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

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20 Concise Monograph SeriesImpact on the GI tract specific toprobioticsLactose malabsorptionAs discussed in the section on “Bacterial fermentationand metabolism”, many micro-organisms fermentlactose, the sugar found in milk and products made frommilk. Although infants rely on lactose, which contributesas much as 10% of the energy in breast milk, manypopulations around the world have a high proportion ofadults who are unable to digest this sugar. In humans, andin fact in all mammals, expression of the enzyme lactaseis down-regulated in adulthood with the exception ofsome population groups, particularly those of Europeanorigin. Lactose intolerance is a condition in which thecolonic fermentation of undigested lactose results ingastrointestinal effects such as abdominal pain, bloating,borborygmi or laxation. There is evidence that the livebacteria of yogurt are able to compensate for the lackof endogenous lactase in the human gut by digestinglactose. The typical measure of improved lactosedigestion is a reduction in breath hydrogen excretion(breath hydrogen is usually raised when undigestedcarbohydrate reaches the colon and is fermented). Thisimproved digestibility reduces the symptoms related tolactose intolerance in some lactose malabsorbers.Impact on the immune responsesGerm-free animals have, as mentioned, anunderdeveloped immune system and GI epithelium,resulting in reduced resistance to infection comparedwith conventional animals. It is thus accepted thatcommensal organisms are vital for the maturation ofthe immune system. The potential for probiotics andprebiotics to impact immune responses and to reducethe risk of infections has been the subject of a number ofhuman studies (discussed below). Such results, combinedwith evidence from mechanistic studies showing changesin certain immune parameters, support the notion thatthe effect of probiotics and prebiotics on the immunesystem can translate into measurable health benefits, butdefinitive evidence is lacking.Gastrointestinal infectionThe small intestine is the main target of many GIinfections such as rotavirus, S. typhimurium and someE. coli types. As early as 1916, it was reported thatS. typhimurium was cleared from the GI tract of healthycarriers of the organism when members of the normalgut microbiota were introduced. Probiotics have longbeen associated with a purported ability to counteractpathogenic bacteria and so recently several potentiallybeneficial strains have been tested in controlled studies.The first-line of treatment for the symptoms of diarrhoeais oral rehydration – and no other dietary treatmentshould be substituted for this, especially in infants.However, in established conditions, some probioticscan be used as an adjunct under medical supervisionwhere appropriate. Certain probiotics seem to be mosteffective in improving symptoms when the diarrhoea isthe result of a viral (rather than bacterial) infection, if theyare used early in the course of the infection and are givenin sufficient amounts. In terms of reduced susceptibilityto infection, some studies have found decreases in therisk of infection in infants (mainly in developing countries)and in institutionalised or hospitalised elderly. Efficacyis clearly strain related, i.e. some strains are effectiveand others not. In addition, there is some evidence thatspecific probiotic strains, and some prebiotics, mayreduce the risk of traveller’s diarrhoea.Some antibiotics can significantly disrupt commensalbacteria, resulting in side effects such as antibioticassociateddiarrhoea (AAD). The estimated incidenceof AAD is as high as 25% for some antibiotics and thiscan lead to patients failing to complete the course of
Probiotics, Prebiotics and the Gut Microbiota 21treatment. There is evidence that specific probiotics canreduce the risk of AAD and, indeed, several meta-analysesconclude that there may be as much as a halving of therisk of AAD in adults or the elderly, although the effect isless consistent in children. The observed effects relate toa limited number of specific probiotic strains. In the caseof prebiotics, it has been shown that FOS administrationfollowing an antibiotic treatment reduced the reoccurrenceof AAD from more than 30% in the controlgroup to less than 10% in the prebiotic group. As this wasnot associated with a decrease in subjects testing positivefor C. difficile, this could suggest that the prebiotic had astabilising effect on the microbiota, supporting a return ofeubiosis.Clostridium difficile infection is a frequent cause ofdiarrhoea in institutionalised populations, for examplein hospitals and in long-term care homes. It is oftenassociated with antibiotic use but it can occur as a resultof other risk factors such as age greater than 65 yearsor a compromised immune system owing to illness,medication or GI surgery. So far, the results of researchto investigate whether probiotics can reduce the risk ofC. difficile infection or reduce the severity or duration ofsymptoms in adults are promising but further confirmatorystudies are needed.A bacterium known as Helicobacter pylori is present inthe stomach of a small proportion of young adults butin as many as 50% of those aged 60 years and over. Itcolonises the mucous layer next to the gastric epitheliumand in some people can cause acute gastritis (i.e. pain,bloating, nausea and vomiting) and can lead to chronicgastritis and peptic ulcers. Treatment involves longtermadministration of strong antibiotics and althoughprobiotics do not speed the eradication of H. pylori, somehave been shown in several studies to reduce the sideeffects of treatment and may result in less disturbance ofthe microbiota.The microbiota in pre-term infants is restricted anddiffers in composition from those in healthy, full-terminfants. In particular, potentially beneficial bifidobacteriaare not well established in the pre-term neonatal gut.The microbiota is further challenged by environmentalbacteria from the hospital milieu and the common useof antibiotics in pre-term infants, putting this populationat increased risk of necrotising enterocolitis (NEC).Although the use of probiotics is not yet established inclinical practice, several trials have shown that specificprobiotic strains can reduce the risk of NEC. Additionalstudies are needed to clarify the preferred strain and doserecommendations. Furthermore, the use of live microbesin such a susceptible population makes confirmation ofsafety for this use a prime objective.Other infectionsThere has been a number of studies on various age groupsto investigate the potential for probiotics to impact thesusceptibility to upper respiratory tract infection (URTI)and its duration and symptoms. Studies were conductedwith a range of different strains; some strains reducedincidence, some reduced duration and most had effectson symptoms. The evidence is promising, but the rangeof strains and the variation in age groups and studydesign prevent any firm conclusions. Evidence for aneffect of prebiotics is limited to a recent, large, long-termstudy in which infants consuming formula supplementedwith a specific GOS/long-chain FOS combination wereless prone to upper URTI and associated fever than werethose infants fed formula without a prebiotic.There has also been interest in the use of probiotics inurogenital medicine. Certain probiotic strains have beenshown to improve recovery from bacterial vaginosisduring antibiotic treatment. Potential mechanisms for theeffect include anti-microbial antagonism, restoration ofbalanced microbiota or an enhanced immune response.
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