The effects of inulin-type fructans on the production as well as the composition of the pool of SCFAs have been evaluated (see Chapter 6, Section 188.8.131.52), the main conclusions being that the colonic fermentation of inulin-type fructans produces large amount of total SCFAs and lactic acid with, as a consequence, the acidification of the large-bowel content (see Chapter 6, Section 184.108.40.206). Such an acidic environment prevents the proliferation of potentially pathogenic bacteria. It is one of the major consequences of the prebiotic effect of inulin-type fructans (see Chapter 9 and especially Section 9.4.1 and Section 9.4.2), which balances the competitive interactions between all the genera, species, and even strains of microorganisms.
Such a fermentation of inulin-type fructans also produces a high proportion of butyrate, one of the most physiologically relevant bacterial metabolites.162 Indeed, it is believed that butyrate protects against colon carcinogenesis by inhibiting colon cell proliferation, by inducing proliferation, and by promoting apoptosis.163 In Spra-gue-Dawley rats treated with the chemical carcinogen dimethylhydrazine (DMH), a treatment known to stimulate apoptosis, Hughes and Rowland have reported that both oligofructose and inulin HP (5% w/w in diet as pretreatment for 3 weeks) further stimulate the defense mechanism above the levels induced by DMH alone thus strengthening the defense capacity against tumor formation.164
Another important mechanism of defense in a balanced colonic microflora is the production of bacteriocins. In particular, it has been demonstrated that the growing Bifidobacterium infantis has an inhibitory effect towards E. coli and Clostridium perfringens.165 Further studies showed that eight species of bifidobac-teria could variously excrete an antimicrobial substance with a broad spectrum of activity affecting the genera salmonella, listeria, campylobacter, and shigella.166 By virtue of their prebiotic effect, inulin-type-fructans selectively stimulate the growth of bifidobacteria and possibly other health-promoting bacteria thus reinforcing the capacity of the colonic microflora to prevent the development of potentially pathogenic microorganisms. A recent publication has demonstrated that it is indeed the case by showing that the inhibitory effect of Lactobacillus plantarum and Bifido-bacterium bifidum on E. coli, Campylobacter jejuni, and Salmonella enteritis is increased if the probiotics are grown in vitro in the presence of oligofructose, inulin, or a mixture of oligofructose and inulin.167
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