Formation of carboxylic acids in rats and humans given indigestible carbohydrates Effects of monomeric composition, solubility, molecular weight and probiotics

Detta är en avhandling från Applied Nutrition and Food Chemistry, Lund University

Sammanfattning: There is increasing evidence that fermentation in the large intestine is important for health, and that health-promoting effects are mediated by fermentation products such as butyric and propionic acid. As the formation of carboxylic acids (CAs) from various carbohydrates reaching the colon has been shown to vary, it is of great interest to identify food factors that could be of importance to this process. Few studies have concentrated on the effects of the physico-chemical properties of the carbohydrates and the effects of probiotics. The present work was an attempt to increase our knowledge in this area. For this purpose, a rat model was set up. A human model was also used to investigate how the colonic concentration of carboxylic acids in healthy subjects would be influenced by dietary supplementation with a ?-glucan-enriched oat bran, and how the concentration varied over three consecutive days. The degree of polymerization (DP) and the solubility of the fructo-oligosaccharides were of great importance for CA formation, while the monomeric composition was of less importance. Fructo-oligosaccharides with a low DP generated high levels of butyric acid throughout the colon, whereas those with a high DP gave high levels of propionic acid. A low solubility of the fructo-oligosaccharides was related to a lower degree of caecal fermentation, and more extensive formation of butyric acid in the distal part of colon. Lactitol and lactulose yielded high proportions of acetic acid and low proportions of butyric acid. The probiotic bacteria affected both the pattern of CAs and the site of release in the hindgut of rats. However, the type of indigestible carbohydrate had more pronounced effects on the profiles and concentrations of CAs in the caecum than the probiotic added. In the distal part of the colon, on the other hand, the combination of pre- and probiotics was of great importance for the results. Bb-12 in combination with inulin of low solubility specifically increased the caecal pool of propionic acid in the rats. The amount of lactic acid, however, increased in the distal colon, which may have been due to the limited amount of substrate in relation to the number of probiotics in this part. Together with lactitol, Bb-12 reduced the concentration of CAs in the distal colon, indicating an accelerated absorption of CAs. The concentration of CAs was higher throughout the hindgut of rats fed pectin and Bb-12. With UCC500 and pectin, the formation of CAs shifted from the caecum to the distal part of the colon, while the combination with lactitol stimulated formation of CAs in the caecum. Together with inulin of high solubility, none of the probiotics investigated increased the proportion of butyric acid and propionic acid at any place in the hindgut of rats. The proportion of lactic acid was generally higher, however, and similarly for succinic acid in rats fed UCC500. Rats fed GG had the lowest body weight gain and the highest caecal tissue weight. The caecal pH in rats fed GG and Bb-12 was lower than expected from the concentration of CAs, indicating that these strains reduce the formation of alkaline components e.g. ammonia. ?-glucan-enriched oat bran increased the faecal concentration of CAs in humans after 8 weeks, indicating an increased concentration in the distal colon also. No significant differences in CA concentrations could be seen for different days, which supports the adequacy of the experimental periods. An experimental design, with 20 subjects and a dietary supplementation with the test food product for 8 weeks, thus appears suitable for screening of potential differences in faecal carboxylic acid patterns.