**3. Background on prebiotics**

*Prebiotics and Probiotics - Potential Benefits in Nutrition and Health*

impacts on human and animal health.

**absorption**

among other factors [1].

regulate satiety [3].

We thus start by describing the environment within the digestive tract – digestive enzymes, systems for carbohydrate absorption, and the microbial communities therein. We then discuss various types of prebiotics, with a particular emphasis on differences in subunits and bond structure. These differences, coupled with differences in microbial enzymes and transport systems, contribute to differences in efficacy, selectivity, and dose between prebiotics. We then focus on prebiotics derived from xylan, arabinan, and mannan, differentiating them from "conventional" prebiotics that rely on subunits of common 6-carbon sugars. Finally, we discuss results from clinical and animal trials with these novel prebiotics, discussing

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

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

**2.2 Microbial communities in the digestive tract**

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,

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

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

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

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 mannooligosaccharides (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"

to utilize these prebiotics. Such prebiotics are less likely to directly feed undesirable bacteria if these bacteria do not have the right transport systems and enzymes. Below, we describe the different chemical structures of prebiotics, along with the enzymes and transport systems responsible for their utilization.
