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 that colonize the large bowel by acting as substrate for them. They were first identified and named by Marcel Roberfroid in 1995. As a functional food component, prebiotics, like probiotics, are a conceptual intermediary between foods and drugs. Depending on the jurisdiction, they typically receive an intermediate level of regulatory scrutiny, in particular of the health claims made concerning them for marketing purposes.
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. 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.
- non-digestible and resistant to breakdown by stomach acid and enzymes in the human gastrointestinal tract
- selectively fermented by intestinal microorganisms
- selectively target and stimulate the growth and activity of beneficial bacteria
Thus, consumption of prebiotics must result in an improvement of the health of the host. Based on the previous classifications, plant-derived carbohydrate compounds called oligosaccharides 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 (GOS). Other dietary fibers also fit the definition of prebiotics, such as resistant starch, 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.
Most prebiotic research has focused on the effects that prebiotics confer on Bifidobacteria and Lactobacillus. 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 selectively to ferment prebiotic fiber based on the enzymes characteristic of the bacterial population. Thus, Lactobacilli prefers inulin and fructooligosaccharides, while Bifidobacteria displays specificity for inulin, fructooligosaccharides, xylooligosaccahrides and galactooligosaccharides. A product that stimulates bifidobacteria is described as a bifidogenic factor, a concept that overlaps, but is not identical with, being prebiotic. 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 metabolism to break down substrates. The bifidobacterial genome contains many genes that encode for carbohydrate-modifying enzymes as well as genes that encode for carbohydrate uptake proteins. The presence of these genes indicates that Bifidobacteria contain specific metabolic pathways specialized for the fermentation and metabolism of plant-derived oligosaccharides, or prebiotics. These pathways in Bifidobacteria ultimately produce short chain fatty acids, which have diverse physiological roles in body functions.
Prebiotic sources must be proven to confer a benefit to the host in order to be classified as a prebiotic. Fermentable carbohydrates derived from fructans and xylans are the most well documented example of prebiotics, and galactooligosaccharides are enzymatically synthesized from lactose. However, there are additional endogenous prebiotics and exogenous food sources that can be classified as prebiotic sources. Additionally, functional foods containing prebiotic food ingredients serve as an additional prebiotic food source. However, the FOS and inulin content in food sources is very low, meaning it is difficult to consume sufficient prebiotics from food alone.
An endogenous source of prebiotics in humans is human breast milk, which contains oligosaccharides structurally similar to GOS, referred to as human milk oligosaccharides (HMOs). These HMOs were found to increase the Bifidobacteria bacterial population in breastfed infants, and to strengthen the infant immune system. Furthermore, HMOs play a role in the establishment of a healthy intestinal microbiota composition of newborns.
Indigestible carbohydrate compounds classified as prebiotics are a type of fermentable fiber, and thus can be classified as dietary fiber. However, not all dietary fiber can be classified as a prebiotic source. In addition to the food sources highlighted in the following table, raw oats, unrefined barley, yacon, and whole grain breakfast cereals, are also classified as prebiotic sources.
|Top 10 Foods Containing Prebiotics|
|Food||Prebiotic Fiber Content by Weight|
|Raw, Dry Chicory Root||64.6%|
|Raw, Dry Jerusalem Artichoke||31.5%|
|Raw, Dry Dandelion Greens||24.3%|
|Raw, Dry Garlic||17.5%|
|Raw, Dry Leek||11.7%|
|Raw, Dry Onion||8.6%|
|Raw Wheat bran||5%|
|Whole Wheat flour, Cooked||4.8%|
While there is no broad consensus on an ideal daily serving of prebiotics, recommendations typically range from 4 to 8 grams (0.14–0.28 oz) for general digestive health support, to 15 grams (0.53 oz) or more for those with active digestive disorders. Given an average 6 grams (0.21 oz) serving, below are the amounts of prebiotic foods required to achieve a daily serving of prebiotic fiber:
|Food||Amount of food to achieve 6 g serving of fructans|
|Raw Chicory Root||9.3 g (0.33 oz)|
|Raw Jerusalem Artichoke||19 g (0.67 oz)|
|Raw Dandelion Greens||24.7 g (0.87 oz)|
|Raw Garlic||34.3 g (1.21 oz)|
|Raw Leek||51.3 g (1.81 oz)|
|Raw Onion||69.8 g (2.46 oz)|
|Cooked Onion||120 g (4.2 oz)|
|Raw Asparagus||120 g (4.2 oz)|
|Raw Wheat Bran||120 g (4.2 oz)|
|Whole Wheat Flour, Cooked||125 g (4.4 oz)|
|Raw Banana||600 g (1.3 lb)|
Functional food applications
The use of prebiotics, specifically GOS, as a fundamental ingredient in the creation of functional foods has been seen in the following food sources:
- fermented milks/yogurts
- sports/health drinks
- energy bars
- baby foods
- sugar-free candy/chewing gum
- breakfast cereals
- bread/baked goods
- meat products
- pet foods
Preliminary research has demonstrated potential effects on calcium and other mineral absorption, immune system effectiveness, bowel acidity, reduction of colorectal cancer risk, inflammatory bowel disease (Crohn's disease or ulcerative colitis) hypertension and defecation frequency. Prebiotics may be effective in decreasing the number of infectious episodes needing antibiotics and the total number of infections in children aged 0–24 months.
While research demonstrates that prebiotics lead to increased production of short-chain fatty acids (SCFA), more research is required to establish a direct causal connection. Prebiotics may be beneficial to inflammatory bowel disease or Crohn's disease through production of SCFA as nourishment for colonic walls, and mitigation of ulcerative colitis symptoms.
The immediate addition of substantial quantities of prebiotics to the diet may result in an increase in fermentation, leading to increased gas production, bloating or bowel movement. Production of SCFA and fermentation quality are reduced during long-term diets of low fiber intake. Until bacterial flora are gradually established to rehabilitate or restore intestinal bacteria, nutrient absorption may be impaired and colonic transit time temporarily increased with an immediate addition of higher prebiotic intake.
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