Prebiotics were first identified in the mid 1990’s. Scientists narrowed the meaning of the term to “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.” However, this wasn’t a narrow enough definition. In order to truly nourish the beneficial microbes in our colon, the prebiotic needed to have three major characteristics. First, it needed to withstand the chemical conditions in gut, namely its high acidity. Second, the bacteria in our colon needed to be able to ferment it. Third, it needed to markedly benefit a selective set of bacteria; namely those which were most beneficial to human health. The scientist who first classified prebiotics has stated that only two substances properly conform to this definition: namely inulin and oligofructose. Galacto-oligosaccharides, fructooligosaccharides are also widely accepted as prebiotics.
These two substances occur in high concentrations in many plants. According to a study conducted in 1999, the average American diet provided an average of 2.6 grams of inulin, and 2.5 grams of oligofructose a day. The main source of prebiotics was in wheat, which provided about 70% of average inulin intake, with onions accounting for an additional 25% of prebiotic intake. There is no consensus about how much a prebiotic material a healthy adult should eat each day, and in fact standardized ‘dosages’ are often frowned upon because of each person’s unique digestive tract, which can encourage or stunt the use of prebiotics.
Theoretically speaking, a healthy community of symbiotic microbes in a host’s gut could confer a number of benefits. In general, prebiotics have been shown to effectively nourish bifidobacteria and lactobacilli across a number of species. These in turn have the potential to aid in a variety of healthy processes, such as absorbing minerals, supporting immune function, alleviating symptoms of bowel diseases and a reduced risk of colon cancer.
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. Research shows that 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.