Content and Distribution of Short-Chain Fatty Acids in the Hindgut of Rats Fed Various Sources of Indigestible Carbohydrates

Abstract: There is increasing evidence that short-chain fatty acids (SCFA) formed by colonic fermentation of indigestible carbohydrates have beneficial physiological effects. Butyric acid is the main energy substrate for the colonic mucosa and has been implicated as important in the prevention and treatment of diseases of the colonic mucosa, such as distal ulcerative colitis and cancer. Propionic acid, on the other hand, has been reported to inhibit hepatic synthesis of cholesterol from acetic acid. In the present work, the fermentability and the SCFA formed from various indigestible carbohydrates were investigated using balance experiments in rats. Emphasis was put on the complexity of the food matrix, including the effect of combined versus single substrates. Composite common foods as well as mixtures versus single sources of indigestible carbohydrates were evaluated. Flour from variously heat-treated red kidney beans (Phaseolus vulgaris) was found to be a comparatively good source for caecal production of butyric acid (~18% of total SCFA). No effect of the treatments performed, i.e. milling and freeze-drying beans before or after boiling, and autoclaving and freeze-drying beans with and without retention of the cooking water, was found on the formation of SCFA in the caecum. However, the bean flour containing the highest amount of resistant starch (RS) led to an increase in butyric acid concentration in the distal colon, which not could be seen with diets containing the higher amounts of oligosaccharides and non-starch polysaccharides. When composite foods containing indigestible carbohydrates (instant potato powder, apples and oat flakes) were compared with commercial dietary fibre fractions from similar sources (potato fibre, apple pectin and a beta-glucan-enriched oat fibre) it was found that the fermentability of the fibre fraction was generally higher than for the indigestible carbohydrates in the corresponding food. All substrates, except apple pectin, generated intermediate to high proportions of butyric acid in the caecum (15-22 versus 8%). Rats fed the composite foods generally exhibited a higher proportion of butyric acid in the distal colon than rats fed the corresponding fibre fraction. Potato powder, i.e. the product with the highest content of RS, gave a high proportion of butyric acid in the distal colon (19%). Potato powder and potato fibre both resulted in high concentrations of butyric acid in the distal colon. The combining of different indigestible carbohydrates affected both the pattern of SCFA and the site for release. A mixture of guar gum and pectin gave a higher proportion and a higher caecal pool of butyric acid than the single substrates. Furthermore, mixing high-amylose maize starch with wheat bran gave higher faecal excretion of SCFA (184 micromol per day) than the single substrates (77 and 116 micromol for high-amylose maize starch and wheat bran, respectively), suggesting that the incorporation of wheat bran delayed the fermentation of high-amylose maize starch to the distal part of the gastrointestinal tract. The pattern of SCFA formed from RS sources i.e. raw potato starch and high-amylose maize starch was found to be dependent on the adaptation time, which may explain some of the inconsistent results found in the literature. The proportion of butyric acid formed from these RS sources increased with increasing length of adaptation time (2-6 w). However, the increase in the proportion of butyric acid with raw potato starch was faster than with high-amylose maize starch.