The human microbiome is a complex, dynamic ecosystem of diverse microbes, encompassing more than 1000 bacterial species, and is closely intertwined with overall health.1-4 Inquiry into the health benefits associated with ingesting beneficial probiotic bacteria spans many decades. Modern research has provided insight on the therapeutic nature of how probiotics restore balance to the colonic microflora to promote gastrointestinal health and subsequently exert other health-related benefits.5, 6
Gibson and Roberfroid (based on earlier research by Fuller in 1991) first established the criteria for whether bacteria qualify as probiotics (beneficial or “good” gut microbes): 5, 7
- The probiotic must be capable of being prepared in a viable manner and on a large scale (e.g., for industrial purposes).
- During use and under storage, the probiotic should remain viable and stable.
- It should be able to survive in the intestinal ecosystem.
- The host animal should gain beneficially from harboring the probiotic.
Major gut bacterial genera include Bacteroides, Clostridium, Lactobacillus, Eubacterium, Enterococcus, Escherichia, Fusobacterium, Bifidobacterium,and Enterobacter.8 Current research on dietary probiotics largely focuses on the Lactobacillus and Bifidobacterium genera.
Colonic bacteria metabolize and ferment dietary components into absorbable nutrients.2, 9 Some colonic bacteria are responsible for the synthesis of vitamins, while others provide protection against pathogens and are vital to maintaining healthy gut epithelial cells. A healthy, balanced gut bacterial ecosystem is associated with averting diarrheal illness, obesity, and diabetes, and there is particular interest in the role gut bacteria play in a healthy immune system.6, 10 But the human gut bacterial composition changes with aging1, 11 and fluctuates with the food we ingest,8 as well as with our general state of health. It is therefore important to get acquainted with our gut bacteria, including the “food” they eat to survive and thrive.
One of the major food sources for probiotics and other colonic bacteria are dietary complex carbohydrates, such as oligosaccharides and polysaccharides, that largely escape hydrolysis and digestion in the stomach and small intestine. These are called nondigestible oligosaccharides, or simply NDO.7 The criteria for an NDO to qualify as a prebiotic are as follows:12, 13
A prebiotic is a selectively fermented ingredient that allows specific changes, both in the composition and/or activity in the gastrointestinal microflora that confer benefits upon host well being and health.
For an NDO to qualify as a prebiotic, it must selectively promote the growth of one or more beneficial bacterial species with known health benefits. Prebiotics manipulate the human colonic microflora by selective modulation of the gut’s bacterial populations (i.e., they stimulate the growth of beneficial bacterial populations). Ingesting “colonic foods” such as NDO ensures that probiotics and other beneficial indigenous microbes flourish.
Several NDO are known to promote the growth of colonic Bifidobacteria strains. Bifidobacteria are recognized to exert favorable effects on the host by stimulating the immune system, and some strains are also involved in the gastrointestinal synthesis of folate, a vitamin vital in several metabolic pathways.14, 15 A prebiotic that promotes the growth of Bifidobacteria is referred to as a bifidobacterial-promoting substrate.
Manufacturing of Prebiotic NDO
Several food sources are rich in NDO, including fruits (bananas and tomatoes), vegetables (asparagus, onion, garlic, beet, artichokes, and leeks), and grains (rye, oats, barley).16 Commercial sources of NDO are available as dietary supplements and food ingredients.
Prebiotics are manufactured via enzymatic processes; for example, NDO can be enzymatically prepared by connecting carbohydrate monomers to generate “longer” oligosaccharides, or they can be prepared by enzymatic decomposition of polysaccharides into smaller oligosaccharides.17 The production of NDO allows suppliers to offer concentrated amounts of NDO in milligram-to-gram quantities per serving.
Chemical Nature of NDO
Oligosaccharides are polymers comprising linked carbohydrate monomers (i.e., monosaccharides). NDO differ from one another by the kind and number—generally 2 to 10—of monosaccharides that make up the NDO, as well as in the types of chemical bonds linking the monosaccharides. Variation in the kind of monosaccharides and types of bonds results in variation between NDO.
This variation is relevant, because there are preferences for certain gut bacteria (and their enzymes) to ferment certain bond types, which in turn stimulates bacterial growth (i.e., increase the population). Not all NDO are equivalent, and ingesting a probiotic but neglecting to ingest the proper NDO or mixture of NDO might not provide sustained health effects. For this reason, dietary supplements often include a mixture of prebiotics and probiotics. These products are referred to as synbiotics.
Colonic bacteria ferment and degrade NDO into smaller, absorbable metabolites, which are then utilized by biological processes. The fermentation of NDO generates soluble short-chain fatty acids, including acetate, propionate, and butyrate; several gases, including hydrogen, carbon dioxide, hydrogen sulfide, and methane; and other metabolites, such as ethanol, pyruvate, lactate, and succinate.11, 18-20 These fermentation end-products are absorbed into the blood stream and further metabolized by various tissues and organs.3, 7, 12 Additionally, short-chain fatty acids inhibit the growth of pathogens by lowering the colonic pH.
Assessing and Comparing the Prebiotic Potential of NDO
Bioavailability, metabolism, and fermentation studies on NDO provide information about whether specific NDO arrive in the gut intact. Marketers can compare the outcome of these studies to glean insight into an NDO’s prebiotic potential.
Over the past two decades, researchers have worked to evaluate the prebiotic potential of NDO.11, 12, 13, 18, 21 Gibson and Roberfroid stipulated that an NDO must meet the following criteria before it can be considered a prebiotic:6, 13
- It resists gastric acidity, hydrolysis by mammalian enzymes, and gastrointestinal absorption.
- The NDO is fermented by the intestinal microflora.
- It selectively stimulates the growth and/or activity of intestinal bacteria associated with health and well being.
To be considered a prebiotic, an NDO must reach the colon intact so it can be fermented by beneficial gut bacteria. Additionally, it must also only promote the growth of such beneficial bacteria, and not harmful bacteria.
Ahead, we look at evaluations of the prebiotic potential of various NDO by experts in the field.11, 13, 18, 21 Note that while the scientific research on the prebiotic nature of NDO is continuously being published, the expert opinions of leading scientists in the field of pre- and probiotics regarding the prebiotic status of NDO is only updated periodically in scientific reviews.4, 7, 12, 13, 18, 21
Fructooligosaccharides (FOS) are oligofructans and inulins and are often referred to as inulin-type prebiotics.4, 21, 22 These NDO comprise linked fructose monosaccharides.7, 18, 22 Research shows that oligofructans and inulins, 1) resist digestive system degradations, 2) are fermented in the colon by most strains of Bifidobacteria and several other colonic bacteria, and 3) selectively stimulate the growth of Bifidobacteria while simultaneously inhibiting the growth of pathogenic species such as Clostridium, a known toxin producer.7, 11, 13, 21 Based on these and other findings, FOS qualify as a prebiotic.
Additionally, in January 2014, FOS gained approval in Europe for a health claim regarding reducing blood sugar levels after meals when used as a sugar substitute in foods and beverages.
Trans-galactooligosaccharides (TOS), also referred to as galactooligosaccharides (GAO), largely comprise linked galactose monosaccharides.16, 23 Research shows that TOS, 1) reach the colon intact, 2) are metabolized by several Bifidobacteria, Lactobacilli, Enterobacteria,and Streptococci species, and 3) stimulate the growth of both Bifidobacteria and Lactobacilli in humans, while decreasing the population of several species, including Bacteroides and Candida.21, 24 TOS meet the criteria for a prebiotic.
Lactulose is described as a galactosyl fructose disaccharide.13 Research shows that lactulose, 1) resists intestinal degradation, 2) is fermented by several strains of Bacteroides, Bifidobacterium bifidum, Clostridium perfringens, and strains of Lactobacilli, and 3) selectively promotes the growth of Bifidobacterium, Lactobacilli, and Streptococcus strains, while inhibiting the growth of Bacteroides, Clostridium, Enterobacterium, and coliforms.13, 21, 24 Based on these outcomes, lactulose meets the criteria for a prebiotic.