The Role of Prebiotics in Disease Prevention and Health Promotion

Chapter 12

The Role of Prebiotics in Disease Prevention and Health Promotion Rabin Gyawali1, Nwadiuto Nwamaioha1, Rita Fiagbor1, Tahl Zimmerman1, Robert H. Newman2, Salam A. Ibrahim1 1Food

and Nutritional Sciences Program, North Carolina A&T State University, Greensboro, NC, United States; 2Department of Biology, North Carolina A&T State University, Greensboro, NC, United States

1. CONCEPT OF PREBIOTICS Gibson and Roberfroid1 defined prebiotics as “a non-digestive food ingredient that beneficially affects the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon that can improve the host health.” Later, these authors revised the concept and proposed a new definition for prebiotic as “a selectively fermented ingredient that allows specific changes; both in the composition and/or activity in the gastrointestinal microbiota that beneficially affects the host health.”2,3 The Food and Agriculture Organization of the United Nations defines prebiotics as “a nonviable food component that confers a health benefit on the host associated with modulation of the microbiota.”4 The International Scientific Association for Probiotics and Prebiotics has proposed a new definition that highlights the scope of prebiotics not only for the colon health but also for the physiological benefits to other body sites.5 Although most of the current definitions of prebiotics focus on the modulation of gut microbiota to improve host health, the precise definition of prebiotics has always been the subject of debate. Further advances in our understanding of diet–microbiome interaction could change the current definitions of prebiotics and strengthen its importance as a valuable nutritional and therapeutic agent. In recent years, prebiotics have been gaining a great deal of attention, as these ingredients provide health and physiological benefits beyond the nutrient contribution alone. The major prebiotics are identified as inulin and oligosaccharides. Oligosaccharides are further grouped into fructooligosaccharides (FOS) and galactooligosaccharides (GOS). The most popular and widely used prebiotics include fructans, inulin, FOS, and GOS, which are generally regarded as safe.6,7 Prebiotics are often synthesized from disaccharides that include trans-galactooligosaccharides. These carbohydrates are nondigestible but can be fermented by the intestinal flora and therefore meet the prebiotic criteria. Most of the available data on prebiotics effects found in the scientific literature relate to inulin and oligofructose, which are found in many vegetables including onion, garlic, leek, asparagus, Jerusalem artichoke, and chicory root.8 Particularly, inulin-type fructans are the best-documented prebiotics and are considered to be an important substrate for their effect on intestinal microbiota, especially bifidobacterial populations.9 Inulin comprises 2–60 fructose molecules joined by β2-1 osidic linkages. Due to the nature of this type of linkage, inulin escapes digestion in the upper gastrointestinal tract, remains intact and selectivity fermented by colonic microbiota. In fact, several studies have demonstrated that the consumption of inulin resulted in increased fecal bifidobacterial populations.8–11 Types and sources of some common prebiotics are listed in Table 12.1. Not all dietary carbohydrates are prebiotics. To consider any food ingredients to be a prebiotic, the following criteria must be met by in vitro and in vivo tests.1,3,9 Resists gastric acidity, hydrolysis by mammalian enzymes, and absorption in the upper gastrointestinal tract Is fermented by intestinal microflora l  Selectively stimulates the growth and/or activity of intestinal bacteria potentially associated with health and well-being l l

Dietary Interventions in Gastrointestinal Diseases. https://doi.org/10.1016/B978-0-12-814468-8.00012-0 Copyright © 2019 Elsevier Inc. All rights reserved.

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TABLE 12.1  Prebiotics and Their Source Prebiotics

Sources

References

Inulin

Present in a range of natural foods, including chicory, onion, garlic, Jerusalem artichokes, tomatoes, and bananas

Crittenden and Playne109

Fructooligosaccharides (FOS)

Occur naturally in fruits and vegetables (asparagus, endive, sugar beet, garlic, chicory, onion, Jerusalem artichokes, bananas, tomatoes). FOS is prepared commercially from chicory in a hydrolysis reaction using inulinase and may also be derived in an enzymatic synthetic reaction via transfer of fructosyl units from sucrose molecules. Other sources: cereals, wheat, barley, rye, and honey

Bird et al.,27 Crittenden,110 and Sangeetha et al.111

Galactooligosaccharides

Legumes, nuts, soy beans and soy products, peas, rapeseed meal, lentils, Chickpeas/hummus, green peas, lima beans, kidney beans

Niba et al.131 and Iacovou et al.112

Fructans

Naturally occurring oligosaccharides found in onions, bananas, wheat, artichokes, garlic, and other whole foods. They are also extracted from chicory or manufactured from sucrose for use in the food industry

Chow113 and Anadón et al.9

Resistant starch granules

Raw potatoes, bananas

Niba et al.131

Pectins

Apple, sugar beet pulp

Niba et al.131

β-Glucans

Oats and barley

Arena et al.90 and Shigwedha et al.72

Psyllium

Psyllium husk (plant)

Shigwedha et al.72

Isomaltooligosaccharides

Produced commercially from the enzymatic action of α-amylase, pullulanase, and α-glucosidase on cornstarch

Kohmoto et al.114

Lactulose

Galactofructose isomerization product derived from lactose

Conway28 and Saunders and Wiggins115

Milk oligosaccharides

Human and cow’s milk. They may also produce synthetically from lactose syrup using β-galactosidase

Crittenden,110 Kolida et al.,116 and Venema18

2. MODULATION OF GUT MICROBIOTA Prebiotics have been shown to be an ideal substrate for the health-promoting bacteria that are present in the colon, especially bifidobacteria and lactobacilli. Prebiotics modulate (manipulate) the colonic microflora by stimulating these health promoting bacteria and inhibiting undesirable bacteria such as Clostridium and Bacteroides.12 Prebiotics such as nondigestible oligosaccharides play a major role in the colonic microbiota composition by promoting the saccharolytic activity of the microbiota. These oligosaccharides differ in lengths, solubility, and sugar composition and thus form a diverse source of substrate that alters the gut microbiota and its activities and thereby provide health benefits to the host.13 Typically, there are two patterns of prebiotic metabolism. In the first pattern, Bifidobacterium spp. possesses cell-associated β-fructofuranosidase activity through cell-associated enzyme exoglycosidases. The enzymatic hydrolysis of monosaccharides from the nonreducing end of the oligosaccharides is taken up by the cells.14 The second mechanism that occurs in some microbiota spp. is through uptake of oligosaccharides by probiotics followed by intracellular metabolism.15 Prebiotics can also influence many aspects of bowel function through fermentation.16 The majority of the colonic microbiota such as bifidobacteria and bacteroides can ferment starch. In a study conducted with rats, it was shown that starch increased large bowel butyrate levels and the numbers of bifidobacteria and lactobacilli.17 It has been reported that the oligosaccharides in human breast milk are also considered to be prebiotics. These oligosaccharides can facilitate the growth of bifidobacteria and lactobacilli in the colon in breast-fed neonates.18 Administration of short-chain GOS and long-chain FOS as a prebiotic mixture (9:1) to newborns resulted in the prevention of allergies and infections that lasted beyond the intervention period.19 Clinical study also revealed that administration of oligosaccharides increased stool frequency and stool softness in infants, which is similar to the results observed in human milk–fed infants. This study thus demonstrated the prebiotic effects of early microbial colonization and their health-promoting effects on infants.20 The prebiotic effects

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of whole grain breakfast on gut modulation were reported by Carvalho-Wells et al.21 These authors found that corn-based whole grain breakfast cereal mediated a bifidogenic modulation of the gut microbiota in a study conducted with 31 subjects. Similarly, in a randomized, controlled trial, banana consumption showed prebiotic effects. Mitsou et al.22 found increased bifidobacterial populations in the fecal samples of subjects who consumed two bananas per day. In a study conducted on obese women, supplementation of inulin-type fructans was shown to alter the gut microbiota, which led to moderate changes in key metabolites associated with obesity and diabetes.23 Results from the recent study on the effect of a dietary intervention with prebiotic inulin-type fructans on colonic microbiota also revealed increased bifidobacterial spp. in fecal samples.24 This study consisted of two 4-week intervention periods with consumption of 12 g of inulin per day. However, the minimum dose of inulin-type prebiotics needed to observe an increase in gut bifidobacterial populations appears to be at least 2.5 g per day.25 It is well established that the human gut harbors diverse microbes that play a fundamental role in the well-being of their host. It is also clear that prebiotics can modulate the composition of human gut microbiota to the benefit of the host. However, most of the studies on the modulation of the gut microbiota using prebiotics are based on distal colon (human fecal samples) analysis. Such studies cannot truly represent the details of the changes occurring in upper regions of the colon.15 Thus, future research that incorporates new molecular techniques would help us to better understand how prebiotics influence the overall microbial composition in the gut.

3. PREBIOTICS EFFECTS IN HUMAN HEALTH 3.1 Production of Short-Chain Fatty Acids Short-chain fatty acids (SCFAs) are organic acids produced by the intestinal microbial fermentation of prebiotics, mainly undigested dietary carbohydrates, specifically resistant starches and dietary fibers. Basically, SCFAs are products of the metabolic actions of the microbiota and their interactions with their nutrient supply. SCFAs are 2- to 5-carbon weak acids that include acetate (C2), propionate (C3), butyrate (C4), and valerate (C5)26 and are used as an energy source by the colonic mucosa. The prebiotics literature has focused primarily on nondigestible oligosaccharides, but other fermentable carbohydrates such as dietary fibers are also considered as a major group of prebiotics. Such fermentable fibers have several health-promoting properties that include modulation of the gut microbial community and immune system support. Fig. 12.1 shows the role of dietary fiber in the GI tract. Intestinal microbiota ferments the undigested prebiotics or fibers that reach the colon and produces SCFAs (i.e., acetic, propionic, butyric acid) and gases (i.e., carbon dioxide, methane, hydrogen). These acids are important regulators of colonic physiological processes and are essential components for normal bowel function.27 The process of fermentation also produces gases (i.e., carbon dioxide, methane, hydrogen). Often, production of gas can cause undesirable discomfort such as pain and bloating by promoting gas retention. SCFAs are either absorbed or further metabolized by colonocytes, hepatocytes, or peripheral tissues. These prebiotics also influence stool bulk as a result of increased microbial biomass and thus play a crucial role in maintaining normal bowel function. Microbes

FIBER Fermentation

Stool bulking

Osmotic load

SCFAs (Butyrate, propionate, acetate) Luminal pH

Gas production (CH4, H2, CO2) Pain, bloating, flatulence

Microbiome changes Acceleration of transit time

Increased biomass

Effects on inflammation and permeability

FIGURE 12.1  The mechanism of action of fiber on intestinal transit time and visceral hypersensitivity. SCFAs, short-chain fatty acids. (Based on Eswaran S, Muir J, Chey WD. Fiber and functional gastrointestinal disorders. Am J Gastroenterol 2013;108:718.)

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in the bowel also contribute to mineral absorption and lipid metabolism because of the lowered pH due to the production of SCFAs.16,28,29 The types of SCFA production depend on the microbiota stimulated by prebiotics. For example, inulin has been shown to increase acetate and butyrate levels, whereas GOS increase the production of acetate and propionate, and only acetate is produced by xylooligosaccharides.30 The hypocholesterolemic effect of prebiotics is primarily attributed to organic acids. Particularly, butyrate is known to inhibit cholesterol synthesis and to provide a source of energy for human epithelial cells, whereas propionate inhibits the syntheses of fatty acids in the liver and helps to lower the rates of triacylglycerol (TAG) secretion. Propionate also reduces the rate of cholesterol synthesis, thereby lowering plasma cholesterol levels.31 Among these SCFAs, propionate and butyrate inhibit the growth of colon tumor cells and histone deacetylases (HDACs). HDACs are a part of enzymes that have major roles in several biological processes, primarily through the repressive influence of HDACs on transcription. In addition, butyrate is taken up by the large intestinal cells (colonocytes), protects against tumor formation in the gut, causes apoptosis, and protects the body from genotoxic carcinogens by enhancing the expression of enzymes involved in detoxification.32 The SCFAs of microbial fermentation of prebiotics or dietary fibers play an important role in the maintenance of intestinal homeostasis and overall health status. The National Academy of Sciences Institute of Medicine recommends that adults consume at least 20–35 g of dietary fiber per day in meals to receive a protective effect with regard to several gastrointestinal diseases and related ailments including colon cancer. However, the average American’s daily intake of dietary fiber is only 12–18 g.33 Because high-fiber diets are important in the prevention and management of several chronic diseases, the inclusion of high-fiber or prebiotic foods in the diet is warranted.

3.2 Colon Cancer The diversity of human microflora and their preference for carbohydrates or proteins substrates may attenuate or promote the risk of colon cancer. Lactic acid bacteria that ferment carbohydrates in the colon help prevent the onset of colorectal cancer (CRC) through the production of anticarcinogenic metabolites such as SCFAs that reduce colorectal pH, a factor associated with a diminished risk of colon cancer in various populations.34 Conversely, bacteria with a preference for proteins may increase the risk of colon cancer by producing toxic protein metabolites such as ammonia and peptides. Thus, an altered gut microbiota can trigger pathogenic responses such as chronic inflammation and immune suppression.35 Prebiotics are foods that cannot be broken down by human enzymes in the small intestine but rather are fermented further down in the gut by bacteria.36 By this definition, prebiotics are selectively fermented food ingredients that are resistant to gastric acid, gastrointestinal absorption, and hydrolysis by mammalian enzymes.2 Generally, only indigestible carbohydrates, primarily oligosaccharides, some polysaccharides, fructans (inulin, FOS), lactulose, fiber, and resistant starch used by selected colon microflora to impart health and well-being to the host are called prebiotics. The efficacy of prebiotics is dependent on the type of carbohydrate being fermented by the intestinal microflora and, to some extent, the dose ingested.37 Inulin and oligofructose specifically stimulate the growth of bifidobacteria in the human gut. Significant decreases in the population of colon bifidobacteria have been observed when administration of inulin and FOS ceased in various human trials.38 One of the primary roles of prebiotics is to positively modulate the gut microbiome by stimulating the proliferation of beneficial gut bacteria, improving the overall metabolic activity of the colon while inhibiting the growth of pathogenic bacteria which can produce carcinogenic enzymes. Other health-promoting actions of prebiotics include the production of SCFAs, modulating gene expression in the colon, increasing micronutrient intake in the colon, regulating xenobiotic metabolizing enzymes, and improving immune responses.35 Primary SCFAs such as butyrate, acetate, and propionate are acidic metabolites from the fermentation of indigestible carbohydrates in the intestinal lumen. The most obvious effect of SCFAs is the lowered luminal pH that provides protection against the growth of pathogens. Increased production of SCFAs through the synbiotic action of prebiotic and probiotics results in the downregulation of inducible nitric oxide synthase and cyclooxygenase-2 (COX-2) enzymes, which have been implicated in the onset of colon carcinogenesis.35 Butyrate, the most important SCFA in terms of colonic health, is obtained primarily from the anaerobic fermentation of dietary fiber,34 which enables otherwise lost energy from fiber to be salvaged. On the average, SCFAs contribute about 5%–15% of the total calories required by humans. Butyrate is utilized as the predominant energy (70%) source for the epithelium of the colonic epithelial cells. Butyrate is a healthy fermentation metabolite because it plays a role in maintaining colonic homeostasis and is associated with several biological properties in the gut.39 One of the first observed roles of butyrate was its effect on DNA methylation and regulated gene expression. Although the impact of this function in preventing CRC is not yet clearly understood, butyrate plays a role in the replication of normal cells while suppressing transformed or mutated cells through apoptosis, an anticarcinogenic effect.34 Butyrate also acts as HDAC inhibitors, which causes hyperacetylation of histones, increasing accessibility

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of transcriptional factors to nucleosomal DNA and modification of oncogene expression.40 Other likely intracellular targets of butyrate include modulation of kinase signaling, alteration of DNA methylation, and selective inhibition of the phosphorylation of histone. These effects underlie the ability of butyrate to control gene expression and cell cycle. Butyrate can alter the components of the plasminogen/plasmin system (PPS) to cause decreased plasminogen activator activity. Increases in the serum level of the components of PPS are positively associated with invasive tumors and colonic cancer. Studies on colonic cancer cells have shown that butyrate promotes the activity of glutathione S-transferase, an enzyme that detoxifies xenobiotics. By inhibiting the decay-accelerating factor, butyrate can mediate the growth and proliferation of tumor cells.39 Another proposed anticarcinogenic mechanism of prebiotics is the binding action of dietary fiber to active genotoxicants known as heterocyclic amines (HCAs). This facilitates the transport of mutagenic HCAs out of the colon through excretion. Dietary fiber helps to create a bulky stool, resulting in a decrease in fecal transit time through the colon, so the contact time between fecal mutagens to interact with the intestinal epithelium is reduced.41 The binding action of prebiotics to HCAs may be due to the interaction between the free functional groups of mutagens and dietary fiber. For example, the treatment of mice with inulin and FOS significantly diminished the appearance of aberrant crypt foci (ACF), a preneoplastic lesion located in the mucosa, which can undergo abnormal proliferation to become malignant. Through prevention of oxidative stress, reduced catalase activity, and changes in microbiota, prebiotics can mediate the appearance of this lesion.42 Epidemiological studies on human subjects showing association between prebiotics, diminished levels of ACF, and CRC are yet to be confirmed. Detailed studies using advanced techniques are needed to establish such valid.35

3.3 Inflammatory Bowel Disease Inflammatory bowel disease (IBD) is chronic disease that affects the gastrointestinal tract (GI). IBD occurs in two major forms as Crohn’s disease or ulcerative colitis, characterized by periods of flares and remission. Crohn’s disease appears as discontinuous and transmural lesions on the gut cell wall. Unlike ulcerative colitis, Crohn’s disease can affect the entire wall of the intestine, whereas ulcerative colitis is associated with lesions restricted to the colon and rectum.43 The causes of IBD are still unclear, but it is hypothesized that IBD occurs due to an extreme immune response to endogenous microbes in genetically predisposed individuals. Epidemiological studies have shown associations between human gut microflora and the induction of IBD.44 Commensal bacteria resident in the lumen of the intestine play a role in immune tolerance. Intestinal homeostasis (microbial equilibrium required to maintain health) may be disrupted due to a loss of epithelium barrier function, alterations in the innate immune cells (macrophages and dendritic cells) that provide initial responses to invading bacteria, and disruption of lymphocyte function in the lamina propria and mesenteric lymph nodes (MLN).45 Under normal conditions, when the intestinal mucosa is simulated by gut microflora, a low-grade physiological inflammatory response occurs. Interactions between gut microorganisms and the mucosal immune regions are important in the regulation of the gut immune system. IBD occurs when there is excessive or unfavorable immune response against commensal microorganisms in the gut.46 The modulation of the gut microbiota can be achieved through several approaches, one of which is the use of prebiotics. As stated earlier, prebiotics are selectively fermented ingredients that impart specific desirable changes on the gut microflora. The most important product of the pre/probiotics relationship (synbiotics) is the formation of SCFA, especially butyrate. Studies have shown that there is an association between the gut microbiota and the onset of IBD. The most inflamed areas in patients with IBD are those with the highest bacterial activity, especially areas with high activities of proteobacteria and actinobacteria. This observation was also linked to decreased levels of SCFA in feces, particularly butyrate, which inhibits the production of proinflammatory cytokines and increases the synthesis of antimicrobial peptides and mucin. SCFAs activate the peroxisome proliferator-activated receptor γ, a known antiinflammatory transcription factor in experimental IBD.47 In patients with ulcerative colitis, intake of prebiotics such as FOS led to reduced fecal calprotectin, an inflammatory marker.48 In vitro studies have shown that some strains of bifidobacteria are immunoregulatory and can increase the release of dendritic cell interleukin (IL) 10.49 A schematic illustration of intestinal mucosa in healthy and IBDaffected individuals is illustrated in Fig. 12.2. Germinated barley foodstuff (GBF), a prebiotic rich in glutamine and hemicellulose, has been reported to have unique properties that offer prolonged remission in people with mild to moderate ulcerative colitis. GBF is converted to butyrate and preferential nutrients for colonocytes when acted on by bifidobacteria and eubacteria.51 Patients who were treated with GBF therapy experienced lower relapse rates than those treated with only conventional medications. Treatment with GBF lowered the serum level of both IL-6 and IL-8, tumor necrosis factor α, and C-reactive proteins (CRPs).52,53 Current research for the treatment of IBD is oriented toward a combined therapy of both prebiotics and probiotics (synbiotic

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FIGURE 12.2  Schematic illustration of intestinal mucosa in healthy and inflammatory bowel disease (IBD)–affected individuals (Matijašić et al.50). (A) Healthy individual and (B) individual with IBD. Note that in healthy individuals, the epithelium is covered by a thick mucus layer. Commensal bacteria inhibit the proliferation of pathogens, thus maintaining intestinal homeostasis. Mucosal dysbiosis (disruption of commensal colonization and proliferation of pathogens) in individuals with IBD results in thinning of the protective mucosal layer, thereby reducing the integrity of the mucosal layer. The mucosal layer is accessible to bacteria, which leads to inflammatory responses by the host immune system. IgA, immunoglobulin A.

combinations). Common synbiotic combinations include Lactobacillus rhamnosus and inulin, bifidobacteria and FOS, and lactobacilli with either inulin or FOS. Patients with active ulcerative colitis who were treated with conventional therapy, supplemented with a combination of Bifidobacterium longum and inulin, in a randomized control trial showed a significantly better clinical activity index compared with those patients treated with a placebo.54 Additional research on the use of synbiotic combinations as novel IBD interventions in humans is currently underway.

3.4 Cardiovascular Disease According to the World Health Organization (WHO),55 by 2030, cardiovascular diseases (CVDs) will remain the leading causes of human death, affecting approximately 23.6 million people around the world (WHO). Unhealthy diets that are high in fats, salt, and free sugar and low in complex carbohydrates, fruits, and vegetables can lead to an increased risk of CVDs. A high level of low-density lipoprotein (LDL) has been strongly associated with CVD risk. Increased triglyceride-rich lipoproteins, high levels of LDL-cholesterol, and low levels of high-density lipoprotein (HDL)-cholesterol are the major lipid profile in people with CVD. Results from several studies suggest that intake of plant-based prebiotics could help improve gut health and reduce the occurrence of CVD. Prebiotics are utilized by the intestinal microbial population to produce SCFAs (acetate, butyrate, propionate), which may lead to reduced incidences of CVDs and improvement of lipid profiles.31,56,57 Prebiotics are known to reduce the cholesterol level in two different ways: (1) by enhancing cholesterol excretion via feces and (2) by producing SCFAs on selective fermentation by the gut microbiota.58 Several studies have suggested that adequate intake of prebiotics consistently lowers the risk of CVD and coronary heart disease by reducing LDL levels.8,33,59,60 These results can be attributed to the beneficial role of fibers in reducing CRP levels, apolipoprotein levels, and blood pressure, all of which are biomarkers for heart disease.59 These authors suggested that dietary fiber is protective against high CRP and observed an inverse relationship between the intake of total dietary fiber and CRP concentrations. The consumption of prebiotics such as inulin and FOS has also been linked to a modification of the serum levels of triglycerides and cholesterol in rodents.61,62 When rats were fed with prebiotic oligofructose, a reduction in cholesterol levels was observed but only after long-term feeding (16 weeks). In another study, healthy men

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and women were administered with 10 g inulin for 3 weeks, and a reduction in fasting TAG and hepatic lipogenesis was observed, but there were not significant effects on plasma cholesterol concentrations.62 Although, some of the studies do support the beneficial effects of prebiotics on lipid levels, the positive results observed in animal studies are not consistent or reproducible in human subjects. Dietary intervention with prebiotics, especially the fructans inulin and oligofructose, reduced serum triacylglyceride in human and animal studies.63 Causey et al.64 reported that a daily intake of 20 g of inulin significantly reduced serum triglycerides and noted an increase in serum HDL-cholesterol compared with the control group. Similarly, Dikeman et al.65 found that inclusion of inulin in the diets of rats increased the level of excretion of fecal lipids and cholesterol compared with that of rats in the control group, suggesting reduced cholesterol absorption. Davidson et al.66 observed a decrease in both total and LDL-cholesterol in hypercholesterolemia human subjects when administered 18 g/day of inulin for 3 weeks. In summary, the prebiotic benefits on CVD have been determined largely from animal studies. Additional, well-conducted human clinical studies are needed to validate prebiotic benefits in the prevention of CVD.

3.5 Type II Diabetes and Glycemic Control Diabetes is a chronic metabolic disorder associated with high blood glucose (simple carbohydrates) that afflicts 422 million in adults worldwide. According to the WHO global report on diabetes, type II diabetes (T2D) is seen as the main factor contributing to the astronomical rise in the total number of diabetics and is driven by high incidences of obesity. Unfortunately, it has been projected that diabetes will be the seventh leading cause of death by 2030.67 Nutrition therapy has proven to be effective in the management and prevention of T2D across most populations. Therefore, understanding the role of food in the metabolic process is important as food intake habits can either reduce or increase the risk of contracting diabetes or complicate an existing condition.68 The effect of food on regulating glycemia and metabolic disorders is a key to the prevention and management of T2D. A variety of factors including type of carbohydrate, amount of dietary fiber, form of food, and specific food component affect the role of carbohydrates on blood glucose by directly influencing the digestion and absorption process. It is equally important to note that foods with low glycemic index tend to release carbohydrates slowly into the blood, thus making these foods effective in regulating blood glucose and insulin responses. The intake of dietary fiber has a negative correlation with body mass index (BMI). However, glycemic index and glycemic load are positively correlated with BMI. Thus, the intake of a prebiotic diet or higher dietary fiber intake is associated with lower body weight and, consequently, reduced risk of T2D.69,70 Prebiotics can alter the gut microbiota composition associated with several metabolic syndromes including obesity and T2D. An increase in bifidobacterial spp. was observed in high-fat oligofructose (OFS) treated mice. This increase in the bacterial population is positively correlated with improved glucose tolerance. These results would thus suggest that a prebiotic diet could reduce levels of endotoxemia, which are associated with an increased risk of diabetes.71 The management and prevention of T2D is linked with dietary changes, particularly the intake of low glycemic index foods such as those rich in dietary fiber. One of the physiological and beneficial effects of the prebiotics or dietary fiber is reduced postprandial blood glucose and/or insulin levels.72 It has been reported that regular consumption of fiber has the potential to reduce glucose absorption rates, prevent weight gain, and increase beneficial nutrients and antioxidant levels, thereby helping to prevent diabetes.8 Hopping et al.73 reported a significantly lower risk of diabetes in people who consumed more than 15 g fiber/day revealing an inverse relationship between dietary fiber consumption and the development of T2D. Similarly, in another study, participants receiving high amounts of insoluble fiber (>17 g/day) or cereal fiber (>8 g/day) had less risk of developing T2D.74 Similarly, Pourghassem et al.75 evaluated the effects of inulin supplementation (10 g/day) on blood glycemic control in women with T2D and found improved glycemic indices. Thus, a diet containing prebiotic properties (nondigestible carbohydrates) could alter the gut microbiota and help to improve metabolic disorders associated with obesity and T2D.76

3.6 Weight Management One of the beneficial effects of prebiotics is enhancement of feelings of satiety and the concomitant reduction of energy intake that can result in better weight management.72 In addition, gut microbiota plays an important role in weight management by modulating the gastrointestinal (GI) tract. Changing the bacterial makeup of the GI can have different effects on appetite and metabolism, thus indirectly impacting weight gain. Reimer et al.77 found that the body fat in rats fed with a high prebiotics diet was significantly lower than the body fat of those fed with high protein or the control diet. This same effect was observed in human studies in which an increase in prebiotics intake was associated with weight loss as a result of

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the decrease in energy intake. In a randomized, double-blind, placebo-controlled trial conducted for 12 weeks, the effects of FOS supplementation on body weight were measured in overweight and obese adults. Results of this study showed a reduction in body weight with FOS, whereas an increase in body weight was observed in the control groups.8 Similarly, in a clinical study with healthy adults, effects of different dosages of dietary supplementation with wheat dextrin (prebiotic fiber) on satiety were investigated. Wheat dextrin increased short-term satiety and decreased hunger feelings compared with placebo.78 A recent randomized, double-blind, placebo-controlled trial of children with overweight and obesity investigated whether 8 g oligofructose-enriched inulin/day or placebo (maltodextrin) for 16 weeks could help control appetite. Results of this study showed improved appetite ratings and a significantly higher feeling of fullness compared with the placebo diet79 indicating that the prebiotics diet also had the potential to help with appetite regulation in obese children. The results of these studies support the inclusion of adequate amounts of prebiotics in every meal as an effective method for managing weight.

3.7 Immune Function Stimulation of the immune system is possible by improving gastrointestinal microflora and by selectively promoting the growth of probiotics and/or inhibiting pathogenic microorganisms.72 This can be achieved by the regular consumption of certain types of prebiotics that support gut-associated modulation of microbiota and immune function. The use of prebiotics can increase the body’s resistance to intestinal and extraintestinal pathogens through modulation of the immune system, resulting in reduction in allergic diseases.80 For example, during fermentation, prebiotic oligosaccharides can indirectly modify the colonic pH and help to release substances that can affect intestinal cells or the mucosal immune response.80 Roller et al.81 reported that the synbiotic association of inulin with probiotics improved the immune system responses of polypectomized and resected colon cancer patients as evidenced by modulation of the gut-associated immune system itself. It has also been reported that prebiotics can modulate the immune system either directly or indirectly to reduce the risk and severity of bowel infections and inflammatory conditions, such as IBD, and functional bowel disorders.82 The prebiotic effect of a carbohydrate such as resistant starch (chemically modified starch used in the food industry) is of great interest in human health promotion, as resistant starch is not absorbed in the intestine. Consequently, such starches are able to manipulate the gut microbiota and microbial products (e.g., SCFAs) and optimize health by promoting the immune response and suppressing potential pathogens.27 Some types of prebiotic fibers have also been shown to play an important role in improving immune function via production of SCFAs. The addition of SCFAs to parenteral feeding has been shown to increase T helper cells, macrophages, and neutrophils and increase the cytotoxic activity of natural killer cells in animal studies.8 In an infants study, oligofructose consumption was found to reduce febrile illness associated with diarrhea or respiratory events and also reduced antibiotic usage.83 Prebiotic β-glucans has also been shown to interact with immune cells and to stimulate the immune system directly. Konikoff and Denson84 reported that intake of a mixture of FOS and inulin produced a significant reduction in disease severity indices, proinflammatory immune markers, and calprotectin. Calprotectin is an abundant neutrophil proteins present in the colonic mucosa and may contribute to the pathogenesis of IBD. Prebiotics such as oligosaccharides also induce antimicrobial effects via their selective stimulation of indigenous beneficial microflora that secretes antimicrobial compounds, modulate immune function, and compete with pathogens for receptors.15 In recent years, human milk oligosaccharides (HMOs) have also received much attention and are considered to be prebiotics. These prebiotics are potent inhibitors of bacterial and viral adhesion to epithelial surfaces, and as a result, breast-fed infants are often at a lower risk of intestinal infections. In addition, these HMOs also contribute to the body’s natural defenses against infection by promoting a proliferation of probiotic bifidobacteria and lactobacilli in intestinal microflora.11,18 The optimum daily requirement for prebiotics is estimated to be between 35 and 50 g/day, which is necessary for proper intestinal function and for positive effects on the immune system such as lowering the risk of allergies in both infants and adults. A better understanding of the probiotic strains and metabolites that influence immune system will allow for the development of novel prebiotics for the prevention and treatment of immunological disorders.

4. SYNBIOTIC APPROACH A synbiotic is defined as a “mixture of probiotics and prebiotics that beneficially affects the host by improving the survival and activity of beneficial microorganisms in the gut.”85 Synbiotics are those products in which the prebiotic compound

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selectively favors the growth of probiotics and their metabolite production. Synbiotic effects can occur in two ways by improvement in the host’s health after ingestion of a mixture of prebiotics and probiotic strains or by the promotion of indigenous beneficial microflora such as bifidobacteria after ingestion of prebiotics alone. Studies carried out on humans and animals on the effectiveness of synbiotics have proven synergistic health effects in the hosts. Synbiotics are designed to beneficially affect the host by the following:1,86 Improving survival and implantation of probiotics in the colon Selectively stimulating the growth or activating the metabolism of health-promoting bacteria (probiotics) in the colon l Improving the microbial composition of the GI tract l l

Bouhnik et al.87 investigated the effects of inulin and Bifidobacterium spp. in healthy volunteers and found an overall increase in bifidobacterial counts in fecal samples. However, with the prebiotic alone, there was no improvement in the bifidobacterial population. Significant improvement in LDL/HDL cholesterol ratios was observed in healthy women following consumption of synbiotic yogurt (Lactobacillus acidophilus + B. longum + FOS) for 7 weeks.88 Synbiotic treatment of Bifidobacterium breve, Lactobacillus casei Shirota, and GOS for over 1 year led to an increase in fecal SCFA levels, increase in fecal bifidobacteria and lactobacillus concentrations, and improvement in the rate of body weight gain in patients with short bowel syndrome.89 Arena et al.90 studied the in vitro synbiotic effects of cereal β-glucans and probiotic lactobacilli strain on the immunomodulatory effects on host cells. In vitro results showed synergistic effects in the modulation of the transcriptional level of several immune-related genes, leading to an overall enhanced antiinflammatory effect. In addition, application of synbiotics has been shown to enhance the barrier function of the epithelium and to stimulate the immune system. This effect was demonstrated by Rafter et al.,91 who observed increased transepithelial resistance of the Caco-2 (colon adenocarcinoma cells) cell monolayer when treated with fecal water taken from polyp patients receiving synbiotics. Similarly, Nowak et al.92 investigated the antigenotoxicity of both probiotic and nonprobiotic lactic acid bacteria against human fecal water, which induces DNA damage in colon Caco-2 cells. The results of this study showed that fermented inulin induced stronger DNA repair in cells pretreated with mutagens than nonfermented inulin. These results suggest that prebiotics can strongly inhibit DNA damage when used in combination with lactic acid bacteria, which may be attributable to carbohydrate type, SCFA yield, and the ratio of the end products of prebiotic fermentation. The efficacy of synbiotic products on in vivo animal studies is also promising. When rats were fed a mixture of inulin and probiotic strains (L. rhamnosus GG, Bifidobacterium lactis BB-12), Angelo et al.93 found a reduction in the incidence of colonic tumors on azoxymethane challenge compared with rats fed with probiotic strains alone. In another study, the synbiotic combination of resistant starch and B. lactis resulted in an apoptotic response to a genotoxic carcinogen in the distal colon of rats.94 According to these authors, resistant starch might have acted as a metabolic substrate creating the appropriate conditions for probiotic strains to exert a proapoptotic response to DNA damage by a cancer initiator in the rats’ colons. Bomba et al.95 demonstrated that the combination of Lactobacillus paracasei and maltodextrin decreased Escherichia coli colonization in the jejunum of piglets. The authors also observed an increase in lactobacilli and bifidobacteria and a decrease in Clostridium and enterobacteria when piglets were fed with a synbiotic combination of L. paracasei with FOS. Patel et al.32 demonstrated that an increase in the myoelectric activity of the small intestine and colon in animal studies might help to reverse age-related intestinal motility decay. Such effects could help to reduce the incidence of constipation commonly seen in elderly people. Studies have also shown the beneficial role of synbiotic administration in animal health as evidenced by an increase in the number of beneficial bacteria and a decrease in the pathogen loads. A synergistic effect of probiotics and prebiotics in the reduction of foodborne pathogenic bacteria was also reported by Bomba et al.96 The mechanism of synbiotic effects is via modulation of the metabolic activity in the intestine along with maintenance of the intestinal biostructure, development of beneficial microbiota, and the concomitant inhibition of potential pathogens present in the GI tract. As a result, a reduction in undesirable metabolites plus inactivation of nitrosamines and carcinogenic substances occurs. This further leads to positive effects on the host’s health as a result of increased levels of SCFAs, ketones, carbon disulfides, and methyl acetates.97 A number of synbiotic approaches have been explored because of their beneficial effects. Some of the in vivo human and animal studies are presented in Tables 12.2 and 12.3. These in vivo studies suggest that trial outcomes of prebiotics vary significantly depending on the condition being treated, the strains of probiotics employed, and the types of prebiotics used. However, synbiotic products provide the potential for development of new types of prebiotics that could target specific probiotic strains for the enhancement of human health. Thus, a mixture of prebiotics and probiotics could be an excellent strategy for the formulation of probiotic supplements and other food products.

TABLE 12.2  Synbiotic Efficacy Shown in Human Studies Synbiotic Approach

Types of Study

Duration

Outcome

References

Bifidobacterium longum, Lactobacillus acidophilus, and OFS

Sixteen healthy subjects who had not received antimicrobial therapy within the previous 3 months

Twice a day for 30 days

Significantly influenced the metabolic activity of the intestinal microbiota. Increase in short-chain fatty acid (SCFA) (butyrate) was positively correlated with the number of bifidobacteria in feces.

Ndagijimana et al.117

Oligofructose-enriched inulin + Lactobacillus rhamnosus GG and Bifidobacterium lactis BB-12

Double-blind, placebo-controlled trial in 37 cancer patients and 43 polypectomized individuals

12 weeks

Colorectal cell proliferation and genotoxicity were significantly reduced and the intestinal barrier function improved

Rafter et al.91

B. longum with mixture of OFS and inulin

Double-blinded, randomized, controlled trial using 18 patients with active ulcerative colitis

1 month

Improved ulcerative colitis symptomology due to the increased gut bifidobacteria

Furrie at al.54

Bifidobacterium breve, Lactobacillus casei, and galactooligosaccharides

Seven malnourished patients with short bowels suffered with refractory enterocolitis

More than 1 year

Improved the intestinal anaerobic bacteria, suppressed the residence of pathogenic bacteria, and increased SCFAs in the feces

Kanamori et al.89

Synbiotic yogurt containing L. acidophilus 145, B. longum 913, and oligofructose

Randomized, placebo-controlled, and crossover study involving 29 women (19–56 years)

Over a period of 21 weeks (300 g daily)

Significantly increased serum high-density lipoprotein (HDL)-cholesterol by 0.3 mmol/L, leading to an improved ratio of low-density lipoprotein (LDL)/HDL.

Kiessling et al.88

Milk containing L. acidophilus (7–8 log CFU/g) and 2.5% of fructooligosaccharides (FOS)

Randomized, placebo-controlled, doubleblind, and crossover-designed study involving 30 volunteers (33–64 years)

Three weeks (375 mL daily)

A significant decline in total cholesterol, LDL-cholesterol, and LDL/HDL ratio

Schaafsma et al.118

6 × 109 CFU of Streptococcus thermophilus, B. lactis, L. acidophilus (2 × 109 of each strain), 10 mg of zinc/day, and 0.3 g of FOS

Randomized double-blind prospective study (65 children aged 6–12 months)

600 mL of supplemented and control diet for 7 days

Reduced the severity and duration of acute gastroenteritis by close to 15 h of administration

Shamir et al.119

5 × 1010 CFU of B. lactis B94 plus 900 mg inulin or placebo

Children between the ages of 2 and 60 months (n = 156) with acute diarrhea

Once a day for 5 days

Synbiotic treatment shortened the duration of acute watery diarrhea by an average of 31 h.

İşlek et al.120

Synbiotic formulation containing Lactobacillus paracasei B21060, arabinogalactan, and xylooligosaccharides

Double-blind, randomized, placebo-controlled trial in children with acute diarrhea (n = 107, aged 3–36 months)

One sachet dissolved in 50 mL of water twice a day for 5 days

Children in synbiotic group showed a significant reduction in the duration of diarrhea

Passariello et al.121

Combination of GOS/FOS (9:1) and B. breve M16-V

Double-blind, placebo-controlled multicenter trial (90 infants with atopic dermatitis, aged 0–7 months)

12 weeks

Reduced severity of atopic dermatitis in a subgroup of infants with elevated IgE levels

Van der Aa et al.122

Fermented milk supplemented with B. lactis Bi-07, L. acidophilus NCFM, and isomaltooligosaccharide

Randomized controlled trial with 100 healthy adults

480 g/d for 2 weeks

Increased fecal bifidobacteria, lactobacilli, and decreased fecal enterobacilli compared with the control group

Wang et al.123

Synbiotic soya-based product fermented with L. acidophilus La-5, Bifidobacterium animalis BB-12, and S. thermophilus

A randomized, double-blind placebo-controlled trial with 36 normocholesterolemic men

100 g/d for 8 weeks

A significant reduction in LDL-cholesterol and sLDL:HDL ratio in the synbiotic group

Bedani et al.132 (2015)

L. casei, L. rhamnosus, S. thermophilus, B. breve, L. acidophilus, B. longum, Lactobacillus bulgaricus, FOS

Randomized, double-blind, placebocontrolled study on 38 subjects (men and women above 18 years) with insulin resistance syndrome

Twice a day for 28 weeks

Levels of fasting blood sugar and insulin resistance improved significantly

Eslamparast et al.124

L. acidophilus, Bifidobacterium bifidum, oligofructose

Randomized, double-blind, placebo-controlled study on 20 patients (50–60 years) with type II diabetes

Two daily dosage (100 mL) for 2 weeks

Increased HDL-C and reduced fasting glycemia

Moroti et al.125

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TABLE 12.3  Synbiotic Efficacy Shown in Animal Studies Synbiotic Approach

Types of Study

Duration

Outcome

References

Resistant starch and Bifidobacterium lactis

Male rats (n = 96, 5 weeks of age)

4 weeks

Significantly facilitated the apoptic response to a genotoxic carcinogen in the distal colon

Le Leu et al.94

Lactobacillus acidophilus ATCC 4962, fructooligosaccharides (FOS), mannitol, and inulin

Twenty-four hypercholesterolemic male pigs

8 weeks

Reduction of plasma total cholesterol, triacylglycerol, and lowdensity lipoprotein cholesterol compared with the control.

Liong et al.126

Combination of 1.71% (w/v) Lactobacillus casei ASCC 292, 4.95% (w/v) FOS, and 6.64% (w/v) maltodextrin

Twenty-four hypercholesterolemic male rats

6 weeks

Lower total cholesterol and triglycerides compared with the control

Liong and Shah127

Fermented milk supplemented with B. lactis Bi-07, L. acidophilus NCFM, and isomaltooligosaccharide

Forty pathogen-free mice

14 days

Increased lactobacilli and bifidobacteria, significantly increased delayed-type hypersensitivity, plaque-forming cells, and halfhemolysis values after intervention with fermented milk.

Wang et al.123

Bifidobacterium breve in combination with transgalactosylated oligosaccharides

Mice infected with intestinal Salmonella Typhimurium

7 days

Combination of bifidobacteria with prebiotic excludes intestinal Salmonella more effectively compared with individual treatments

Asahara et al.128

L. casei and dextran

Completely randomized design study with 58 Holstein dairy cows

1 year

Increased milk production in cows fed with synbiotic diet compared with the group fed with basic diet

Yasuda et al.129

A mixture of seven probiotic strains (Lactobacillus spp., Bifidobacterium longum, Streptococcus thermophilus, Enterococcus faecium) and a blend of FOS + arabinogalactans

Healthy cats (n = 12) and dogs (n = 12) of different breeds and ages

21 days

Synbiotic diet leads to an increased abundance of probiotic bacteria in the feces of cats and dogs

Garcia-Mazcorro et al.130

5. INSIGHT INTO PREBIOTICS EFFECT ON THE GROWTH OF HARMFUL BACTERIA Previous studies conducted in our laboratory and the studies reported in literature suggest that over-the-counter drugs and antibiotics that are prescribed for pain relief and to treat bacterial infections, respectively, not only kill harmful bacteria but also affect the viability and functionality of beneficial ones.98–101 It is clear that modifying the gut environment through medications affects gut microbiota and this includes both harmful and beneficial species. However, there is nearly a complete lack of information about the effect of prebiotics, i.e., do prebiotics promote the growth of harmful bacteria? Fig. 12.3 illustrates this unanswered question. The mechanism whereby prebiotics promote specific probiotic strains and how probiotic strains selectively feed on certain prebiotics has been widely reported in literature. However, a question that has barely been addressed is whether harmful species are also promoted in the gut. Our gut harbors a vast ensemble of microbes that exert either harmful or beneficial effects to the host. Usually, prebiotics support the growth of beneficial bacteria. However, it is also possible that harmful bacteria can utilize these prebiotics for growth and could compete with beneficial ones for the adhesion sites. If the number of potentially harmful bacteria becomes greater, higher incidences of chronic illnesses may appear. Some bacteria such as Salmonella, Shigella, Clostridia, Staphylococcus aureus, Candida albicans, Campylobacter jejuni, E. coli, Veillonella, and Klebsiella are disease causing and have detrimental effects if overspread because of gut dysbiosis. In vitro studies have demonstrated that these bacteria can ferment inulin, FOS, and other indigestible oligosaccharides, all of which are rich sources of nutrients needed for their growth.102–104 Peterson et al.105 demonstrated the ability of Salmonella to ferment prebiotic. This study examined whether Salmonella Typhimurium infection in mice could be prevented by administration of dietary carbohydrates. Interestingly,

162  SECTION | III  Probiotics, Prebiotics, and Synbiotics in Intestinal Functions

Higher number of beneficial bacteria

Lower number of beneficial bacteria

Antibiotics

Over-the-counter (OTC) drugs

X

X

X X

Antibiotics and OTC drugs not only kill the harmful bacteria but also negatively affect the population of beneficial bacteria

In the case of prebiotics?

Prebiotics support the growth of beneficial bacteria

Harmful bacteria

?

FIGURE 12.3  Schematic representation showing possible effects of medical drugs and prebiotics on gut microbiota. Up and down arrows indicate higher and lower number of bacterial population, respectively. Cross (X) indicates bacterial cells that are susceptible to the drugs.

the results showed that the mice fed with diets containing FOS or xylooligosaccharides had significantly higher numbers of S. Typhimurium in the liver, spleen, and MLN when compared with the mice fed with the cornstarch-based control diet. Rada et al.106 isolated clostridial species from infant feces and tested their growth in media containing seven types of commercially available prebiotics and found that clostridial species were able to metabolize all prebiotics tested. However, enhanced growth of clostridia was on media containing raffinose and lactulose. Based on these results, Rada et al.106 raised a valid concern about whether bifidobacteria-deficient infants should be supplemented with prebiotics. The influence of FOS on Clostridium difficile was experimentally determined in in vivo study by Gaskin et al.107 Authors found that higher populations of C. difficile in mice treated with antibiotic cefoxitin and FOS. Although, study showed reduction in the titers of C. difficile toxin A when FOS was given with the antibiotic, it also raises the questions concerning interactions of nondigestible oligosaccharides and antibiotics in the gut. Further in vivo studies will be required to determine the exact mechanism, whereby these prebiotics help pathogens proliferate. The studies references above are mainly based on in vitro tests. We certainly cannot derive any conclusions about the overall impact of prebiotics on human health on the basis of very few findings. However, more research should be carried out to answer the question of how prebiotics, meant only to promote beneficial bacteria, may have harmful effects as well. In addition, we would recommend that any study into the effects of prebiotics on the health of the gut or on the promotion of healthy bacteria should include, as a matter of course, the effects of these supplements on harmful bacteria. Complete studies of the ratio of beneficial to the harmful bacteria (dysbiosis) need to be carried out. To understand the potential negative consequences of prebiotics, the following questions need to be addressed. 1. Can dysbiosis (higher number of harmful bacteria than beneficial ones) be directly linked to diseases initiation and progression, thereby affecting our health? 2. As a result of gut dysbiosis, is the growth suppressing effect of probiotics on harmful bacteria diminished? 3. What is the effect of prebiotics on harmful bacteria in the context of a patient taking medication? and 4. Do changes in dietary patterns (e.g., synbiotic diets) help restore microbiota in a way that favors beneficial species?

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Whether prebiotics promote the growth of bad bacteria remains the subject of future research. If the outcomes of such study favor the assumptions we made, then “what types of prebiotics to what extent” could promote the growth of harmful bacteria will remain a debatable topic requiring further human clinical trials.

6. CONCLUSIONS AND FUTURE DIRECTIONS Diet rich in prebiotics could play a major role in gut microflora modulation, thereby providing several beneficial effects in disease prevention and health promotion. Furthermore, taking prebiotic as a means to modify microbiota, which is already present in gut, seems to be a more practical approach than taking probiotics. Although there are several literatures suggesting the beneficial effects on host health, there are still some inconsistent results between humans and animals. Thus, prebiotic research needs to be conducted thoroughly on humans and only those prebiotics that have been clinically tested can be recommended. Development of novel prebiotics that are target and site-specific will further enhance their efficacy, and as a result, additional health benefits could be achieved. Furthermore, identifying the methods in which certain prebiotics that served as an improved probiotics delivery systems to the specific sites within the gastrointestinal tract appears to be an important research topic. In our earlier study,56,133 we reported that certain plant-derived components (prebiotics) contain minerals such as iron (Fe2+). Bacteria need Fe2+ for growth, and harmful bacteria need to compete successfully for Fe2+ in the highly iron-stressed environment. Fe2+ is known to be a growth-promoting factor for Bifidobacterium spp., which is dominant group of intestinal microbiota. When Fe2+ is limited in the growth environment, Bifidobacterium spp. that have a tendency to be better at iron scavenging make Fe2+ unavailable to harmful bacteria. Thus, growth inhibition of these competing harmful bacteria by depriving them of iron is an indirect effect of prebiotics on gut microbial community. Based on this mechanism, additional work can be carried out on the discovery of novel prebiotics with dual activity that can improve the viability and functionality of probiotics while inhibiting the presence of harmful bacteria. Our literature review revealed that poor health conditions linked to the various microbiotas in our gut could be manipulated or altered by the applications of prebiotics and synbiotics. There is a strong relation between the diet and human gut microbiota. It has been shown that switching from a low-fat, plant-based diet to a “western” (high-fat, high-sugar) diet can shift the structure of microbiota within a single day.108 While prebiotics supplement products are easily available on today’s market, the consumers are advised to opt for more plant-based diets rich in prebiotics.

ACKNOWLEDGMENTS This work was supported by Agriculture research and the Department of Family and Consumer Sciences at North Carolina Agriculture and Technical State University through the USDA Evans–Allen Program, project number NC.X-291-5-15-170-1. The authors would also like to thank Jarrow Formulas (formulator and supplier of pre- and probiotics supplements, California, USA) for supporting the food microbiology laboratory throughout the years.

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Beneficial health effects of low-digestible carbohydrate consumption. Br J Nutr 2001;85:S23–30. 61. Nakamura Y, Natsume M, Yasuda A, Ishizaka M, Kawahata K, Koga J. Fructooligosaccharides suppress high-fat diet-induced fat accumulation in C57BL/6J mice. Biofactors 2017;43:145–51. 62. Letexier D, Diraison F, Beylot M. Addition of inulin to a moderately high-carbohydrate diet reduces hepatic lipogenesis and plasma triacylglycerol concentrations in humans. Am J Clin Nutr 2003;77:559–64. 63. Tuohy KM, Fava F, Viola R. ‘The way to a man’s heart is through his gut microbiota’–dietary pro-and prebiotics for the management of cardiovascular risk. Proc Nutr Soc 2014;73:172–85. 64. Causey JL, Feirtag JM, Gallaher DD, Tungland BC, Slavin JL. Effects of dietary inulin on serum lipids, blood glucose and the gastrointestinal environment in hypercholesterolemic men. Nutr Res 2000;20:191–201. 65. Dikeman CL, Murphy MR, Fahey GC. Dietary fibers affect viscosity of solutions and simulated human gastric and small intestinal digesta. J Nutr 2006;136:913–9. 66. Davidson MH, Maki KC, Synecki C, Torri SA, Drennan KB. Effects of dietary inulin on serum lipids in men and women with hypercholesterolemia. Nutr Res 1998;18:503–17. 67. WHO. Global report on diabetes. October 9, 2017. Retrieved from: http://www.who.int/diabetes/global-report/en/. 68. Russell WR, Baka A, Björck I, et al. Impact of diet composition on blood glucose regulation. Crit Rev Food Sci Nutr 2016;56:541–90. 69. Maki KC, Phillips AK. Dietary substitutions for refined carbohydrate that show promise for reducing risk of type 2 diabetes in men and women. J Nutr 2015;145:159S–63S. 70. Murakami K, Sasaki S, Okubo H, Takahashi Y, Hosoi Y, Itabashi M. Dietary fiber intake, dietary glycemic index and load, and body mass index: a cross-sectional study of 3931 Japanese women aged 18-20 years. Eur J Clin Nutr 2007;61:986. 71. Cani PD, Neyrinck A, Fava F, et al. 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166  SECTION | III  Probiotics, Prebiotics, and Synbiotics in Intestinal Functions

83. Saavedra J, Tschernia A. Human studies with probiotics and prebiotics: clinical implications. Br J Nutr 2002;87:S241–6. 84. Konikoff MR, Denson LA. Role of fecal calprotectin as a biomarker of intestinal inflammation in inflammatory bowel disease. Inflamm Bowel Dis 2006;12:524–34. 85. Tuohy KM, Probert HM, Smejkal CW, Gibson GR. Using probiotics and prebiotics to improve gut health. Drug Discov Today 2003;8:692–700. 86. Roberfroid M. Prebiotics and synbiotics: concepts and nutritional properties. Br J Nutr 1998;80:S197–202. 87. Bouhnik Y, Flourie B, Andrieux C, Bisetti N, Briet F, Rambaud J. Effects of Bifidobacterium sp fermented milk ingested with or without inulin on colonic bifidobacteria and enzymatic activities in healthy humans. Eur J Clin Nutr 1996;50:269–73. 88. Kiessling G, Schneider J, Jahreis G. Long-term consumption of fermented dairy products over 6 months increases HDL cholesterol. Eur J Clin Nutr 2002;56:843. 89. 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