Relationship between structure and function of dietary fibre: a comparative study of the effects of three galactomannans on cholesterol metabolism in the rat.

Male adult rats were fed on diets containing 80 g/kg galactomannans with different galactose (G): mannose (M) ratios/kg. The galactomannans were compared with purified cellulose (Solkaflok) and the animal were also fed on a basal diet free from fibre. All diets contained cholesterol (10 g/kg) and sodium cholate (2 g/kg). The three galactomannans were fenugreek gum (1G:1M), guar gum (1G:2M) and locust-bean gum (1G:4M). In comparison with the fibre-free and Solkaflok diets, all three galactomannans lowered the concentrations of cholesterol in both liver and blood plasma. The galactomannans also decreased the rate of hepatic synthesis of cholesterol. Dietary galactomannans increased caecal volatile fatty acids, particularly propionic, increased the weight of the caecum and its contents and increased the amount of water in the faeces. The increase in propionic acid production was significantly related to a decrease in caecal pH, but not to changes in plasma cholesterol or hepatic cholesterol synthesis. These effects were significantly influenced by chemical composition and structure of the galactomannan; they were most evident when the proportion of galactose in the galactomannan was highest (i.e. fenugreek gum). The three galactomannans also differed markedly in their effects on the viscosity of the digesta, but the galactomannan which gave the highest viscosity was least effective in lowering plasma cholesterol. A separate experiment with perfused loops of small intestine in vivo showed that the most effective galactomannan, fenugreek gum, had no direct effect on cholesterol absorption.

A viscous fibre (methylcellulose) lowers blood glucose and plasma triacylglycerols and increases liver glycogen independently of volatile fatty acid production in the rat.

1. Adult male rats were maintained on diets containing 80 g methylcellulose/kg of low (25 cP), medium (400 cP) and high (1500 cP) viscosity. 2. After 10 d, the viscosity of stomach and caecal contents was found to have increased in proportion to that of the dietary fibre. Concentrations of volatile fatty acids in caecal digesta were lowest with the high-viscosity fibre but acetate was the major acid present with all three diets. Acetate was the only acid found in significant quantities in hepatic portal venous plasma and concentrations of this acid were unaffected by diet. 3. Concentrations of glucose in arterial blood were low with the medium- and high-viscosity diets while the content of liver glycogen was high. These effects of fibre were not directly on glucose absorption as the intestines were net removers of the hexose at the time of sampling. 4. Hepatic lipogenesis and plasma triacylglycerol concentrations were both higher in rats fed on the low-viscosity fibre. Plasma cholesterol concentrations, hepatic cholesterol synthesis and faecal bile acid excretion were not altered by dietary fibre viscosity. 5. We conclude that the effects of dietary fibre on carbohydrate absorption and storage and fatty acid synthesis are a function of the viscosity of the fibre in solution, high viscosity slowing the digestion and absorption of nutrients in the small intestine. Large-bowel microbial fermentation is not of direct significance to these events. In contrast, effects of fibre polysaccharides on sterol metabolism seem not to be related to their rheological properties.

Gelling agents.

The characteristic physical properties of food gels result from the ability of certain proteins and polysaccharides to form continuous three-dimensional molecular networks. Development of new products based on gels, and improvements to existing ones, require a basic understanding of the intermolecular forces by which network cross-linkages are formed and stabilized. In this review new methods for studying food gels are described. The mechanisms by which molecular networks are formed by the gelling agents most frequently used by the food industry are discussed, with emphasis on the role of the solvent (water) and added solutes such as salts and sugars. More complex mixed gelling systems are discussed in some detail because of their practical importance and recent significant developments in this area.

A mechanism for the hypocholesterolaemic activity of saponins.

1. Saponins are steroid or triterpene glycosides which occur in a number of important food plants, including such staples as soya beans (Glycine max) and chickpeas (Cicer arietinum). They are known to be hypocholesterolaemic. 2. Some saponins form an insoluble complex with cholesterol which prevents its absorption from the small intestine. Others cause an increase in the faecal excretion of bile acids, an indirect route for elimination of cholesterol. 3. We have investigated the effects of different saponins on absorption of the bile salt sodium cholate from perfused loops of small intestine, in vivo, in the rat. Purified saponins from soapwort (Saponaria officinalis), soya beans and quillaia (Quillaia saponaria) reduced the rate of absorption of the bile salt; soya-bean and soapwort saponins substantially so but quillaia saponin to a much lesser extent. 4. These results were explained by the formation of large mixed micelles by bile acid and saponin molecules in aqueous solution. These aggregates can have molecular weights in excess of 10(6) daltons, consequently the bile acid molecules incorporated in them are not available for absorption. 5. Control of plasma cholesterol and nutrient absorption through dietary saponins could provide substantial health and nutritional benefits in humans.

Effects of dietary saponins on fecal bile acids and neutral sterols, plasma lipids and lipoprotein turnover in the pig.

Four young mature male pigs, 110 to 120 kg of body weight, were maintained on a low (0.01%) cholesterol diet. A double changeover design was used so that at any time two pigs received additionally 20 g/day of saponins as a 0.33% solution in drinking water. Saponins raised concentrations of fecal bile acids and neutral sterols and increased the contribution of primary acids to excretion. Neither the concentration of total plasma cholesterol nor low-density and high-density lipoprotein cholesterol were affected by saponins. There was also no change in either absolute or fractional catabolic rate of low-density or high-density lipiproteins. The data are discussed in relation to the effects of cholestyramine on plasma cholesterol and bile acid excretion in the pig and to the possible role of saponin-containing foods in the control of plasma cholesterol in man.

Increased thermal stability of proteins in the presence of sugars and polyols.

Sugars and polyols stablize proteins against heat denaturation. Scanning calorimetry was used to obtain a quantitative estimate of the degree of stabilization. Solutions of ovalbumin, lysozyme, conalbumin, and alpha-chymotrypsinogen were heated at a constant rate, and the temperature of the maximum rate of denaturation was estimated (Tm). Addition of a sugar or polyol raised Tm. The magnitude of the stabilizing effect (delta Tm) depended on both the nature of the protein and the nature of the sugar or polyol, ranging from 18.5 degrees C for lysozyme at pH 3 in the presence of 50% (w/w) sorbitol to 0 degrees C for conalbumin at pH 7 in 50% glycerol solution. It is argued that this stablization is due to the effects of sugars and polyols on hydrophobic interactions. The strength of the hydrophobic interaction was measured in model systems in sucrose and glycerol solutions. Sucrose and glycerol strengthened the pairwise hydrophobic interaction between hydrophobic groups; however, they reduced the tendency for complete transfer of hydrophobic groups from an aqueous to a nonpolar environment. The extent of stabliziation by different sugars and polyols is explained by their different influences on the structure of water. The difference between the partial molar volume of the sugar or polyol and its van der Waals volume was used as a rough quantitative measure of the structure-making or structure-breaking effect. There was a linear relationship between this quantity and delta Tm.

Effects of saponins on bile acids and plasma lipids in the rat.

1. The effects of feeding isolated saponins on plasma lipid concentrations and on concentrations of biliary and faecal bile acids and neutral sterols were studied in the rat. 2. The animals were given one of four diets, i.e. a standard low-cholesterol synthetic diet, the diet + 10 g saponins/kg, the diet + 10 g cholesterol/kg, the diet + 10 g cholesterol + 10 g saponins/kg. 3. Saponins partially reversed the hypercholesterolaemia caused by the high-cholesterol diet and increased both the rate of bile acid secretion and the faecal excretion of bile acids and neutral sterols. The proportionate contribution of the primary bile acids (particularly chenodeoxycholic) to faecal excretion was also increased by saponins. 4. The results are discussed in relation to the hypothesis that saponins act by inducing the adsorption of bile acids by dietary fibre.

Adsorption of bile salts from aqueous solution by plant fibre and cholestyramine.

1. Adsorption of bile salts by dietary fibre is believed to promote their excretion and hence to reduce the serum cholesterol level in man and experimental animals. 2. We have tested a number of plant fibre fractions and other related materials for their ability to adsorb bile salts from aqueous solution. The "insoluble" plant fractions were from "dry grain" (a residue from brewing), apple, wheat bran, lucerne (Medicago sativa), soya beans, mung beans (Phaseolus mungo), chick peas (Cicer arietinum), rolled oats, spinach (Spinacia oleracea), sunflower seeds, sawdust and sheep faeces. The other materials were cholestyramine, pectin and lignins prepared from wheat bran and from sawdust. 3. Only cholestyramine and the fibre from lucerne, soya beans, mung beans, chick peas, spinach, and sunflower seeds adsorbed enough of either sodium cholate or sodium deoxycholate for adsorption to be detectable. 4. This result conflicts with a report that the lignin component of dietary fibre is responsible for adsorption of bile salts. 5. Adsorption of bile salts, when it occurs, may depend on the presence of saponins bound to the fibre.