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Department of Nutritional Sciences, Faculty of Medicine, University of Toronto (S.P., A.E., V.V.)
Clinical Nutrition and Risk Factor Modification Centre and Division of Metabolism and Endocrinology, St. Michael's Hospital (S.P., A.E., V.V.), Toronto, Ontario
Department of Agricultural, Food and Nutritional Science, University of Alberta (F.T., T.V.), Edmonton, Alberta, CANADA
Address reprint requests to: Vladimir Vuksan, PhD, University of Toronto, Faculty of Medicine, Department of Nutritional Sciences, Toronto, ON, M5S 3E2, CANADA. E-mail: v.vuksan{at}utoronto.ca
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ABSTRACT |
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FOOTNOTES
ABSTRACT
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
ACKNOWLEDGMENTS
REFERENCES |
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Design: In an acute, randomized, double-blind, crossover study, 11 healthy subjects (gender: 5M:6F; age: 34 ± 5 years; BMI: 23 ± 0.8 kg/m2) were randomly assigned, on three separate occasions, to consume one of three fiber-matched treatments along with a 75g oral glucose drink. The enzymatically processed β-glucan (Oat-A) differed from β-glucan processed through the aqueous method (Oat-B) solely with regard to viscosity. Finger-prick capillary blood samples were obtained at fasting and at 15, 30, 45, 60, 90 and 120 min after the start of the test drink. The viscosities of the fiber drinks were determined (Paar Physica UDS200 viscometer).
Results: Rheological measurements demonstrated that Oat-A had a significantly higher viscosity than Oat-B and control at 5, 15, 30, 60, and 120 min (p < 0.001). The incremental area under the glucose curve (AUC) on Oat-A was 19.6% and 17% lower than that of Oat-B and control, respectively (p < 0.01).
Conclusions: This study shows that processing oat β-glucan through enzymatic, rather than by aqueous methods, preserves the viscosity and improves postprandial glycemic control.
Key words: β-glucan, viscosity, postprandial glycemia, healthy individuals
Abbreviations: M = males • F = females • BMI = body mass index
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INTRODUCTION |
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ABSTRACT
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
ACKNOWLEDGMENTS
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In order to increase the physiological effectiveness of oat β-glucan concentrates, a unique technology has been developed to increase their β-glucan content while maintaining its native molecular weight and thus, viscosity. The current study investigated oat β-glucan concentrates produced by two processing methods: the first is an aqueous process conventionally used to extract β-glucan by solubilizing it in large quantities of water; the second is a novel process which is an alcohol-based enzymatic technique used to concentrate the native cell walls containing the β-glucan. While the aim of both procedures is to remove the starch and protein components of the oats, the second process aims to obtain a fiber concentrate of high viscosity by preserving the native state of β-glucan, whereas the β-glucan can be degraded upon solubilization in the first process due to high shear and endogenous enzyme activity.
The purpose of this study was to examine the intra-fiber variability between these two methods of β-glucan concentration by (1) investigating the effect of two sources of oat β-glucan concentrates on postprandial glycemia in healthy individuals and to (2) explore the levels of viscosity required to achieve glucose-lowering effects.
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SUBJECTS AND METHODS |
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FOOTNOTES
ABSTRACT
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
ACKNOWLEDGMENTS
REFERENCES |
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Treatments
Subjects received a total of three treatments in a double-blind, randomized order. The tests included 300 ml of an oral glucose drink (75g-Glucodex®, Chambly, QB) diluted with 150 ml of water added to either 10.17 g of Oat-B (6g of β-glucan; concentrate obtained by an aqueous extraction process), 11.05 g of Oat-A (6g of β-glucan; concentrate obtained by an alcohol-based enzymatic process) or a combination of 8 g fructooligosaccharide and 3 g of wheat bran (control). Seventy-five grams of glucose was the standard dose administered according to WHO guidelines. The treatments were almost identical in energy and macronutrient composition. The beverages were prepared by slowly adding the fiber to 150 mL of water while stirring to ensure that it was mixed uniformly and then adding 300 mL of Glucodex®. The drink was administered to subjects in the mixing cup immediately following preparation. Subjects were asked to consume the entire beverage over 5 min. The weighing, mixing and preparation of the three treatments were performed by a blinder independent from the study, who also performed the randomization using a randomization table. Subjects were blinded by providing them with the treatments in two equal portions so that they were not left to gel and they were allotted 5 min to consume the treatment drinks.
Protocol
Subjects attended the Clinical Nutrition and Risk Factor Modification Centre at St. Michael's hospital on three separate mornings following a 10–12 h overnight fast. A minimum of 3 days separated each visit to eliminate any carry-over effects. Subjects were instructed to maintain the same dietary and exercise patterns the evening before each visit. To ensure that these instructions were followed, subjects completed a questionnaire detailing pre-session information about their diet and lifestyle patterns. Before the beginning of each test, subjects' height (m) and weight (kg) were measured and each subject gave approximately 250µL of a fasting finger-prick capillary blood sample, using a Monoejector Lancet device (Owen Mumford Ltd., Woodstock, Oxon, England). One of the three treatments was then administered. Additional finger-prick blood samples were obtained at 15, 30, 45, 60, 90, and 120 min after the start of the treatment. All samples were collected in tubes containing fluoride oxalate and immediately frozen at –20°C pending analysis.
Blood Glucose Analysis
All samples were analyzed within three days of collection. The glucose concentration of each was determined by the glucose oxidase method using a YSI 2300 Stat glucose/L-lactate analyzer, model 115 (Yellow Springs, Ohio).
Viscosity Determination
Corresponding amounts of the fiber ingredients were blended into 15 mL of water followed by 30 mL of Glucodex®. In order to illustrate the viscosity changes over time, drinks were held in a water bath at 37°C for up to 2 h (duration of glycemic response testing). Throughout this holding period, tubes containing the beverage samples were inverted 2–3 times every 10 min to provide a gentle mix in an effort to mimic body conditions, rather than continuous stirring with a magnetic stirrer since the mixing is rather vigorous even at the lowest setting. Samples were removed from the water bath at the end of the holding time, centrifuged (3000 x g for 10 min) and the viscosity of the supernatant was determined using a Paar Physica UDS 200 viscometer (equipped with a cup and bob double gap geometry and Peltier heating system) at 37°C and different shear rates. The samples were centrifuged to remove insoluble solids due to the sensitivity of the narrow double gap geometry of the rheometer. Therefore, the viscosity of the whole drink may be higher; however, the viscosity of the supernatant as determined in this test represents the soluble fraction thickened mainly by β-glucan.
Statistical Analyses
A power calculation was carried out in order to select the number of subjects required to participate in the study. Differences in postprandial blood glucose were detected with 80% power at a level of p < 0.05 (two-tailed) by the number of subjects. Blood glucose curves were plotted as the incremental change in blood glucose over time and the positive incremental area under the curve (AUC) was calculated geometrically for each subject, ignoring areas below the fasting blood glucose value [6]. Incremental blood glucose concentrations were used to control for baseline/fasting differences between the treatments. Statistical analyses were then performed using the Number Cruncher Statistical System (NCSS statistical software, Kaysville, Utah). Repeated measures two-way analysis of variance (ANOVA) assessed interactive and independent effects of treatment (Oat-A vs. Oat-B vs. control) as well as viscosity (Oat-A vs. Oat-B vs. control). Pairwise differences in AUC and incremental glycemia at each time point (15, 30, 45, 60, 90, 120 min) between the treatments were assessed by repeated measures one-way ANOVA adjusted for multiple comparisons with the Tukey-Kramer multiple comparison test. All results were expressed as mean ± SD and considered statistically significant at p < 0.05.
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RESULTS |
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FOOTNOTES
ABSTRACT
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
ACKNOWLEDGMENTS
REFERENCES |
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Viscosity measurements for the 3 treatments are shown, where the centipoise (cps) was used as a unit of absolute viscosity (Fig. 1). Rheological measurements demonstrated that Oat-A had a significantly higher viscosity than Oat-B and control at 5, 15, 30, 60 and 120 min (p < 0.001) using two-way repeated measures ANOVA.
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DISCUSSION |
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Consumption of β-glucan with a meal increases the viscosity of the food bolus in the alimentary tract [8]. The increased viscosity is thought to reduce the absorption rate of the digested nutrients from the small intestine by resisting the convective effects of intestinal contractions and thus decreasing the postprandial glucose and insulin peaks [13]. This effect is thought to be proportional with the level of fiber viscosity [3,9], defined as the resistance to flow, or "stickiness", that enables fibers such as β-glucan to form a viscous slurry in the gastrointestinal tract [14,15]. Increased viscosity of the intestinal contents also impedes bulk diffusion from the lumen to the surface of the small intestine, due to an increase in thickness of the unstirred water layer [10], and perhaps due to delayed gastric emptying [11].
Oats was one of the first functional foods approved for health claim by the United States Food and Drug Administration (FDA) because it has been studied extensively. Specifically, it is claimed that the consumption of oat and oat products containing 0.75 grams of β-glucan per serving can reduce the risk of heart disease [12]. The FDA accepts that viscosity is a major physicochemical property responsible for the physiological effects of soluble fiber [16]. This recognition is based on the fact that not all clinical studies evaluated by the FDA [12] resulted in positive outcomes and that physicochemical properties of β-glucan can be significantly affected by processing treatments. There is growing interest in extracting β-glucan from oats and barley so that it can be incorporated into a variety of food products, since it is not realistic to consume the recommended 3g β-glucan/day [12] based on oats alone over extended periods of time.
Conventional methods of concentrating β-glucan involve aqueous extraction processes whereby large quantities of water are required to solubilize β-glucan. The resulting β-glucan concentrates from these conventional methods show lower viscosity upon resolubilization in water, as demonstrated in the current study with Oat-B. This may result from the high shearing involved in processing and endogenous enzymes present in the grains, such as cellulase and β-glucanase which hydrolyze the β-glucan molecule, altering its molecular weight and therefore, its ability to form viscous solutions. Thus, the quality of β-glucan is degraded. A recently developed novel technology concentrates β-glucan using an alcohol-based enzymatic technique. In this process, grain flour is slurried in aqueous ethanol and treated with special enzyme cocktails to hydrolyze protein and starch. Subsequently, the β-glucan enriched cell wall fiber particulates that are freed from starch and protein are recovered by simple filtration techniques. In the presence of alcohol, β-glucan is not solubilized and remains intact within the cell wall. This technology yields fiber concentrates with superior β-glucan characteristics (i.e. higher molecular weight and viscosity) than those produced through aqueous techniques as demonstrated with Oat-A. Since β-glucan is not solubilized from cell walls, potential hydrolysis by endogenous enzymes such as cellulose and β-glucanase is minimized. As a result, β-glucan concentrations of up to 60% (w/w on a dry weight basis) can be obtained by this technology that preserves the physicochemical properties of native β-glucan. Viscosity measurements thus confirmed our hypothesis that Oat-A had a higher viscosity than Oat-B and control (Fig. 1) and that there was a reduction in postprandial blood glucose (Figs. 2 and3).
A review of clinical studies shows that there have been less clear relationships between polysaccharide viscosity and glycemic response. The data from this study are in agreement with Wood et al. [17] indicating that rheological properties such as viscosity are more important in determining the physiological effects of fiber than the type and quantity of fiber, however, this study examines the glycemic-lowering effects of β-glucan concentrated with an enzymatic technology that specifically targets the preservation of viscosity. The findings have shown that this concentration process can be successful in maintaining the viscosity and physiological effects of native β-glucan without any degradation. There have been previous reports relating viscosity and the dose of β-glucan to glycemic control, however, this study emphasizes that even with the same dose of β-glucan, viscosities may differ due to the type of processing method - a factor that may influence the amount and properties of β-glucan. Our data is similar to data presented in other literature; however, there is inadequate information on the bioavailability of β-glucan, which makes comparison among the studies difficult. Selecting the process of concentration is important in the preservation of the β-glucan molecule. Beta-glucan is a cell wall component of cereal grains and exists in appreciable amounts in barley (up to 15%, w/w) and oat (up to 7%, w/w). A number of clinical studies have demonstrated the positive health benefits (i.e. lowering of blood cholesterol, regulation of blood glucose levels and stimulation of the immune system) of increasing the level of oat or barley β-glucans to physiologically effective concentrations in the human diet.
Implications of these findings are promising. The effect of various processing conditions on the physicochemical properties of oat β-glucans such as viscosity and how such effects influence hyperglycemia and conditions including hyperlipidemia and other risk factors for diabetes and cardiovascular disease are certainly important topics for further research. As well, there is a growing interest in extracting β-glucan from oats so that it can be incorporated into a variety of food products, however, findings in the liquid form may not be universally applied to the solid form. Little is known about the behavior of β-glucan in different food matrices and recent findings only show an inconsistent effect of β-glucan when administered in different forms [18]. This is subject to future investigations. To our knowledge, this is the first study to compare two oat β-glucan concentrates with similar chemical compositions, but different viscosities. This study emphasizes the importance of processing methods in the production of oat β-glucan concentrates in preserving the viscosity of β-glucan, thus increasing the efficacy of such products in reducing postprandial glycemia.
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ACKNOWLEDGMENTS |
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TOP
FOOTNOTES
ABSTRACT
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
ACKNOWLEDGMENTS
REFERENCES |
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FOOTNOTES |
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TOP
FOOTNOTES
ABSTRACT
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
ACKNOWLEDGMENTS
REFERENCES |
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Disclosure: None of the authors had a conflict of interest.
Received March 18, 2006. Accepted September 6, 2006.
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TOP
FOOTNOTES
ABSTRACT
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
ACKNOWLEDGMENTS
REFERENCES |
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  K. R. Juvonen, A.-K. Purhonen, M. Salmenkallio-Marttila, L. Lahteenmaki, D. E. Laaksonen, K.-H. Herzig, M. I. J. Uusitupa, K. S. Poutanen, and L. J. Karhunen Viscosity of Oat Bran-Enriched Beverages Influences Gastrointestinal Hormonal Responses in Healthy Humans J. Nutr., March 1, 2009; 139(3): 461 - 466. [Abstract] [Full Text] [PDF]
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