Oligofructose and inulin are completely fermented mainly, if not exclusively, in the colon (at least in humans) and, consequently, they are not recovered to any significant extent in the feces. As a result of colonic fermentation, excretion of breath H2 is significantly increased. As discussed above (see Section 18.104.22.168), if this is a valid marker of intestinal fermentation, it cannot be considered a quantitative measurement of the extent of fermentation.
In adequate experimental models (e.g., heteroxenic rats harboring human fecal flora) it has been reported that oral ingestion of oligofructose and inulin leads to an increased production of total SCFAs and a reduction in pH in the cecum. Moreover, these data point to increased amount and/or molar ratio of butyric acid in cecum content. However, in humans no significant effects on fecal SCFAs has been reported, but, as discussed above (Section 22.214.171.124), this is not a valid model to study both quantitatively and qualitatively SCFAs production by colonic carbohydrate fermentation. Indeed, only a small proportion (probably not more than 5%) of all SCFAs produced in the large bowel is excreted in feces. In one study on rats using inulin, a high proportion of propionic acid was recorded. Although inulin, like oligofructose, is completely fermented in the colon, the degradation may be slower than with oligofructose, which might give rise to another SCFAs pattern. Whether the nature of the carbohydrate determines its fermentability in the colon is a question that has barely been addressed. Van Laere et al. produced a range of oligosaccharides with different sugar compositions and molecular sizes.102 They tested their fermentation in a batch system with several strains of bifidobacteria, clostridia, bacteroides, and lactobacilli. Fermentability differed with oligosaccharide structure. Oligofructose, inulin, and xylooligosaccharides were extensively fermented, except by the clostridia, while few species were able to breakdown arabinoxylans under the conditions of the experiment. Linear oligosaccharides were catabolized to a greater degree than those with branched structures while bifidobacteria utilized low DP carbohydrate first, and bacteroides, those with a high DP. Metabolic collaboration among species was evident in carbohydrate breakdown. Both the structure of the carbohydrate and the bacterial species present in the ecosystem are probably important factors in controlling fermentation of carbohydrates.
Oligofructose and inulin are thus both nondigestible and fermentable carbohydrates. Like all dietary substrates that become fermented in the intestine but mostly in the large bowel, they may produce intestinal discomfort and at very high doses (30 g/d or higher) they can even become laxative (because of osmotic effect). From the studies available it can be concluded that, on average, the most frequent intestinal side effects are flatulence and bloating and that these symptoms are mild or moderate, but a small percentage (1-4%) of the adult population may experience more severe reactions. However, it must always be kept in mind that these conclusions concern all types of nondigestible but fermentable dietary substrates and not only inulin-type fructans. As a result of the assessment of the available data, it can be concluded that for a wide proportion of the adult population (probably 95% or more), a daily dose of 10 g inulin-type fructans will cause no unusual intestinal side effects. For about three quarter of these adults, even a dose of 20 g/d will be acceptable. Little information is presently available concerning the acceptability of nondigestible but fermentable carbohydrates, in general, and inulin-type fructans, in particular, in other age groups. Still, a test with oligofructose has shown that children 10 to 13 years old ingested daily doses of 3 g, 6 g, and 9 g without reporting any undesirable side effects.103
These various and idiosyncratic effects of inulin-type fructans on intestinal symptomatology are difficult to explain. Individuals widely vary in their responses to fermentation of carbohydrates, and the stoichiometry of fermentation differs from carbohydrate to carbohydrate, especially as a function of chain length and monosac-charide composition, suggesting that, in general, molecules with longer chain length are fermented more slowly and with less net H2 excretion.49 This hypothesis is supported by Brighenti et al., who compared breath H2 production after consumption of the same amount of lactulose (short-chain molecule), inulin (medium-chain molecules) and resistant starch (long-chain molecules producing) by healthy subjects and showed they exhaled 4.7, 19.1 and 26.6 ppm H2/h/g respectively.104 These findings were broadly reflected by in vitro fermentation studies and suggest that, in general, molecules with longer chain length are fermented more slowly and with less net H2 production than short-chain molecules.
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