fiber concentration was not correlated with any of the postprandial parameters studied. The results supports the body of research concluding that postprandial glycemia depends more on product viscosity than fiber concentration, but that viscosity is not the only determinant of postprandial response to a 50 g available carbohydrate load. (Editors comments)