Inulin and fructooligosaccharides modulate the response of the immune system to illness, through the stimulation of bifidobacteria and lactobacilli, and the improvement of the general microfloral balance in the colon (Watzl et al., 2005; Yasui et al., 1992). In this role, prebiotics have been shown to promote the production of macrophages, lymphocytes, and antibodies, in particular the local production of immunoglobulin A (IgA)-positive cells in the intestines and cecal mucosa (Bornet, 2001; Hosono et al., 2003; Kadooka et al., 1991; Roberfroid, 2005; Yasui et al., 1992). In addition, thriving bifidobacteria and lactobacilli populations help to strengthen the mucosa-blood barrier in the intestine lining. They do this by outcompeting pathogens for adhesion sites on the intestinal lining, producing short-chain fatty acids that nourish cells in the mucosal layer, lowering intestinal pH to levels unfavorable to pathogens, and releasing bacteriocins against pathogens (Anon., 2006; Wang and Gibson, 1993). Fructooligosaccharides may therefore help to prevent a wide range of illness and disease conditions, including ulcerative colitis, symptoms of inflammatory bowel diseases, and E. coli O157 infection (Hidaka et al., 2001; Kanauchi et al., 2003; Oike et al., 1999; Wolf et al., 2003). The ability of inulins to stimulate the immune system has also led to interest in their use as vaccine adjuvants (Cooper, 1995; Silva et al., 2004).
Newborn babies and infants have underdeveloped immune systems. Breast milk contains natural prebiotic oligosaccharides that stimulate bifidobacteria and lactobacilli, which tend to be more prevalent in the gastrointestinal flora of breast-fed babies. Infant formula and cow's milk are deficient in oligosaccharides, a factor holding back the development of an infant's immune system. The addition of fructooligosaccharide prebiotics to infant formula can increase gastrointestinal bifidobacteria and lactobacilli counts (Boehm et al., 2002; Knol et al., 2000; Moro et al., 2002), while daily fructooligosaccharide supplements (e.g., 2 gday-1) can lower the prevalence of sickness (e.g., vomiting, diarrhea) in infants (Saavedra et al., 1999; Vandenplas, 2002; Waligora-Dupriet et al., 2005).
By boosting the immune system, it has been claimed that fructooligosaccharide supplements reduce the risk of colorectal cancer developing (Kowhi et al., 1978, 1982; Pool-Zobel et al., 2002). In mice and rats, for example, fructooligosaccharides reduced colon carcinogens and the occurrence of colon tumors (Pierre et al., 1997), while dietary inulin and fructooligosaccharides suppressed chemically induced tumors (Taper and Roberfroid, 2002) and reduced genotoxic damage to the colonic epithelium in rats (Rowland, 1998). The release of the short-chain fatty acid butyrate, from fermenting inulin and fructooligosaccharides, may play a role in suppressing colon cancer. Butyrate has been shown to have a direct antiproliferation effect on tumor cells in vitro (Kruh, 1982), while the release of butyrate has been correlated with a protective effect against colon cancer in experimental studies with rats (Bornet, 2001; McIntyre et al., 1993). Moreover, inulin injections can prolong the survival of melanoma-bearing mice (Cooper and Carter, 1986). However, more proof is needed before dietary fibers such as inulin can be said to definitely prevent colorectal cancer (Baron, 2005; Park et al., 2005).
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