Mechanisms behind the cholesterol-reducing effect of cereal soluble dietary fibres: (1,3:1,4) beta glucan & arabinoxylan

Purnima Gunness (2011). Mechanisms behind the cholesterol-reducing effect of cereal soluble dietary fibres: (1,3:1,4) beta glucan & arabinoxylan PhD Thesis, Centre for Nutrition and Food Sciences, The University of Queensland.

       
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Author Purnima Gunness
Thesis Title Mechanisms behind the cholesterol-reducing effect of cereal soluble dietary fibres: (1,3:1,4) beta glucan & arabinoxylan
School, Centre or Institute Centre for Nutrition and Food Sciences
Institution The University of Queensland
Publication date 2011-12
Thesis type PhD Thesis
Supervisor Prof Mike gidley
Dr Bernadine Flanagan
Total pages 138
Total colour pages 14
Total black and white pages 124
Language eng
Subjects 111103 Nutritional Physiology
090803 Food Nutritional Balance
Abstract/Summary Epidemiological, clinical and animal studies have shown a positive relationship between diets rich in soluble dietary fibres (SDF) such as -glucan (βG), pectin, guar gum, psyllium and arabinoxylan (AX), and reduced serum cholesterol leading to a decreased risk of cardiovascular disease (CVD). Three mechanisms have been proposed to explain the cholesterol-reducing effects of SDF: prevention of bile salts (BS) reabsorption from the small intestine leading to an excess faecal BS excretion; reduced glycemic response leading to lower insulin stimulation of hepatic cholesterol synthesis; and fermentation products of SDF, mainly propionate causing a reduced hepatic cholesterol synthesis via enzyme inhibition. The most convincing evidence has been reported for the BS “binding” mechanism; however the nature of the interactions between the BS-cholesterol mixed micelles and SDF are poorly defined. Therefore in this thesis, three physicochemical mechanisms have been postulated and in vitro experiments carried out to test these hypotheses on simple systems made up of mono-molecular BS micelles and different sources and molecular weights SDFs. These hypotheses are: Hypothesis 1: SDF increases the barrier properties of the unstirred layer between BS micelles & intestinal absorptive cells Hypothesis 2: SDF & BS are associated / complexed at a molecular level Hypothesis 3: SDF forms a local matrix that entraps BS micelles. Further, an animal experiment in which pigs were used as a human model was carried out to investigate whether a wheat soluble arabinoxylan rich fraction (AXRF) added to a typical Western diet lowers blood cholesterol and triglycerides by the same mechanisms as βGs. The passage kinetics of a model bile salt and complete porcine bile across a dialysis membrane of 20 KDa MWCO in the presence and absence of two cereal SDF polysaccharides barley βG and wheat AX has been studied as a model for passage across the unstirred water layer that lines the small intestine. A first-order kinetic analysis was used to calculate the rate coefficients (K) which quantified the effectiveness of barley βG and wheat AX in retarding the transport of bile salts. For both the model bile salt and porcine bile, steady state viscosity was shown to be more related to rate coefficients than concentration. These results suggest that both polymers delay the passage of BS, with effectiveness determined by solution viscosity. The 2nd and 3rd hypotheses have been studied using 13C nuclear magnetic resonance (NMR) spectroscopy and small angle X-ray scattering (SAXS). Investigation of molecular interactions were carried out between model BS (taurochenodeoxycholate: TCDC or glycochenodeoxycholate: GCDC) micelles with each of bG and AX. The results showed that the SDFs interact differently with the BS depending not only on the source but also on their molecular weights. In the presence of oat and barley bG, small chemical shift changes were observed in the BS resonances, but not in the bG resonances, without any apparent change in line widths. The SAXS results showed that the X-ray scattering of the BS and bG were not additive and consistent with changed dimensions of bG in the presence of BS. The combined results from NMR and SAXS suggest that the BS micelles form dynamic interactions leading to temporary adsorption to the polymer. In contrast, in the presence of low and medium molecular weights wheat AX, no systematic chemical shift changes for either TCDC/GCDC or AX resonances were found, but line broadening with decreased intensities of the TCDC/GCDC signals was observed. The SAXS results showed that the scattering of the BS and AX were additive with no change in the dimensions of either component in the presence of the other. These results suggest that the AX polymers form local networks entrapping the micelles and restricting their mobility. High molecular weight barley and oat βG and rye AX form both close molecular interactions and network with the micelles. In the animal trial, 40 male pigs were separated by weight into 4 groups and each group was fed on one of 4 experimental diets for 4 weeks. They were then anesthetised and blood and intestinal samples were collected. The diets were comprised of a typical Western diet containing low or high red meat content with and without added wheat arabinoxylan-rich fraction (AXRF). The blood samples were analysed for total BS in the jugular (JV) and hepatic portal (HP) veins while total cholesterol (TC), high density lipoprotein cholesterol (HDL-C) and triglycerides (TAG) were analysed in the JV. The intestinal samples taken from 4 sections of the small intestine and the caecum were analysed for individual BS and fatty acids (FA) apparent digestibility. The pigs fed the AXRF diets had significantly decreased blood BS from both locations (P-values: HPV=0.003; JV=0.001) and TAG (P=0.02) but with no effect on cholesterol (P-values: TC=0.28; LDL-C=0.45), the latter probably because the Western-style diet did not induce hypercholesterolemia. The reduced blood TAG levels were consistent with the reduced/delayed TAG digestion in the small intestine found in the presence of AXRF (P-values: total fat=0.02; SFA=0.08; MUFA=0.01; unsaturated FA=0.02).
Keyword (1,3:1,4)-beta-D-glucan
arabinoxylan
cholesterol
bile salt
binding
13C NMR
passage kinetics
SAXS
fatty acid profile
triglycerides
viscosity
Additional Notes Coloured pages: 16, 26, 27, 28, 32, 48, 58, 59, 61, 63, 64, 65, 88, 89 Landscape pages: 18, 26, 27, 28, 46, 83, 85, 88, 89, 116 to 123

 
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Created: Fri, 01 Jun 2012, 13:11:22 EST by Purnima Gunness on behalf of Library - Information Access Service