Effects of varying the content and proportions of gum arabic and cellulose on caecal volatile fatty acid concentrations in the rat

Abstract

Adult male rats were fed a synthetic diet containing 5, 10 or 15% by weight of gum arabic (GA) or cellulose (C). Food consumption over the 10 d experimental period was similar in rats fed the 5C and 15C and all the GA diets but was raised in the 10C group. Final body weights were similar in the 5C, 10C and 15C groups and were equally lower in rats fed the GA diets. The wet weight of caecal digesta was the same with all three C diets while digesta wet weight rose with increasing dietary GA content and diarrhoea was observed with the 15GA diet. Concentrations of total caecal VFA were similar in the 5C and 5GA groups and were depressed equally by the 10C and 15C diets. Total VFA did not change in the 10GA rats but were depressed in the 15GA group. Calculation of the mass of caecal VFA showed that this was increased with the 10GA diet but substantially reduced with that containing 15GA, suggesting that VFA were not causative of diarrhoea. In animals fed diets containing mixtures of both fibres to a level of 15% by weight, caecal digesta was the same with 10C+5GA and 7.5C+7.5 GA diets but increased with 5C+10GA. Total VFA concentrations and pool size were unaffected by changing from 10C+5GA to 7.5C+7.5GA but the concentration and mass of propionate was increased. In rats fed 5C+10GA VFA concentration was greatly depressed although their mass was unchanged from the 10C+5GA group. We conclude that VFA concentrations are influenced by the type and concentration of dietary fibre and that fibre mixtures favour production of propionate. Below a certain level of fermentable fibre digesta mass seems to reflect enhanced fermentation but above that level, the presence of unfermented fibre. Differences between the present experiments and previous ones may be due to changes in the large bowel microflora.