INTRODUCTION OF PREBIOTICS

I.                  BACKGROUND

     A complex community of microorganisms inhabits the mammalian gastrointestinal tract from mouth to anus, but the colon is, by far, the main site of this microbial colonization. As a consequence, progress in biology, physiology, and nutrition has considerably broadened our view of the function and the pathophysiological roles of the intestine, especially the large bowel. This organ is no longer viewed solely as a storage vessel that produces feces and eventually absorbs water and a few other simple molecules of both nutritive and endogenous origin. Indeed, recent researchs has convincingly shown that the large bowel and its microbiota form a strong symbiotic association and interact with each other to play major roles not only in colonic function but also in whole body physiology including endocrine activities, immunity, and even brain function.

Bifidobacteria and Lactobacillus live in great concentrations at the lower region of the small intestine and predominantly in the large intestine. These beneficial bacteria take a role as guards against harmful microbes living in the large intestine, keeping them from invading the small intestine. The small intestine functions to digest and absorb the majority of nutrients. When the body's immunity declines the risk of infection increases, it is essential to keep the immune capacity high for preventing or decreasing infection. Bifidobacteria and Lactobacillus have recently been shown to increase gut and overall immunity.

There are several aspects of an individual's health and diet that can adversely affect the gut microflora. High levels of fat in the diet can negatively affect the level of Bifidobacteria in the gut, as these beneficial bacteria are sensitive to increased levels of faecal bile acids, which are directly related to the amount of fat in the diet. Fatty diets may increase the amounts of bile acids in the faeces, and consequently increase their inhibitory effect to Bifidobacteria. The nature of the diet may also indirectly influence the gut conditions, which affect the activity of Bifidobacteria. Disorders of gastric function or intestinal motility may disturb the normal microbial balance.

Numerous studies have shown that an imbalance of friendly to unfriendly gut bacteria (too few friendly bacteria) can cause or aggravate various health conditions. Moreover, supplements aimed at increasing the number of friendly bacteria have been shown to help combat many types of diarrhea, irritable bowel syndrome, eczema, ulcerative colitis; reduce the incidence of canker sores and vaginal yeast infections; and exert positive effects on the immune system. Friendly gut bacteria consist of Lactobacillus acidophilus, L. bulgaricus,L. reuteri, L. plantarum, L. casei, B. bifidus, S. salivarius, S. thermophilus and the yeast Saccharomyces boulardi.

II. OBJECTIVES

The main objective of this paper is to showcase :

1.      Brief introduction of prebiotics

2.      Health benefits of prebiotics

3.      Prebiotics in food applications

III. INTRODUCTION

     The term “prebiotic” was coined in 1995 by Gibson and Roberfroid,although prebiotics were recognized as early as the 1950s when György and coworkers described “bifidus factor”, a bifidogenic substance that selectively promoted the growth of bifidobacteria (called Lactobacillus bifidus at that time). Human milk and colostrums were found to contain large amounts of “bifidus factor”. In the 1970s and ’80s, Japanese investigators pioneered the use of digestion-resistant saccharides to favorably modify the intestinal microbiota using fructooligosaccharides, galactooligosaccharides, and lactulose The 1980s and ’90s saw a marked increase in the use of probiotics to favorably modify the intestinal microbiota and a concomitant growth in interest in using prebiotics to achieve the same goal. In contrast to the probiotic strategy for microflora modification by providing living microorganisms, the prebiotic strategy seeks to stimulate the growth and/or enhance the metabolic activity of the healthful bacteria already colonizing the intestines. Prebiotics offer the ability to enhance the healthful strains in a person’s unique community of bacteria including beneficial strains not available as probiotics, such as Eubacterium species.

In 1995, Gibson and Roberfroid defined a prebiotic as a “nondigestible 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, and thus improves host health.” This definition only considers microbial changes in the human colonic ecosystem.Later, it was considered timely to extrapolate this into other areas that may benefit from a selective targeting of particular microorganisms and to propose a refined definition of a prebiotic as (Gibson et al.2004): “a selectively fermented ingredient that allows specific changes, both in the composition and/or activity in the gastrointestinal microflora that confers benefits”

By definition, a prebiotic classifies as a specific colonic nutrient. Most potential prebiotics are carbohydrates, but the definition does not exclude non-carbohydrates to be used as a prebiotic. Accordingly, the key characteristics that serve as criteria for classification of a compound as prebiotics are resistance to digestive processes in the upper part of the gastro intestinal tract and selective fermentation by one or a limited number of the microorganisms in the intestinal microbiota, especially the colonic microbiota, thus giving these a proliferation advantage and consequently modifying the microbiota composition.

In order for a food ingredient to be classified as a prebiotic , it must :

 1) be neither hydrolyzed nor absorbed in the upper part of the gastrointestinal tract.

 2) be a selective substrate for one or a limited number of beneficial bacteria commensal to the colon which are stimulated to grow and/or are metabolically activated.

3) consequently, be able to alter the colonie flora in favor of a healthier composition.

4) induce luminal or systemic effects that are beneficial to the host health.

Prebiotics can be classified as a type of digestion-resistant carbohydrate or dietary fiber. Like all fibers, prebiotics resist breakdown by human digestive secretions and arrive relatively unchanged in the lower regions of the intestinal tract where they can be utilized as an energy source by the resident microflora. What distinguishes prebiotics from other fibers is that prebiotics by definition selectively stimulate the growth of only beneficial microfloral organisms such as lactobacilli and bifidobacteria. A number of important dietary fibers like cellulose and pectin fail to meet this definition. Prebiotic properties have been ascribed to many types of carbohydrates, but they have been best documented for digestion-resistant oligosaccharides (DGOs).

DGOs include

- inulin-type fructans

- galactooligosaccharides

-lactulose

-isomaltooligosaccharides

-xylooligosaccharides

-soy-oligosaccharides

-gentiooligosaccharide

-nigeroligosaccharides (Stephen Olmstead, MD, David Wolfson)

Most prebiotics and prebiotic candidates identified today are nondigestible oligosaccharides. They are obtained either by extraction from plants (e.g.,chicory inulin), possibly followed by an enzymatic hydrolysis (e.g., oligofructose from inulin) or by synthesis (by trans-glycosylation reactions) from mono- or disaccharides such as sucrose (fructooligosaccharides) or lactose (trans-galactosylated oligosaccharides or galactooligosaccharides) .

IV.COMMONLY USED PREBIOTICS

Inulin-type Fructans:

The prebiotic effects of inulin-type fructans are well-established. Inulin-type fructans are linear DGOs composed of fructose moieties linked by β(2à1) bonds. Inulin is arbitrarily defined as a mixture of oligosaccharides with chain lengths of 2-60 fructose molecules, with or without an initial glucose. Inulin-type fructans are storage carbohydrates commonly found in wheat, onions,asparagus, bananas, garlic, artichokes, and leeks. Inulin-type fructans have their greatest effect on intestinal Bifidobacterium populations

Galactooligosaccharides:

Galactooligosaccharides are digestion-resistant oligosaccharides naturally found in both human and cow’s milk. They can also be derived from specific microbial fermentation of lactose or synthesized using the enzyme β-galactosidase and lactose syrup. Galactooligosaccharides selectively augment Bifidobacterium and Lactobacillus numbers within the human intestinal microbiota. Prebiotic applications of galactooligosaccharides are of great interest because of their natural occurrence in human milk. Administration of galactooligosaccharides to formula-fed infants has been shown to engender an intestinal flora similar to that of breast-fed infants

Lactulose:

Lactulose is synthetic galacto-fructose made by the isomerization of lactose. Although technically a disaccharide, lactulose is generally grouped together with the DGOs. Lactulose is not usually present in nature although very small amounts may be found in heat-treated milk products as a result of non-catalyzed isomerization.. The β(1à4) bond in lactulose cannot be split by human intestinal enzymes and is preferentially metabolized by colonic lactic acid bacteria with lactate and short-chain fatty acids as major end products with with significant increases in Bifidobacterium and Lactobacillus numbers and reductions in Clostridium perfringens, Bacteroides, Enterobacteriaceae, and Streptococcus populations

Fig: Different prebiotics