The physico-chemical properties of dietary fibre determine metabolic responses, short-chain Fatty Acid profiles and gut microbiota composition in rats fed low- and high-fat diets

Abstract

The aim of this study was to investigate how physico-chemical properties of two dietary fibres, guar gum and pectin, affected weight gain, adiposity, lipid metabolism, short-chain fatty acid (SCFA) profiles and the gut microbiota in male Wistar rats fed either low- or high-fat diets for three weeks. Both pectin and guar gum reduced weight gain, adiposity, liver fat and blood glucose levels in rats fed a high-fat diet. Methoxylation degree of pectin (low, LM and high (HM)) and viscosity of guar gum (low, medium or high) resulted in different effects in the rats, where total blood and caecal amounts of SCFA were increased with guar gum (all viscosities) and with high methoxylated (HM) pectin. However, only guar gum with medium and high viscosity increased the levels of butyric acid in caecum and blood. Both pectin and guar gum reduced cholesterol, liver steatosis and blood glucose levels, but to varying extent depending on the degree of methoxylation and viscosity of the fibres. The medium viscosity guar gum was the most effective preparation for prevention of diet-induced hyperlipidaemia and liver steatosis. Caecal abundance of Akkermansia was increased with high-fat feeding and with HM pectin and guar gum of all viscosities tested. Moreover, guar gum had distinct bifidogenic effects independent of viscosity, increasing the caecal abundance of Bifidobacterium ten-fold. In conclusion, by tailoring the viscosity and possibly also the degree of methoxylation of dietary fibre, metabolic effects may be optimized, through a targeted modulation of the gut microbiota and its metabolites.

Fig 1. Viscosity vs. shear stress of guar gum with low, medium and high viscosity.

Fig 2. Serum levels (μmol/L) of short-chain fatty acids (SCFA) in rats (n = 7/group) fed low- or high-fat fat diets together with pectin (low- (LM) or high-methoxylated, (HM)) or guar gum (GG, low, medium or high viscosity) for three weeks.

Values are expressed as mean and SD. The fibre-free control groups (low- and high-fat diet) were compared with the corresponding fibre groups and significantly different means are shown as: * P-value < 0.05, ** P-value < 0.01 and *** P-value < 0.001.

Fig 3. Caecal pool (μmol) of short-chain fatty acids (SCFA) in rats (n = 7/group) fed low- or high-fat fat diets together with pectin (low- (LM) or high-methoxylated (HM)) or guar gum (GG, low, medium or high viscosity) for three weeks.

Values are expressed as mean and SD. The fibre-free control groups (low- and high-fat diet) were compared with the corresponding fibre groups and significantly different means are shown as: * P-value < 0.05, ** P-value < 0.01 and *** P-value < 0.001.

Fig 4. Log copy numbers of Akkermansia muciniphila, Bifidobacterium, Lactobacillus and Bacteroides per gram of caecal sample using quantitative PCR.

Mean values of each fibre group (HM and LM pectin, guar gum (GG) low, medium and high viscosity) were compared to the fibre-free control, for each fat-level, and diets marked with different letters (a or b) show significant differences (P<0.05). Mean values of high-fat (HF) groups that were significantly different from corresponding low-fat (LF) groups were marked as follows: * P< 0.05, ** P< 0.01 and *** P< 0.001. Two-way analysis of variance (ANOVA) using adjusted sum of square was performed to test the interaction and main effects of each factor (fat, fibre, fat x fibre) on the caecal microbiota.

Fig 5. PCA biplot to visualize patterns and relationships of investigated biomarkers in rats (n = 7/group) fed low- or high-fat fat diets together with pectin (low- (LM) or high-methoxylated (HM)) or guar gum (low, medium or high viscosity) for three weeks.

Triangles represent each individual rat with different colours for each fibre, LF = low fat; HF = high fat. Different biomarkers are shown in green circles and different bacterial genera in green stars.