Prebiotics: Mechanisms, Sources, and Examples

Glenn Gibson and Marcel Roberfroid first introduced the concept of prebiotics in 1995. They stated that a “Prebiotic is 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 was later redefined in 2008 as “a selectively fermented ingredient that results in specific changes in the composition and/or activity of the gastrointestinal microbiota, thus conferring benefits upon host health.”

Prebiotics are included in a human diet containing different types of fruits, vegetables, and legumes, which are a source of plant polysaccharides and are fermented by microbes. The prebiotics food includes broccoli, carrots, Jerusalem, Psyllium seed husk, garlic, onions, asparagus, wheat, oat, rye, barley, tomatoes, berries, root tubers, and root vegetables(sweet potato, potato), and so on. Nowadays, prebiotics are produced commercially as a supplementary diet.

Mechanism of Action of Prebiotics

Human enzymes such as pancreatic and salivary amylase digest few carbohydrates (present in our diet) by breaking glycosidic linkages present in them, while those not digested act as a substrate for bacterial fermentation. These non-digestible substances include oligosaccharides and polysaccharides which are usually called prebiotics and can withstand digestion and absorption in the small intestine. However, these get fermented by the intestinal flora especially Bifidobacteria and Lactobacillus in the large intestine which positively impacts the gut. 

The enzymes produced by bacteria ferment complex polysaccharides and produce secondary metabolites, short-chain fatty acids (SCFAs) such as propionate, butyrate, acetate, and gases. These by-products encourage the activity and proliferation of good native bacteria in the gastrointestinal tract and reduce pathogenic bacteria. They also protect other body parts, such as the central nervous, cardiovascular, and immune systems.

Criteria for Eligibility as Prebiotic

For the food ingredients to be prebiotic, they need to fulfill the following criteria:

1. Resistant to hydrolysis and not absorbed in the upper part of the gastrointestinal tract
2. Selective substrate for one or few beneficial bacteria that is naturally present in the colon.
3. Beneficial to the host’s health and able to change the flora in order to make it healthier.
4. Encouraged to thrive or metabolically trigger induce luminal or systemic effects that are beneficial to host health.
5. For commercially produced, must have a good storage life at room temperatures such as heat, and dehydration.

Prebiotic Examples

In human diets, prebiotics exists in small concentration and includes oligosaccharide carbohydrates as well as non-carbohydrates that meet the above-mentioned criteria. The most widely known prebiotics includes inulin, galactooligosaccharides, and fructooligosaccharides while other include polyphenols, lactitol, xylooligosaccharides, isomaltooligosaccharides are emerging prebiotics. Some of the widely used prebiotics are:

Galacto-oligosaccharides (GOS)

GOS are composed of two to eight sugar units, two of which are galactose and two disaccharides (containing two galactose units), and one of the sugar units is terminal glucose. The chemical structures vary by branching, chain length, and glycosyl linkages. Food that contains galactooligosaccharides includes lentils, chickpeas, dairy products, beans, and so on. 

Biological activity: It enhances the production of short-chain fatty acids, reduces the count of potentially pathogenic bacteria, promotes normal function of the intestine, lipid and carbohydrate metabolism, and so on. It naturally occurs in breast milk and stimulates the intestinal Bifidobacteria and Lactobacilli in infants. Since it has a positive effect on immune function, it is commercially available and is added to dairy products such as yogurt and infant formulas.  

Inulin

It is a non-digestible water-soluble storage polysaccharide found in many plant-based foods. These are polymers consisting of a linear chain of β-2,1-linked d-fructofuranose molecules terminated at the reducing end by a sucrose-type linkage by a glucose residue. It is difficult for inulin to be absorbed and digested by the human small intestine due to the existence of β-()-D-frutosyl fructose bonds between the fructose unit and the isomeric carbon of inulin, but it can be fermented by the intestinal flora of the human large intestine. Some examples of food that contains inulin include Asparagus, Dandelion root, onions, chicory, burdock, leeks, and so on.  

Biological activity: It increases the apparent calcium and magnesium absorption, relieves constipation, manages blood sugar and blood lipid with diabetes type II in elderly people, suppresses the appetite reduces weight, and enhances gut microbiome such as Bifidiobacteria.

Fructooligosaccharides (FOS)

It is inulin-type fructans, which have low calories and are non-digestible carbohydrates. These are composed of linear chains of fructose units, linked by β-2,1 bond. Food that contains fructooligosaccharides: tomatoes, garlic, onions, wheat, rye, chicory root, seaweed, sugar cane, rice bran, papaya, beetroot peels, and so on. 

Biological activity: It enhances the body’s intestinal flora, lowers the risk of heart disease and certain cancers, relieves constipation, improves lipids in hyperlipidemia, stops the production of putrefactive substances in the gut, and promotes healthier digestion. They have a number of advantageous physiological effects, including less carcinogenicity, improved mineral absorption in the intestine, and decreased levels of triacylglycerols, phospholipids, and serum cholesterol.

Advantages of Prebiotics

Prebiotics has more positive effects on human health. Some of these advantages are mentioned below:

1. These affect mainly the gastrointestinal tract as well as distant organs such as the central nervous system, immune system, and cardiovascular system, as their degradation forms majorly SCFAs which enter the blood circulation by diffusing through gut enterocytes. 
2. The fermented product of prebiotics is mostly acids which decreases the pH of the gut from 6.5 to 5.5 that alters the population of gut microbes. Lactobacilli and Bifidobacteria decrease the harmful bacteria. As an example, the colonization of Salmonella in the epithelium is prevented due to the adhesion of mannose to Salmonella.
3. Inhibits the activities of spoilage bacteria like Clostridium spp., such as an increase in Bifidobacteria in the gut, harmful fermentation products are less likely to occur.
4. In the intestinal epithelium, butyric acid reduces the growth of lesions such as adenomas and carcinomas in the gut.
5. Gut microbiota affects the central nervous system activity through the “gut-brain axis”. Some prebiotics, like FOS and GOS, regulates synaptic proteins, neurotransmitters (such d-serine), and brain-derived neurotrophic factors. 

Limitations of Prebiotics

The intake of prebiotics also has a few limitations which are mentioned below:

1. Prebiotics may negatively impact lipid profiles by generating certain SCFAs, namely acetate. Acetyl-CoA, which is a substrate for the synthesis of fatty acids in hepatocytes, can be produced from acetate. This may explain why blood levels of cholesterol and triglycerides rose following rectal acetate infusion.
2. Prebiotics are fermented in the colon and have an osmotic effect in the intestinal lumen. They could cause bloating and flatulence.
3. High doses cause diarrhea and abdominal pain.
4. Large daily doses have lately been linked to a rise in gastroesophageal reflux.

References 

1. Davani-Davari, D., Negahdaripour, M., Karimzadeh, I., Seifan, M., Mohkam, M., Masoumi, S. J., … Ghasemi, Y. (2019). Prebiotics: Definition, types, sources, mechanisms, and clinical applications. MDPI, 8(3), 1–27. https://doi.org/10.3390/foods8030092
2. Gibson, G. R., & Roberfroid, M. B. (1995). Dietary modulation of the human colonic microbiota: Introducing the concept of prebiotics. Journal of Nutrition, 125(6), 1401–1412. https://doi.org/10.1093/jn/125.6.1401
3. Rahim, M. A., Saeed, F., Khalid, W., Hussain, M., & Anjum, F. M. (2021). Functional and nutraceutical properties of fructo-oligosaccharides derivatives: a review. International Journal of Food Properties, 24(1), 1588–1602. https://doi.org/10.1080/10942912.2021.1986520
4. You, S., Ma, Y., Yan, B., Pei, W., Wu, Q., Ding, C., & Huang, C. (2022). The promotion mechanism of prebiotics for probiotics: A review. Frontiers in Nutrition, 9(2), 1–22. https://doi.org/10.3389/fnut.2022.1000517                                                                                              
5. https://atlasbiomed.com/blog/inulin-prebiotic-fiber/
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