Dietary Interventions and Irritable Bowel Syndrome – What Really Works?

Teodora Surdea-Blaga; Anamaria Cozma-Petrut; Dan Lucian Dumitraşcu


Curr Opin Gastroenterol. 2021;37(2):152-157. 

In This Article

Interventions That Modulate Gut Microbiota

Gut dysbiosis plays a role in the pathogenesis of IBS as suggested by the risk of developing IBS following bacterial, viral and parasitic infections, the description of an altered gut microbiota or bacterial overgrowth in IBS and the association of IBS severity with unique microbiota signatures.[35] Moreover, reports showed that IBS patients who did not respond to LFD or a first-line IBS diet suffered from severe dysbiosis.[5] Such clinical observations have promoted interest in the use of probiotics, prebiotics and synbiotics to improve IBS symptoms.


Probiotics are 'live microorganisms that, when administered in adequate amounts, confer a health benefit on the host'.[36] Probiotics may alleviate IBS symptoms by targeting gut motility and visceral hypersensitivity, through modulation of the gut microbiota and its metabolic activity, as well as interactions with the intestinal immune system, central and enteric nervous systems.[32]

Numerous RCTs evaluated the efficacy of probiotics in IBS. Various single or multistrain formulations, containing especially Bifidobacteria (B.) and Lactobacilli (L.), in different doses and treatment durations were used. Meta-analyses have demonstrated that probiotics may be effective and safe to reduce global IBS symptoms and individual symptoms (e.g., abdominal pain, bloating, flatulence), but the inter-trial heterogeneity did not allow for any definitive conclusion to be drawn with regard to optimal strain or species.[37–39] Concerning the optimal dose of probiotics, a network meta-analysis assessing the efficacy of different probiotic protocols in IBS, suggested that 109–1010 CFU/day may be a reference range.[40]

Several meta-analyses emphasized that the effects of probiotics are highly strain specific[41] and that multistrain formulations are better than single-strain formulations in IBS.[37,38,40] In contrast, one meta-analysis suggested that single-strain probiotics, rather than combinations, are more efficient.[39] In fact, recent RCTs reported improvement in IBS symptoms with single-strain formulations, such as L. acidophilus DDS-1, B. lactis UABla-12[42] and Saccharomyces cerevisiae CNCM I-3856.[43]

Nonviable bacterial strains (e.g., heat-inactivated B. bifidum MIMBb75) have been recently studied and showed significant improvement in overall symptom response and individual IBS symptoms. Nonviable bacteria might have theoretical advantages over viable strains in terms of reduced risk of probiotic-related sepsis and longer shelf-life.[44]

Furthermore, gut microbiome differences between IBS subtypes were observed by next-generation sequencing. There is potential to advance metagenome-guided interventions aimed at personalized microbial therapy.[45] Nevertheless, probiotic therapy does not appear to permanently restore the microbiota, probiotics requiring repeated administrations for long term benefit, because symptoms quickly return once treatment stopped.[35] Therefore, probiotic therapy in IBS, although promising, is hampered by limited knowledge in terms of which probiotic to use (species, strains, individual or combination administration), as well as dose and duration of treatment. More high-quality RCTs of robust design are warranted to better define these aspects.

Given the relatively low level of evidence, the recent American Gastroenterological Association guidelines on probiotics[46] advised against the use of probiotics in IBS, expect for research purposes, unlike the recommendations of other working groups in previous years.[33,47]


Prebiotics are typically nondigestible polysaccharides, 'selectively utilized by host microorganisms conferring a health benefit'.[36] Mechanistic rationale for using prebiotics in IBS includes their potential bifidogenic effect and the putative anti-inflammatory and immunomodulatory properties of fermentation metabolites, such as SCFA.[12] Despite these aspects, a recent meta-analysis indicated that although prebiotics increased bifidobacteria, they did not improve gastrointestinal symptoms or QoL in patients with functional bowel disorders.[48] The same study reported that prebiotic doses lower than 6 g/day improved flatulence, but higher doses did not. Flatulence was significantly worsened with inulin-type fructans [e.g., inulin, oligofructose, fructo-oligosaccharides (FOS)], but was improved with noninulin-type fructans [e.g., galactooligosaccharides (GOS), guar gum].[48] These results emphasize the importance of considering both the type and the dose of prebiotics in future clinical research. Interesting findings are provided by a recent RCT that found improvements in global IBS symptoms following combined LFD and β-GOS supplementation. However, β-GOS did not prevent the decrease in bifidobacteria resulting from a LFD.[49] In contrast, a recent high-quality study found equal efficacy for a prebiotic and LFD in terms of symptoms improvement with microbiota changes occurring with the LFD alone.[50] To conclude, the evidence for the use of prebiotics in IBS is still limited and further studies in this domain are warranted.


Probiotics and prebiotics may also be combined as synbiotics, which act synergistically or complementarily to promote the survival and activity of beneficial organisms in the gut.[36] Few RCTs have explored the therapeutic potential of synbiotics in IBS. One trial in patients with IBS-D investigated a synbiotic containing two strains of Lactobacillus, three strains of Bifidobacterium, and a short-chain FOS, and found improvements in the severity scores of flatulence, bowel habits and global IBS symptoms.[51] Another RCT showed that adding Bacillus coagulans spores and inulin to LFD was superior to LFD alone in alleviating IBS symptoms.[52] Likewise, in IBS-C patients, a synbiotic containing L. helveticus and polydextrose, alleviated constipation-related symptoms and reduced intestinal transit time. However, the effectiveness of the synbiotic was comparable to the probiotic alone.[53] To conclude, the evidence for the use of synbiotics in IBS is still limited and further well designed research is required.