Prebiotics are compounds in food that foster growth or activity of beneficial microorganisms such as bacteria and fungi. [1] The most common environment concerning their effects on human health is 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 help growth or activity of advantageous bacteria in the colon by acting as substrates for them. [1] They were first identified and named by Marcel Roberfroid in 1995. [1] [2] 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, resistant starch from grains and beans, and inulin from chicory root.
The definition of prebiotics and the food ingredients that can fall under this classification, has evolved since its first definition in 1995. [3] 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. [3] [4] Further research has suggested that selective stimulation has not been scientifically demonstrated. [5] 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. [3] 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. [6]
Compounds that can be classified as prebiotics must also meet the following criteria: [3] [4] [6]
Thus, consumption of prebiotics may facilitate the health of the host. [7] 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. [8] [4] [9] [10] 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. [7] [3] Fructans are a category of carbohydrate consisting of fructooligosaccharides (FOS) and inulins, while galactans consist of galactooligosaccharides. [3] Resistant starch has been shown to shift the intestinal bacteria, as well as improve biomarkers for numerous health conditions. [11] [12] [13] Other dietary fibers also fit the definition of prebiotics, such as pectin, [14] beta-glucans, [15] and xylooligosaccharides. [16]
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". [17] Consequently, under EFSA rules individual ingredients cannot be labeled as prebiotics, but only as dietary fiber and with no implication of health benefits. [17]
When the prebiotic concept was first introduced in 1995, the primary focus was on the effects that prebiotics confer on Bifidobacteria and Lactobacillus. [3] [4] [18] 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. [19] 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) [20] and the effectiveness and intrinsic strength of the immune system. [21] 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. [22] Thus, Lactobacilli prefer inulin and fructooligosaccharides, while Bifidobacteria display specificity for inulin, fructooligosaccharides, xylooligosaccharides and galactooligosaccharides. [22] Studies have also shown that prebiotics, besides helping growth of beneficial gut bacteria, can also inhibit detrimental and potentially pathogenic microbes in the gut, [9] [4] such as clostridia. [4]
Fermentation is the main mechanism of action by which prebiotics are used by beneficial bacteria in the colon. [7] [4] Both Bifidobacteria and Lactobacillus are bacterial populations which use saccharolytic metabolism to break down substrates. [4] 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, [4] [7] which have diverse physiological roles in body functions. [23] [3]
Prebiotic sources must be proven to confer a benefit to the host in order to be classified as a prebiotic. [3] Fermentable carbohydrates derived from fructans and xylans are one well documented example of prebiotics. [3] Resistant starch from starchy foods are also well documented prebiotics and have historically been the highest source of prebiotics in the diet, as 4-10% of starch in mixed diets has been shown to reach the large intestine. [24] One study reported that individuals consuming a traditional diet in Africa consumed 38 grams of resistant starch/day. [25]
An endogenous source of prebiotics in humans is human breast milk, which contains oligosaccharides structurally similar to galactooligosaccharides, referred to as human milk oligosaccharides. [26] [9] [22] [3] Human milk oligosaccharides were found to increase the Bifidobacteria bacterial population in breastfed infants, and to strengthen the infant immune system. [3] [9] Furthermore, human milk oligosaccharides help establish a healthy intestinal microbiota composition in newborns. [3]
Indigestible carbohydrate compounds classified as prebiotics are a type of fermentable fiber, and thus can be classified as dietary fiber. [4] However, not all dietary fiber can be classified as a prebiotic source. [4] In addition to the food sources highlighted in the following table, raw oats, [18] unrefined barley, [18] yacón, [18] and whole grain breakfast cereals [4] are also classified as prebiotic fiber sources. The predominant type of prebiotic fiber may vary according to the food. For instance, oats and barley have high amounts of beta-glucans, fruit and berries contain pectins, seeds contain gums, onions and Jerusalem artichokes are rich in inulin and oligofructose, and bananas and legumes contain resistant starch. [27]
Food | Prebiotic Fiber 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 Asparagus | 5% |
Raw Wheat bran | 5% |
Whole Wheat flour, Cooked | 4.8% |
Raw Banana | 1% |
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 |
---|---|
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) |
Preliminary research has demonstrated potential effects on calcium and other mineral absorption, [29] immune system effectiveness, [30] [31] bowel acidity, reduction of colorectal cancer risk, [32] inflammatory bowel disease (Crohn's disease or ulcerative colitis), [33] hypertension [34] and defecation frequency. [35] 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. [31] [36]
No good evidence shows that prebiotics are effective in preventing or treating allergies. [37]
While research demonstrates that prebiotics lead to increased production of short-chain fatty acids (SCFA), [38] 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. [39]
The sudden 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. [40] Production of SCFA and fermentation quality are reduced during long-term diets of low fiber intake. [41] Until bacterial flora are gradually established to rehabilitate or restore intestinal bacteria, nutrient absorption may be impaired and colonic transit time temporarily increased with a rapid addition of higher prebiotic intake. [40] [42]
Genetically modified plants have been created in research labs with upregulated inulin production. [43] [44]
Dietary fiber or roughage is the portion of plant-derived food that cannot be completely broken down by human digestive enzymes. Dietary fibers are diverse in chemical composition and can be grouped generally by their solubility, viscosity and fermentability which affect how fibers are processed in the body. Dietary fiber has two main subtypes: soluble fiber and insoluble fiber which are components of plant-based foods such as legumes, whole grains, cereals, vegetables, fruits, and nuts or seeds. A diet high in regular fiber consumption is generally associated with supporting health and lowering the risk of several diseases. Dietary fiber consists of non-starch polysaccharides and other plant components such as cellulose, resistant starch, resistant dextrins, inulins, lignins, chitins, pectins, beta-glucans, and oligosaccharides.
Inulins are a group of naturally occurring polysaccharides produced by many types of plants, industrially most often extracted from chicory. The inulins belong to a class of dietary fibers known as fructans. Inulin is used by some plants as a means of storing energy and is typically found in roots or rhizomes. Most plants that synthesize and store inulin do not store other forms of carbohydrate such as starch. In 2018, the United States Food and Drug Administration approved inulin as a dietary fiber ingredient used to improve the nutritional value of manufactured food products. Using inulin to measure kidney function is the "gold standard" for comparison with other means of estimating glomerular filtration rate.
An oligosaccharide is a saccharide polymer containing a small number of monosaccharides. Oligosaccharides can have many functions including cell recognition and cell adhesion.
Fructose malabsorption, formerly named dietary fructose intolerance (DFI), is a digestive disorder in which absorption of fructose is impaired by deficient fructose carriers in the small intestine's enterocytes. This results in an increased concentration of fructose. Intolerance to fructose was first identified and reported in 1956.
Fructooligosaccharides (FOS) also sometimes called oligofructose or oligofructan, are oligosaccharide fructans, used as an alternative sweetener. FOS exhibits sweetness levels between 30 and 50 percent of sugar in commercially prepared syrups. It occurs naturally, and its commercial use emerged in the 1980s in response to demand for healthier and calorie-reduced foods.
Resistant starch (RS) is starch, including its degradation products, that escapes from digestion in the small intestine of healthy individuals. Resistant starch occurs naturally in foods, but it can also be added as part of dried raw foods, or used as an additive in manufactured foods.
A fructan is a polymer of fructose molecules. Fructans with a short chain length are known as fructooligosaccharides. Fructans can be found in over 12% of the angiosperms including both monocots and dicots such as agave, artichokes, asparagus, leeks, garlic, onions, yacón, jícama, barley and wheat.
Natural growth promoters (NGPs) are feed additives for farm animals.
Synbiotics refer to food ingredients or dietary supplements combining probiotics and prebiotics in a form of synergism, hence synbiotics. The synbiotic concept was first introduced as "mixtures of probiotics and prebiotics that beneficially affect the host by improving the survival and implantation of live microbial dietary supplements in the gastrointestinal tract, by selectively stimulating the growth and/or by activating the metabolism of one or a limited number of health-promoting bacteria, thus improving host welfare". As of 2018, the research on this concept is preliminary, with no high-quality evidence from clinical research that such benefits exist.
Galactooligosaccharides (GOS), also known as oligogalactosyllactose, oligogalactose, oligolactose or transgalactooligosaccharides (TOS), belong to the group of prebiotics. Prebiotics are defined as non-digestible food ingredients that beneficially affect the host by stimulating the growth and/or activity of beneficial bacteria in the colon. GOS occurs in commercially available products such as food for both infants and adults.
Bifidobacterium is a genus of gram-positive, nonmotile, often branched anaerobic bacteria. They are ubiquitous inhabitants of the gastrointestinal tract though strains have been isolated from the vagina and mouth of mammals, including humans. Bifidobacteria are one of the major genera of bacteria that make up the gastrointestinal tract microbiota in mammals. Some bifidobacteria are used as probiotics.
FODMAPs or fermentable oligosaccharides, disaccharides, monosaccharides, and polyols are short-chain carbohydrates that are poorly absorbed in the small intestine and ferment in the colon. They include short-chain oligosaccharide polymers of fructose (fructans) and galactooligosaccharides, disaccharides (lactose), monosaccharides (fructose), and sugar alcohols (polyols), such as sorbitol, mannitol, xylitol, and maltitol. Most FODMAPs are naturally present in food and the human diet, but the polyols may be added artificially in commercially prepared foods and beverages.
Xylooligosaccharides (XOS) are polymers of the sugar xylose. They are produced from the xylan fraction in plant fiber. Their C5 structure is fundamentally different from other prebiotics, which are based upon C6 sugars. Xylooligosaccharides have been commercially available since the 1980s, originally produced by Suntory in Japan. They have more recently become more widely available commercially, as technologies have advanced and production costs have fallen. Some enzymes from yeast can exclusively convert xylan into only xylooligosaccharides-DP-3 to 7.
Isomaltooligosaccharide (IMO) is a mixture of short-chain carbohydrates which has a digestion-resistant property. IMO is found naturally in some foods, as well as being manufactured commercially. The raw material used for manufacturing IMO is starch, which is enzymatically converted into a mixture of isomaltooligosaccharides.
Microbiota-accessible carbohydrates (MACs) are carbohydrates that are resistant to digestion by a host's metabolism, and are made available for gut microbes, as prebiotics, to ferment or metabolize into beneficial compounds, such as short chain fatty acids. The term, ‘‘microbiota-accessible carbohydrate’’ contributes to a conceptual framework for investigating and discussing the amount of metabolic activity that a specific food or carbohydrate can contribute to a host's microbiota.
Bifidobacterium breve is a bacterial species of the genus Bifidobacterium which has probiotic properties. Bifidobacteria are a type of bacteria that live symbiotically in the intestines of humans. They have been used to treat a number of conditions including constipation, diarrhea, irritable bowel syndrome and even the cold and flu. Some of these uses have been backed up by scientific research, but others have not. B. breve is a gram positive, anaerobic, rod shaped organism that is non motile and forms branches with its neighbors.
Psychobiotics is a term used in preliminary research to refer to live bacteria that, when ingested in appropriate amounts, might confer a mental health benefit by affecting microbiota of the host organism. Whether bacteria might play a role in the gut-brain axis is under research. A 2020 literature review suggests that the consumption of psychobiotics could be considered as a viable option to restore mental health although lacking randomized controlled trials on clear mental health outcomes in humans.
The human milk microbiota, also known as human milk probiotics (HMP), encompasses the microbiota–the community of microorganisms–present within the human mammary glands and breast milk. Contrary to the traditional belief that human breast milk is sterile, advancements in both microbial culture and culture-independent methods have confirmed that human milk harbors diverse communities of bacteria. These communities are distinct in composition from other microbial populations found within the human body which constitute the human microbiome.
Kestose is a class of sugars that belongs to a group of fructooligosaccharides.