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.
Some NDO are considered prebiotic candidates because they only meet some of the criteria laid out by Gibson and Roberfroid.13,21 Additional scientific research (especially human studies) to support their prebiotic nature are lacking and would be required in order to upgrade the status of these NDO to prebiotics. Examples of prebiotic candidates are:
Isomaltooligosaccharides (IMO) comprise linked glucose monosaccharides.16 Evidence suggests that a portion of ingested IMO reaches the colon, but that IMO also undergo digestion in the small intestine (jejunum).13, 21 Several colonic bacteria, including some Bifidobacteria and Bacteroides strains, have been shown to metabolize IMO, and human studies have shown that ingestion of IMO promotes the growth of Bifidobacteria. However, according to expert opinion, there is not enough scientific evidence to conclude that IMO qualify as a prebiotic.13, 21
Xylooligosaccharides (XOS) comprise linked xylose monosaccharides.16 According to evaluations in 2004 and 2010, XOS did not meet all of the criteria necessary to qualify as a prebiotic.13, 21 Evidence indicating that XOS resist gastric digestion to reach the colon intact is lacking. There is, however, evidence indicating that Bifidobacteria metabolized XOS, while Lactobacilli do not. Additionally, a human study indicated that XOS increased Bifidobacteria growth, while decreasing Bacteroides. There is no clear indication that XOS selectively stimulate the growth of only beneficial colonic bacteria. According to the 2010 review, there is not enough evidence to qualify XOS as a prebiotic.21
Alpha-glucooligosaccharides (alpha-GOS) comprise linked glucose monosaccharides.13, 16, 20, 25 Research indicates that alpha-GOS are poorly digested in the upper GI tract and largely reach the colon intact. In animal models, alpha-GOS are also fermented by microflora from humans, including Bacteroides, Bifidobacteria, Clostridium, and Lactobacilli. A 2013 study reported the increase of Bifidobacteria populations in an in vitro study.26 As of 2010, GOS do not yet qualify as a prebiotic, and the review concluded that human studies are still needed to evaluate their prebiotic potential.
Soybean oligosaccharides, or SOS, comprise raffinose and stachyose (tri- and tetrasaccharide units known as alpha-galactosides).21, 27 Both of these saccharide derivatives are known to reach the colon intact, thereby resisting gastric digestion. There is evidence to indicate that SOS are fermented by the colonic bacteria, namely Bifidobacteria and Lactobacilli, and that SOS stimulate the growth of Bifidobacteria. Regardless of these findings, the 2010 review stated that there is not enough scientific evidence to support the prebiotic nature of these NDO.20
Human Exposure Studies and Safety
Because NDO are macro-additives (food additives that are added to foods in large quantities (gram amounts) and intended to preplace conventional macronutrients such as carbohydrates, proteins, and fats), general animal toxicological studies, which use test dosages that are substantially higher than doses for anticipated intended human exposure, are impractical; such high doses would likely introduce a significant nutrient imbalance in the animals’ diets, and study results/outcomes will likely not reflect the safety of such an ingredient.28 If animal studies are impractical due to the high dose required, then investigations into digestion, metabolism, and absorption of NDO in human studies become more relevant in assessing safety.
Human studies show that the following are well tolerated: FOS (8–20 g/day),29-34 TOS (3.5–7 g/day),36-38 GOS (5 g/day),3 inulin (16 g/day),39 IMO (10 g/day),40 and a prebiotic candidate not previously mentioned, arabinoxylan-oligosaccharide (AXOS) (10 g/day).41 There are no concerns of adverse events with any of the aforementioned prebiotics, and the overall consensus is that these NDO are safe for human consumption.
Health Benefits of Prebiotics
There are several scientific reviews reporting on the health benefits associated with the consumption of prebiotics in human studies.3, 4, 21, 39, 42 Some of the health benefits highlighted include the following:
- Inulin and TOS (a.k.a., GAO) reduced attacks of diarrhea, and FOS reduced Clostridium difficile–induced diarrhea relapses.
- Inulin reduced endoscopic sores and inflammation in patients suffering from pouchitis.
- TOS increased Bifidobacteria and improved bowel movement frequency, while reducing flatulence and bloating.
- FOS improved calcium absorption in adolescents.
- FOS, inulin, TOS, and lactulose stimulate the growth of beneficial gut bacterial species in adults.
- Several studies have linked obesity with an imbalance in gut bacterial species (obese individuals exhibited elevated levels of Firmicutes, Actinobacteria, and Lactobacillus strains, and reduced Bacteroidetes strains). Weight loss has been associated with reduction in Clostridium and increase in Bacteroides. Supplementation with FOS and inulin-type fructans has shown to reduce body weight in overweight and obese adults as well enhance satiety.
Research shows that both Bifidobacterium and Lactobacillus probiotic strains exert positive effects on immune function and response through various mechanisms, such as reversing mucosal inflammation and improving phagocytic activity.44 Because Bifidobacterium colonic populations decrease as humans age,1, 11 and because Bifidobacterium are associated with numerous health benefits, the consumption of probiotic and prebiotic supplements to help sustain optimal Bifidobacterium colonic populations is essential.45
Current Regulatory Status
Several GRAS notices for NDO have been successfully filed with FDA, without questions from the agency. These NDO are considered safe as ingredients in a broad range of food categories, reflecting the generally accepted opinion that NDOs are safe for daily human consumption.
As of yet, there are no recommended daily intake values for prebiotics, and FDA has not approved any health claims regarding the consumption of prebiotics. The current guidelines recommend that women and men ingest 25 g and 38 g, respectively, of dietary fiber (nondigestible carbohydrates and lignins) per day.
Prebiotics: Feeding Good Health
The health benefits associated with maintaining a healthy and balanced gut microflora are unquestionable. Prebiotics are the main nutrients that feed the human gut bacteria, and prebiotics are important for maintaining a balanced colonic bacterial ecosystem.
It is evident through scientific research that the consumption of prebiotics and probiotics promotes the growth of beneficial gut bacterial species that, in turn, produce beneficial health effects. Companies should evaluate the research and make their choice of prebiotic with care.
Antoinette Y. Odendaal, PhD, wrote this article while serving as a research associate with AIBMR Life Sciences (Puyallup, WA), a consulting firm with expertise in GRAS self-affirmation and FDA notification, as well as regulatory compliance.
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