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The concept of synbiotic was described at first time about 25 years ago, and it involved the combination of probiotics and non-digestible ingredients that could be selectively fermented (prebiotics). Thus, synbiotics were reported as mixtures of “probiotics and prebiotics that beneficially affect the host” [9]. The term was composed by the Greek prefix “syn”, which means “together” and the suffix “biotic”, which means “belonging to life”. However, confusion about synbiotic products remained, in part, because the original definition “mixtures of probiotics and prebi-otics that beneficially affect the host, improving survival and implantation in the gastrointestinal tract, selectively stimulating the growth and/or activation of the metabolism of one or a limited number of health-promoting bacteria, and thus improving the host’s well-being” lacked precision. Furthermore, the inclusion of other agents in the ‘–biotics’ category, including post-biotic [75] and pharmabiotics [76], may further contribute to the confusion. In May 2019, a panel updated the definition of synbiotic to “a mixture containing living microorganisms and substrate(s) used selectively by host microorganisms that confer a benefit to the host’s health”. The panel reported that defining synbiotics simply as a mixture of probiotics and prebiotics could preclude the innovation of synbiot-ics that are produced to work cooperatively. Therefore, the requirement that each component must meet for evidence of its action and the dose established for probiotics and prebiotics individually could represent an obstacle. A complementary synbiotic could not be designed for its parts to work cooperatively and, therefore, must be composed of a probiotic plus a prebiotic (Figure 2.2). Thus, in a complementary synbiotic, the prebiotic and probiotic works independently to achieve at least one health effect [15]. On the other hand, a synergistic synbiotic is where the substrate is designed to be used selectively by the probiotic microorganisms administered together (Figure 2.3). The mixture of the selectively utilized substrate (prebiotic) and a live microorganism (probiotic) works together to achieve at least one health effect [15].

Consumption of a combination of prebiotics and probiotics appropriately selected can improve the beneficial effects of the compounds when used individually. Fructooligosaccharides and inulin can stimulate the growth of bifidobacterial in the intestine (bifidogenic effect) and suppress the activity of undesirable bacteria (Enterococcus faecalis, Escherichia coli, and Proteus genus). Consequently, there is a reduction in the pH values because the beneficial microorganisms produce acids, and an antagonist effect against harmful or pathogenic bacteria may be observed, with decreases in the concentration of toxic metabolites [77].

Figure 2.2 Mechanism of action of a complementary synbiotic.

Figure 2.3 Mechanism of action of a synergistic synbiotic.

The synergistic activity between prebiotics probiotics and was considered efficient because they could improve the implantation and survival of probiotics in the gastrointestinal tract. Some health effects of orally administered combinations of live microorganisms and a prebiotic substrate are reported in Figure 2.4 [15]. Randomized controlled trials have been reported including adults with overweight, metabolic diseases, obesity, non-alcoholic fatty liver disease, and type 2 diabetes mellitus [15]. In addition, the investigation of other outcomes also was conducted, such as surgical infections, irritable bowel syndrome, atopic dermatitis, chronic kidney disease, eradication of Helicobacter pylori infection, and management of gestational diabetes mellitus [15]. However, it is important to note that for a formulation to be called synbiotic, there needs to be evidence to prove the selective use by the host microbiota (complementary synbiotic) or by the live microorganism that is co-administered (synergistic synbiotic) [15].

Synbiotics have as the main benefit the increase in the permanence of probiotics in the gastrointestinal environment. Synbiotics can control infections, which is associated with the formation of compounds during the fermentation process in the large intestine by probiotic cultures [78]. They can improve the functionality of epithelial barriers and modify the ecosystem of bacteria [79]. Prebiotics can improve the microbiota of the large intestine, while probiotics can also act in the small intestine. In an ideal synbiotic product, a synergistic action is observed between beneficial microorganisms (probiotics) and the selected substrate (prebiotics), increasing the activity of the probiotic culture. The increases in the probiotic activity is associated with greater health benefits [80, 81].

Figure 2.4 Synbiotics health benefits tested with human trials. Adapted from Swanson et al. [15].