Photo by http://benvandenbroecke.be/ Copyright, ISAPP 2019.
The concept of a ‘prebiotic’ was put forward in 1995 by Gibson and Roberfroid in their scientific publication called “Dietary modulation of the human colonic microbiota: Introducing the concept of prebiotics”. In the decades that followed, scientific discussions about prebiotics tended to focus on identifying substrates that target health-promoting groups of bacteria in the gut: usually, bifidobacteria and lactobacilli.
The current scientific definition of a prebiotic was developed by a panel of experts in microbiology, nutrition, and clinical research convened by ISAPP in 2016. This current consensus definition is: “a substrate that is selectively utilized by host microorganisms conferring a health benefit”. Thus, the concept includes three essential parts: a substance, a physiologically beneficial effect, and a mechanism.
Prebiotics are frequently equated with dietary fibers, but only a subset of dietary fibers actually qualify as prebiotics (and, according to the broad scientific definition, prebiotics need not be forms of dietary fiber). According to the scientific consensus definition, any candidate prebiotic compound must be shown to act as a substrate for health-promoting microorganisms in the gut or elsewhere; in addition, its beneficial physiological effect on the host should depend (at least in part) on utilization of the compound by microbes. This means a broad range of genera, species, or strains may be targeted by a prebiotic substrate—not just bifidobacteria and lactobacilli. But to qualify under this definition, the prebiotic substance must affect a limited group of microorganisms in the host rather than the entire microbial ecosystem, in order to meet the criterion of being ‘selectively utilized’.
Prebiotics may be present naturally or in synthesized forms. The majority of research into their health effects has focused on isolated substances (to allow for tighter control of substance and dose), and less on prebiotics as part of whole plant foods.
Prebiotics have been studied for certain health effects: for example, reduction of infections, influences on cardiometabolic health, mineral availability, and immune modulation. The most commonly-studied prebiotics are the soluble fibers inulin, fructooligosaccharides (FOS), galactooligosaccharides (GOS), and more recently human milk oligosaccharides (HMOs). To date, while the prebiotic substances with the most well-documented health effects to date are fermentable carbohydrates, the scientific consensus definition allows for a broad range of substances targeting different host niches (such as the mouth, skin, or urogenital tract) to be considered prebiotics, given appropriate scientific support.
See these reviews (here and here) for summaries of prebiotic health benefits. At present, there are no official dietary recommendations for ‘adequate intake’ or ‘recommended daily allowance’ for prebiotics in healthy individuals. Most prebiotics for the gut require an oral dose of 3 grams per day or more to confer a benefit. Typically, around 5 grams is the target for FOS and GOS in the daily diet—and this includes plant sources of prebiotics
The recommended daily amount of fiber is 28 g/day, based on 2000 kcal/day diet.