probiotics, prebiotics and the gut microbiota - International Life ...

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14 Concise Monograph Seriesor if any safety concerns can be defined and excluded,the organism may be granted QPS status. Thus, forany strain of micro-organism that can be unequivocallydemonstrated to be from a qualified QPS group (suchas Lactobacillus or Bifidobacterium), further safetyassessment is limited to tests for antibiotic resistance. Ifa microbe is not covered by QPS, then a comprehensiveassessment of safety is likely to be required before it canbe used in the food supply.Application of probiotics in foodProbiotic organisms are used in a variety of foods, themain category being dairy products, but they are alsopresent as food supplements in capsule or tablet form.Since viability is an essential property of a probiotic, thefinal product must contain an adequate amount of livingprobiotic(s) until the end of its shelf life. A health claim forthe addition of probiotics to foods or food supplementsshould only be made if there are documented benefitsbased on good quality human trials conducted with therelevant food product containing the specific strain thatis the subject of the claim and using relevant endpoints.These studies should also be able to demonstrate thesafe, effective dose of the probiotic organism in food.Like legislation on food safety, regulation of health claimsfor foods varies by country or region and any claims oncommercial products containing probiotics must adhereto requirements, which in some cases include pre-marketapproval of the claim by the regulatory authorities.THE PREBIOTIC CONCEPTDefinition and historyAs mentioned, the Japanese were the first to recognise thevalue of fermentable oligosaccharides, initially in feedingpiglets and later, during the 1980s, with the identificationof human milk oligosaccharides. However, it was notuntil 1995 that the prebiotic concept for modulation ofgut microbiota was introduced. Although a number ofdefinitions have been proposed, there is as yet no fullagreement on a single definition of a prebiotic. The mostrecent was agreed at the 2010 Meeting of the InternationalScientific Association of Probiotics and Prebiotics (ISAPP)(Gibson et al., 2011):“A dietary prebiotic is a selectively fermented ingredientthat results in specific changes, in the compositionand/or activity of the gastrointestinal microbiota, thusconferring benefit(s) upon host health.”Characterisation of prebioticingredientsAlthough not stipulated as a requirement in the definitionof a prebiotic, to date only carbohydrate compoundshave been studied with regard to prebiotic activity.Most research has been carried out on fructans (i.e. thepolysaccharide inulin or fructo-oligosaccharides (FOS)derived from various crops or from sucrose) and galactooligosaccharides(GOS). For these ingredients, selectivefermentation and a shift in the microbiota have beenconfirmed in human studies and they have been linkedto potential health benefits. Candidate or emergingprebiotics require additional evidence in humans beforethey can be fully established as prebiotic. Such candidate
Probiotics, Prebiotics and the Gut Microbiota 15prebiotics include the disaccharide lactulose, furtheroligosaccharides and resistant dextrins, polysaccharidessuch as polydextrose, arabinoxylans and resistant starchesas well as some polyols such as lactitol and isomalt.Some prebiotics occur naturally in foods such as chicory,cereals, agave and milk. However, most foods containonly trace levels, so the approach of refining the activeingredients from these foods crops or of producingthem by synthesis (e.g. enzymatic, chemical or thermalprocesses) has been undertaken in order to attain levels infoods whereby a prebiotic effect may occur.Many prebiotics and candidate prebiotics today fall intothe nutritional and regulatory definition of dietary fibre andare labelled as nutrients of that category. They share withdietary fibre the properties of resistance to digestion and(for some fibres) fermentability, but established prebioticsare distinguished from dietary fibre by the selectivity oftheir fermentation. Note that mono- and disaccharides aretypically not considered as dietary fibre according to EUand CODEX definitions.Criteria for prebiotic selectionPrebiotics have an action complementary to, but distinctfrom, probiotics. Probiotics are exogenous micro-organismsthat are ingested to promote a specific health effect. Incontrast, the prebiotic concept is based on the selectivestimulation of the host’s own beneficial microbiota, theprebiotic being the substrate that is (selectively) fermented,stimulating the growth and activity of the particular microorganismor group of micro-organisms of interest and thusleading to the desired health effect.It is essential to measure the effect of the candidateprebiotic on bacterial growth; it is not enough simplyto know that fermentation has taken place. Although invitro tests can be used to screen potential candidates, theincrease in target microbes must be quantified in humantrials after a short feeding period at acceptable levels ofintake in order to establish prebiotic status. Furthermore,human feeding trials are essential in order to demonstratea health benefit.The main site of action for prebiotics is the colon. Thus,a prebiotic should resist the effects of gastric acidity anddigestive enzymes in order to reach the colon intact. Oncethere, prebiotics confer their purported benefits throughthe stimulation of the growth and/or the metabolicactivities of the bacteria that ferment them. The foremosttarget genera for prebiotic action are bifidobacteria andlactobacilli, although this may change as knowledge ofthe microbial diversity and functionality expands. It can,however, not be excluded that prebiotics have a directeffect on health e.g. through the immune system or animpact on binding of microbes to receptors.Prebiotics and probiotics may be combined into“synbiotics”. In this case, the effects of the two componentsshould be synergistic. The probiotic may be stimulated togrow in the gut by fermenting the prebiotic and/or theprebiotic may support a more favourable gut environmentin which the probiotic may better compete.Application of prebiotics in foodAs noted above some prebiotics or candidate prebioticsare naturally occurring and widely consumed at low levelsin the normal diet. The commercial prebiotic ingredientsGOS and fructans are used in infant foods when their safetyand efficacy has been demonstrated; in some countriesthis may require premarket approval. In foods for generalconsumption, the target level of intake of prebiotic rangesfrom 2 to 20 g per day, depending on the ingredientand the desired effect. These amounts can be readilyincorporated into a variety of foods such as cereals, bread,confectionery, biscuits, yoghurts, table spreads, saucesand drinks. Similarly to the case of probiotics, the healthbenefits of candidate prebiotics need to be demonstratedin clinical trials.
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