Prebiotics

Prebiotics are compounds in food that induce the growth or activity of beneficial microorganisms such as bacteria and fungi. The most common example is in the gastrointestinal tract, where prebiotics can alter the composition of organisms in the gut microbiome.

Dietary prebiotics are typically nondigestible fiber compounds that pass undigested through the upper part of the gastrointestinal tract and stimulate the growth or activity of advantageous bacteria in the colon by acting as substrates for them. They were first identified and named by Marcel Roberfroid in 1995. Depending on the jurisdiction, they may have regulatory scrutiny as food additives for the health claims made for marketing purposes. Common prebiotics used in food manufacturing include beta-glucan from oats and inulin from chicory root.

Definition

The definition of prebiotics and the food ingredients that can fall under this classification, has evolved since its first definition in 1995. In its earliest definition, the term prebiotics was used to refer to non-digestible food ingredients that were beneficial to the host through their selective stimulation of specific bacteria within the colon. Further research has suggested that selective stimulation has not been scientifically demonstrated. As a result of research suggesting that prebiotics could impact microorganisms outside of the colon, in 2016 the International Scientific Association for Probiotics and Prebiotics (ISAPP) produced the following definition of prebiotics: a substrate that is selectively used by a host microorganism to produce a health benefit. In 2021, The Global Prebiotic Association (GPA) defined a prebiotic as a product or ingredient that is utilized in the microbiota producing a health or performance benefit.

Compounds that can be classified as prebiotics must also meet the following criteria:

* non-digestible and resistant to breakdown by stomach acid and enzymes in the human gastrointestinal tract
* fermented by microorganisms on or in the body
* stimulating the growth and activity of beneficial bacteria

Thus, consumption of prebiotics may facilitate the health of the host. Based on the previous classifications, plant-derived carbohydrate compounds called oligosaccharides as well as resistant starch are the main source of prebiotics that have been identified. Specifically, fructans and galactans are two oligosaccharide sources which have been found to stimulate the activity and growth of beneficial bacterial colonies in the gut. Fructans are a category of carbohydrate consisting of fructooligosaccharides (FOS) and inulins, while galactans consist of galactooligosaccharides. Resistant starch has been shown to shift the intestinal bacteria, as well as improve biomarkers for numerous health conditions. Other dietary fibers also fit the definition of prebiotics, such as pectin, beta-glucans, and xylooligosaccharides.

The European Food Safety Authority (EFSA), the regulatory agency for product labeling, differentiates between "prebiotic" and "dietary fiber", stating that "a cause and effect relationship has not been established between the consumption of the food constituents which are the subject of the health claims and a beneficial physiological effect related to increasing numbers of gastrointestinal microbiota". Consequently, under EFSA rules individual ingredients cannot be labeled as prebiotics, but only as dietary fiber and with no implication of health benefits.

Function

When the prebiotic concept was first introduced in 1995, the primary focus was on the effects that prebiotics confer on ''Bifidobacteria'' and ''Lactobacillus.'' With improved mechanistic techniques in recent years, the current prebiotic targets have expanded to a wider range of microbes, including ''Roseburia'' spp., ''Eubacterium'' spp., ''Akkermansia'' spp., ''Christensenella'' spp., ''Propionibacterium'' spp. and ''Faecalibacterium'' spp. These bacteria have been highlighted as key probiotics and beneficial gut bacteria as they may have several beneficial effects on the host in terms of improving digestion (including but not limited to enhancing mineral absorption) and the effectiveness and intrinsic strength of the immune system. Both ''Bifidobacteria'' and ''Lactobacillus'' have been shown to have differing prebiotic specificity and to selectively ferment prebiotic fiber based on the enzymes characteristic of the bacterial population. Thus, ''Lactobacilli'' prefer inulin and fructooligosaccharides, while ''Bifidobacteria'' display specificity for inulin, fructooligosaccharides, xylooligosaccharides and galactooligosaccharides. Studies have also shown that prebiotics, besides stimulating the growth of beneficial gut bacteria, can also inhibit the growth of detrimental and potentially pathogenic microbes in the gut, such as clostridia.

Mechanism of action

Fermentation is the main mechanism of action by which prebiotics are used by beneficial bacteria in the colon. Both ''Bifidobacteria'' and ''Lactobacillus'' are bacterial populations which use saccharolytic