The large intestine, or colon, is the site of several unique functions in the body. Sitting at the terminal end of the digestive tract, the colon employs its large surface area – 300 square meters or about the size of a tennis court – to absorb water and concentrate food residue into feces. The colon also hosts the body’s largest microbial colony. The 1012 microbial cells in each gram of dry content in the colon dwarf the number of bacterial cells that can survive elsewhere in the GI tract where the environment is less hospitable. The immunomodulating effects of the microbiota and its ability to defend against pathogens in food characterize the colon as the largest participant in the body’s immune system. The importance of a well-functioning colon has been underestimated in the past but rapidly is becoming the focus of a growing body of research.
The Role of the Gut Microbiota
Gut bacteria in the colon participate in a number of reactions that impact health. They catalyze two types of fermentation, saccharolytic fermentation of non-digestible carbohydrates (dietary fibers), and proteolytic fermentation of protein residues. The beneficial saccharolytic process generates short-chain fatty acids (SCFAs) that serve to create a more acidic environment in the colon, strengthen the thickness of the intestinal mucosa, enhancing mineral absorption and inhibiting acid-sensitive pathogens. SCFAs also fuel the growth of the microbiota, chemically and mechanically stimulate bowel movements, and “cross-talk” with the brain to signal satiety (Delzenne & Cani, 2011). The bacterial strains that participate in saccharolytic fermentation, primarily bifidobacteria and lactobacillus, further benefit the body by synthesizing vitamins and generating bacteriostatic and bacteriocidal compounds that keep potentially harmful bacteria in check (Roberfroid et al, 2010). In contrast, proteolytic fermentation generates potentially toxic compounds like cresol, indol, scatol, and hydrogen sulfide (DePreter et al, 2007, 2008).
Numerous factors influence the health of the gut microbiota and the balance between beneficial and detrimental bacteria. These include the birth environment (maternal flora and mode of birth), diet, genetics, age, medications, probiotics, and prebiotics (probiotic bacteria are consumed in certain supplemented foods or taken as supplements to introduce beneficial cultures into the colon, they do not become natural inhabitants of the large intestine.) Prebiotics nourish beneficial bacteria that are natural inhabitants of the large intestine and offer a way to selectively stimulate the growth of those bacteria that participate in saccharolytic fermentation. Prebiotics first were defined in 1995 as compounds that stimulate the growth and/or activity of beneficial bacteria that improve the health of the microbiome and contribute to a healthy state of microbiota structure called normobiosis (Gibson et al, 1995). The International Scientific Association for Probiotics and Prebiotics, Nov 2008, London, Ontario developed the definition further: A dietary prebiotic is a selectively fermented ingredient that results in specific changes, in the composition and/or activity of the gastrointestinal microbiota, thus conferring benefit(s) upon host health. (Roberfroid et al, 2010). The International Scientific Association of Probiotics and Prebiotics (ISAPP) meets every few years to reflect on the latest advances in microbiome science. The latest review was published as a consensus statement in 2017 (Gibson et al. 2017), confirming that selective growth is key to the prebiotic concept, a criteria that distinguishes prebiotics from many other substances. The updated definition is as follows:
This distinguishes prebiotics from many other substances. Inulin and oligofructose / fructo-oligosaccharides from the chicory root are the most researched proven plant-based prebiotics. An informative infographic on the health benefits of prebiotics and in which foods they can be found is available on the ISAPP website. More research on prebiotics can be expected in the near future.
Inulin-Type Fructans as Prebiotics
The root of the chicory plant is the source of the inulin-type fructans inulin and oligofructose. Native inulin is a storage carbohydrate characterized by varying chain lengths of fructose molecules. Approximately 30 percent of native inulin occurs in the form of fructose oligosaccharides (FOS), called oligofructose. Regardless of chain length, inulin is not digested in the small intestine and can act as a prebiotic that is selectively fermented by bacteria in the colon.
Research confirms the prebiotic activity of inulin and oligofructose in humans and animals on microbiota composition and growth of the biomass (Bouhnik et al, 2004; Bouhnik et al, 2006). Inulin and oligofructose selectively stimulate the growth and/or activity of beneficial bacteria such as bifidobacteria (Kleessen et al, 1997; Kruse et al, 1999; Menne et al, 2000; Bouhnik et al, 2004; Kolida et al, 2007; Ramirez-Farias, et al, 2009; Marteau et al, 2011; Mendlik et al, 2012) and lactobacilli (Langlands et al, 2004; Ten Bruggengate et al, 2006). The selective fermentation process generates SCFAs, including butyrate, propionate, acetate, and acetaldehyde (De Preter et al, 2013). In the presence of adequate amounts of fermentable fibers and beneficial bacteria, saccharolytic fermentation dominates, suppressing the generation of potentially harmful by-products of protein fermentation (Gibson et al, 1995; De Preter et al, 2007; Scott et al, 2013), in part by suppressing the activity of enzymes such as beta-glucuronidase (De Preter et al, 2008; Slavin & Feirtag, 2011). In one study, a high dose of 15-30g/day oligofructose over 2 weeks was shown to reduce fecal water genotoxicity (Windey et al, 2014).
The increased biomass that results from exposure to the prebiotics inulin and oligofructose adds bulk to the fecal mass (Gibson et al, 1995; Castiglia-Delavaud et al, 1998; Den Hond et al, 2000; Van Dokkum et al, 1999; Scholtens et al, 2006). Furthermore, the high water content of the biomass increases moisture in the stool and acts as a softening agent (Mendlik et al, 2012). As a result, excretion is facilitated.
Bifidobacteria – biomarker for healthy gut flora
Bifidobacteria are the best studied beneficial bacteria and are considered a biomarker for a healthy microbiota. They are abundant in the colonic microflora of healthy, breast-fed infants (Bullen et al, 1976; Beerens et al, 1980), where they have been associated with lower counts of E. coli and other potentially detrimental bacteria. A healthy microbiota in infancy could help improve health later in life by potentially lowering risk of obesity, inflammatory bowel disease, allergies, and certain behavioral disorders in adulthood (Wopereis et al, 2014). Higher bifidobacteria counts in the feces of adults have been associated with improved health and the absence of disease (Rastall et al, 2005).
While several fruits and vegetables contain inulin, chicory root is a highly abundant source, and inulin and oligofructose from chicory root are the best researched among the few scientifically proven prebiotics (Roberfroid 2010). They are particularly effective in stimulating bifidobacteria, as demonstrated in numerous studies in infants, children, healthy adults, and adults with impaired intestinal microflora. The prebiotic benefits of inulin and oligofructose are conferred at an intake of as little as 5 grams per day for adults.
Prebiotics and Health
The numerous health benefits and physiological targets postulated to result from prebiotic effects on selective fermentation include improvement and/or stabilization of gut microbiota composition; improvement of intestinal functions (stool regularity, bulking, consistency); increase in mineral absorption and resultant improvement of bone health; initiation and modulation of the immune response; lower risk of intestinal infections; improved intestinal barrier functions; reduction of metabolic endotoxemia; modulation of GI peptide production, energy metabolism and satiety; and lower risk of obesity, type 2 diabetes, and metabolic syndrome (Crittenden et al, 2006; Roberfroid et al, 2010; Rastall, 2012). Bifidobacteria influence the ability of bacteria colonies to dock at the mucosal cell wall, blocking adhesion of harmful strains and reducing intestinal infections (Crittenden et al, 2006).
A large and growing body of human studies demonstrate that consumption of inulin-type fructans increases the number of bifidobacteria in stool and mucosa samples. Effects have been observed globally, in all age groups, independent of gender and health status, and confirmed for different supplementation periods and varying daily intake levels. Bifidogenic effects are independent of food matrix; regardless of their form, chicory root fibers boost bifidobacteria.
Prebiotics have been shown to benefit the health of infants and children. Breast milk has a naturally high content of non-digestible fermentable oligosaccharides that influence the development of bifidobacteria and other beneficial gut bacteria (Ballard & Morrow, 2013). Fermentable oligosaccharides may be added to infant formula to match those in breast milk. An increase in bifidobacteria can improve stool quality, decrease the risk of gastroenteritis, and reduce the frequency of atopic eczema in children (Roberfroid et al, 2010).
The natural food components inulin and oligofructose are safe for consumption by infants and children. A meta-analysis of 5 randomized clinical trials concludes that prebiotics have a high potential to reduce incidence of overall infections and infections requiring antibiotic therapy (Lohner et al, 2014). Saavedra and Tschernia (2002) observed improved gastrointestinal comfort and disease resistance in a group of 123 healthy young children given a daily dose of 1.1 g oligofructose.
Long-term Maintenance of Benefits
A higher intake of dietary fiber is related to higher activity in the gut due to e.g. higher bulk from non-fermented food residues, signals related to the fermentation process (improved regularity due to increase in gut motoric), gas production (that is absorbed to a high extend and may or may not result in wind or bloating. Bifidobacteria are not promoting gas production. Inulin, having the prebiotic effect, does not have a particular response compared to other fermentable or non-fermentable dietary fibers that do not have a prebiotic effect. Perception of this normal digestive process may be noticeable particularly to individuals who are accustomed to a low fiber diet. Based on the totality of the evidence, 15-20 g/day (intake in several eating occasions spread over the day) of inulin and/or oligofructose generally is well-tolerated, noticing the digestive effect, but not in a disturbing way. Already with 5 g per day the prebiotic effect was substantiated. Any changes in the microflora require at least one week of adoption, so chicory root fiber can be added slowly over time as an effective prebiotic.
Probiotics, i.e. living bacteria that are eaten pass through the body, i.e. they do not become part of the regular permanent microbiota in the gut, but are excreted via the feces.
Prebiotic inulin and oligofrucose promote the natural inhibitants of the gut, the gut microbiota. In particular, the bifido and lactobacillae prefer inulin-type fructans are their feed and grow quickly under these optimized conditions. Of course, if this preferred nutrient is not delivered anymore the bifodoflora decreases again. This means, a regular intake, not necessarily a daily intake, of inulin-type fructans is recommended to have the bifidoflora stay on a high level.