**2. The gastrointestinal tract, gut microbiota, and carbohydrate absorption**

Downstream of the stomach, the digestive tract is comprised of the small intestine and the colon. The colon is often discussed in terms of the proximal and distal regions, which is relevant in the context of prebiotics, considering both rates and locations for bacterial growth. The small intestine is the primary region for drug and nutrient absorption, although some nutrients and metabolites from microbial growth and metabolism are also absorbed from the colon, where the majority of the gastrointestinal bacteria reside. The transit time through the small intestine is very short – only a few hours, whereas the transit time through the colon may be on the order of 30–40 hours or more, depending upon dietary fiber and fluid intake, among other factors [1].

## **2.1 Enzymes and nutrient absorption in the digestive tract**

The small intestine of the human gastrointestinal system contains amylases (from the pancreas) to break down glycogen and starch, a 6-carbon sugar with α-1,4 bonds, and brush border enzymes (lactase, maltase, dextrinase, sucrase) to break down short chain glucooligosaccharides and disaccharides such as sucrose, lactose, and maltose into glucose, fructose, and galactose [2]. Other enzymes aid in the digestion of fats and proteins. The epithelium in the jejunum and ileum of the small intestine is also specifically designed to absorb 6-carbon sugars such as glucose, fructose and galactose via passive, facilitated and active transport systems. Dimers must be broken down into monomers before absorption, and oligomers can persist further into the colon, where they feed microbes that contain enzymes and transport systems to break down complex polymers and oligomers such as xylan and inulin. Short chain fatty acids (SCFAs) can be absorbed from the small intestine, or from the colon if produced as metabolites of bacterial fermentation. It is estimated that >90% of the SCFAs produced in the colon are absorbed, where they can influence, e.g., hepatic regulation of glucose and lipids, and hormones that regulate satiety [3].

### **2.2 Microbial communities in the digestive tract**

The digestive tract is proposed to contain about 1013 bacteria, with 100–300 different taxa and thousands of phenotypes [4]. The dominant gut bacteria identified by 16S RNA are *Actinobacteria, Bacteroidetes, Firmicutes, Proteobacteria, Verrucomicrobia*. The small intestine, where simple carbohydrates are abundant, contains about 1–10000 bacteria per gram of intestinal content, primarily *Clostridium, Bacteroides*, and *Streptococcus* species. Zoetendal et al. [5] sampled

**53**

*Functional Attributes and Health Benefits of Novel Prebiotic Oligosaccharides Derived…*

the small intestine of four healthy subjects, and observed that *Bacteroidetes*, *Clostridium* (clusters XIVa, IV, XI, and IX) and *Proteobacteria* dominated, with some *Actinobacteria* and *Bacilli* also present. Zoetendal et al. [5] provides evidence that the microbiota, particularly *Streptococcus* species, in the small intestine adapts rapidly to changes in dietary intake, particularly carbohydrates, based upon the presence of transport systems and enzymes that quickly and efficiently utilize

The colon, which survives on complex carbohydrates that are not digested or absorbed in the small intestine, contains about 1011 bacteria per gram of intestinal content, mainly *Prevotella, Ruminococcus*, and *Bacteroides*. Mucin degraders such as *Akkermansia mucinophila* are commonly found in the mucous layer. The GI tract, for various reasons, may also contain pathogenic bacteria at levels that may or may not be clinically significant. Furthermore, there is increasing recognition of the impact of the gut microbiota on the efficacy of drugs, since some microbes contain enzymes similar to those in the liver [6, 7], or may initiate breakdown of prodrugs. Such effects have been observed with metformin and L-DOPA, among other drugs, and may account for at least part of the interindividual variation in drug efficacy

Most microbes evolved to process conventional 6 carbon sugars, particularly monomeric glucose, fructose and galactose. A smaller fraction is able to use man-

**3.1 Definition, brief description of different types of prebiotics, their structure,** 

Historically, the definition of prebiotics was specific to oligosaccharides affecting the gut microbiome, and health impacts arising therein. Recently, the International Scientific Association for Probiotics and Prebiotics modified the consensus definition to include other types of compounds that may act as prebiotics, and also included prebiotics that could work outside of the digestive tract [9]. Even so, it is essential that that a prebiotic must *selectively* stimulate the *growth* of beneficial bacteria, and that there must be a health benefit arising from the consumption/ application of the prebiotic. Molecules such as antibiotics that modify the microbiome by acting as antimicrobial agents against undesirable bacteria would not be considered prebiotics, since they do not act as substrates for beneficial bacteria. Fructans such as inulin and fructooligosaccharides (FOS) are most common among prebiotics in the marketplace, although galactooligosaccharides (GOS) produced from lactose are also available, and xylooligosaccharides (XOS) were available in Japan since the 1980s [10]. Recently, forms of resistant starch (RS), isomaltooligosaccharides (IMOS), arabinoxylooligosaccharides (AXOS) and man-

nooligosaccharides (MOS) have become available commercially.

Fundamentally, these prebiotics are all materially different, even though the end goal – promoting growth of beneficial bacteria, is the same. The types of bacteria that can be fed by each of these prebiotics depend upon enzymes and transport systems present in the bacteria, which can vary considerably. The selectivity of a prebiotic is also tied to these enzyme and transport systems; if a high percentage of the bacteria have the necessary enzymes and transport systems, then the prebiotic will feed a diverse array of bacteria, including beneficial bacteria along with undesirable bacteria. Conversely, a highly selective prebiotic may not feed many types of bacteria, because fewer types of microbes have the right microbial "machinery"

*DOI: http://dx.doi.org/10.5772/intechopen.89484*

simple carbohydrates.

and side effects.

**and function**

nose, arabinose, and xylose [8].

**3. Background on prebiotics**

*Functional Attributes and Health Benefits of Novel Prebiotic Oligosaccharides Derived… DOI: http://dx.doi.org/10.5772/intechopen.89484*

the small intestine of four healthy subjects, and observed that *Bacteroidetes*, *Clostridium* (clusters XIVa, IV, XI, and IX) and *Proteobacteria* dominated, with some *Actinobacteria* and *Bacilli* also present. Zoetendal et al. [5] provides evidence that the microbiota, particularly *Streptococcus* species, in the small intestine adapts rapidly to changes in dietary intake, particularly carbohydrates, based upon the presence of transport systems and enzymes that quickly and efficiently utilize simple carbohydrates.

The colon, which survives on complex carbohydrates that are not digested or absorbed in the small intestine, contains about 1011 bacteria per gram of intestinal content, mainly *Prevotella, Ruminococcus*, and *Bacteroides*. Mucin degraders such as *Akkermansia mucinophila* are commonly found in the mucous layer. The GI tract, for various reasons, may also contain pathogenic bacteria at levels that may or may not be clinically significant. Furthermore, there is increasing recognition of the impact of the gut microbiota on the efficacy of drugs, since some microbes contain enzymes similar to those in the liver [6, 7], or may initiate breakdown of prodrugs. Such effects have been observed with metformin and L-DOPA, among other drugs, and may account for at least part of the interindividual variation in drug efficacy and side effects.

Most microbes evolved to process conventional 6 carbon sugars, particularly monomeric glucose, fructose and galactose. A smaller fraction is able to use mannose, arabinose, and xylose [8].
