A number of health benefits attributed to Jerusalem artichoke tubers in human and animal diets are related to its role as a promoter of probiotic activity in the large intestine. Probiotics have been a dietary element for thousands of years. However, the term probiotic only gained its current usage in the early 1970s, as an organism or substance that has a beneficial effect on the balance of microorganisms in the colon. This definition coincided with work revealing the essential role of intestinal microflora in the digestion of food and in promoting general well-being. Today, a probiotic is generally defined as a live microbial feed supplement that beneficially affects the host animal by improving its intestinal microbial balance (Fuller, 1992).
The large intestine is the most heavily colonized region of the digestive system, with up to 1012 bacteria per gram of intestinal content. Around 50 genera are represented, with hundreds of different species and strains occurring. The vast majority of these bacteria are anaerobes, and they range from the beneficial to the pathogenic. The balance of the intestinal microflora greatly influences digestive processes. Beneficial bacteria naturally present in the large intestines include species of Bifidobacterium and Lactobacillus. Probiotics and substrates that promote their activity help shift the balance toward the optimum for these beneficial bacteria, so they constitute at least one third of the total bacterial population (Gibson and Roberfroid, 1995).
The earliest probiotic was probably fermented milk. From biblical times to the present day, yogurts and other fermented dairy products have been recognized as having health-promoting properties. The importance of bacteria in giving fermented milk products these properties was recognized at the beginning of the 20th century (Metchnikoff, 1907). From the 1950s onward, experiments have elucidated the role that ingested beneficial bacteria (good bacteria) play in maintaining a favorable microflora balance, and thereby countering the impact of pathogenic bacteria (bad bacteria). For instance, lactobacilli were identified as the key probiotic ingredient in fermented milk products.
A number of bacteria and yeasts have probiotic activity. However, in commercial applications, bacteria associated with four genera have predominated: Bifidobacterium, Lactobacillus, and, to a lesser extent, Enterococcus and Streptococcus. Collectively, these are called lactic acid bacteria (LAB) in the nutrition literature, although Bifidobacterium and the minor commercial genera are not strictly part of the lactic acid group. The following species are utilized in commercial products: Bifidobacterium breve, B. bifidus, B. infantis, B. lactis, B. longum, B. thermophilum, Lactobacillus acidophilus, L. delbrueckii subsp. bulgaricus, L. helveticus, L. casei, L. fermentum, L. johnsonii, L. plantarum, L. rhamnosus, L. salivarius, Enterococcus faecium, and Streptococcus salivarius subsp. thermophilus (Lee et al., 1999). A number of claims are made for these products, including improvement of the colon microflora balance, stimulation of the immune system, enhanced resistance to bacterial infection, and general health benefits.
Many factors contribute to the effectiveness of probiotic organisms, including their ability to adhere to the lining of the intestine (Crociani et al., 1995). A crucial factor in the survival and proliferation of both native and introduced beneficial bacteria (i.e., bifidobacteria, lactobacilli) in the lower intestines is the availability of a carbohydrate source that has not been digested by the human digestive system and that can be used as a substrate for growth. A range of nondigestible oligosaccharides have been shown to stimulate the activity of bifidobacteria and lactobacilli in the colon (Fuller, 1997). Nondigestible substances that stimulate the growth or activity of beneficial bacteria in the colon are called prebiotics; they have become an important component in probiotic supplements (Gibson and Roberfroid, 1995; Gibson et al., 2005; Tuohy et al., 2005). Jerusalem artichoke tuber extracts are rich in inulins and fructooligosaccharides, and are therefore a potential source of prebiotics.
Probiotic supplements can take the form of powders, dried tablets, pellets or cubes, and pastes or sprays, depending on their use. They usually contain an active bacterial ingredient (probiotic) plus a carbohydrate source that can be selectively fermented by the bacteria (prebiotic). A combination of an active bacterial ingredient and carbohydrate source is also called a synbiotic (Gibson and McCartley, 1998; Roberfroid et al., 2002a). Synbiotic has been slowly accepted as a term, however, and probiotic is still commonly used for mixtures of probiotics and prebiotics (Heasman and Mellentin, 2001). A typical synbiotic (probiotic) supplement may therefore contain a culture of bifidobacteria together with inulins or fructooligosaccharides. Inulin prebiotic supplements are also added to food without a probiotic component to promote the activity of indigenous bifidobacteria. Some probiotic products (e.g., yogurt drinks) have come under critical scrutiny because the probiotic bacteria added, including bifidobacteria, do not survive beyond the stomach to contribute to the colon microflora (e.g., Graham-Rowe, 2006). However, there is no doubt that prebiotic inulin and fructooligosaccharide do reach the colon.
Inulin and fructooligosaccharides are excellent prebiotics because they are not digested or absorbed in the small intestine. The characteristic (1-2)-bonds, which link the fructose units, cannot be degraded by mammalian digestive enzymes (Oku et al., 1984), and therefore reach the colon as intact molecules. Around 85% of ingested inulin survives to the colon, where it acts as a fermentable substrate for the colonic microflora. Inulin and fructooligosaccharides selectively stimulate the growth of bifidobacteria and lactobacilli, an effect not achieved with other types of carbohydrate such as starch or pectin (Gibson et al., 2005; Mitsuoka et al., 1987; Wang and Gibson, 1993). They therefore influence species composition in the colon in favor of beneficial bacteria (Gibson et al., 1995; Gibson and Wang, 1994). A daily intake of around 8 to 10 g of fructooligosaccharides significantly increases bifidobacteria in the large intestine (e.g., Bouhnik et al., 1997; Hidaka et al., 1991; Tuohy et al., 2001). As little as 5 g of inulin a day in the diet can produce an observable bifidogenic effect (Bouhnik et al., 1999; Williams et al., 1994).
By promoting bifidobacteria and other beneficial microorganisms, prebiotics help to suppress harmful microorganisms through competitive inhibition. Thriving populations of beneficial bacteria can outcompete other bacteria for nutritional resources and adhesion sites on epithelial cells lining the intestinal wall. Good adhesion is important to prevent bacteria being removed by host secretions and intestinal flow; the attachment ability of different Bifidobacterium species is correlated with their ability to colonize the large intestine (Crociani et al., 1995). Bifidobacteria also release antibacterial agents, as a result of fermenting inulins (Gibson and Wang, 1994). Lactobacilli secrete bacteriocins — peptides with specific antibacterial action (Dodd and Gasson, 1994). Numbers of pathogenic bacteria in fecal samples are often reduced when inulin and fructooligosaccharide supplements are included in the diet (e.g., Gibson and Wang, 1994). Among the potentially harmful bacteria that have been reported to be suppressed are Clostridium perfingens and C. difficile and pathogenic strains of Escherichia coli, Staphylococcus aureus, Campylobacteria jejuni, Salmonella enteritidis, and Candida albicans (Araya-Kojima et al., 1995; Buddington et al., 2002; Fooks and Gibson, 2002; Gibson et al., 1995; Gilliland and Speck, 1977; Harmsen et al., 2002; Kleessen et al., 1997; Rao, 2001; Wang and Gibson, 1993; Yamazaki et al., 1982). However, not all dietary trials with inulin and fructooligosaccharide supplements resulted in significantly reduced counts of pathogenic bacteria (Roberfroid, 2005).
Prebiotic and synbiotic supplements containing inulins and fructooligosaccharides can be particularly effective after illness and antibiotic treatments, to help bifidobacteria recolonize the large intestine, as bifidobacteria can be eradicated by certain antibiotics (Colombel et al., 1987). They are also particularly helpful for other at-risk groups, such as the elderly, babies and infants, and people traveling or on holiday abroad, as a preventative measure against illness. In addition to their bifidogenic effect, inulin-type fructan prebiotics have been reported to have a range of other beneficial health effects. These claims are supported by data from studies with animal models and from clinical trials, although it should be noted that large variability often occurs between different studies. The main health claims relate to mineral absorption and bone health, decreased blood lipids and heart disease, stimulation of the immune system and disease prevention, and improved bowel function (Boeckner et al., 2001; Roberfroid, 1995; Tungland, 2003).
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