Table 2. A summary of observational and experimental studies describing various dietary treatments for the management of IBS symptoms.

IBS: irritable bowel syndrome; FODMAP: fermentable oligosaccharides, disaccharides, monosaccharides and polyols; IBS-SSS: IBS symptom severity scale; IBS-D: irritable bowel syndrome with diarrhea; FC: fecal calprotectin; GFD: gluten-free diet; NCGS: non-celiac gluten sensitivity; IBS-M: irritable bowel syndrome mixed type; IBS-U: irritable bowel syndrome-unsubtyped; IBS-C: irritable bowel syndrome with constipation; LFD/B-GOS: low fodmap diet/ β-Galactooligosaccharide; LFD: low fodmap diet; aMED: alternate mediterranean diet; MEDAS: mediterranean diet adherence screener; HC: health control; DHQ II: diet history questionnaire II; MD: mediterranean diet; TBD: tritordeum based diet; BIA: bioelectrical impedance analysis; QoL: quality of life; SCFAs: short-chain fatty acids; CNAQ: council of nutrition appetite questionnaire

Author	Type of Study	Sample Size	Inclusion criteria	Results of study	Study weakness
Naseri et al. [59]	Uncontrolled, open-label clinical trial study	30	A diagnosis of IBS according to Rome IV criteria, age 18- 59, absence of other functional gastrointestinal (GI) diseases, normal mucosa on biopsy and colonoscopy. Exclusion criteria: positive history of inflammatory bowel disease, celiac disease, diseases of the liver, GI surgery, cancer, use of non-steroidal anti-inflammatory drugs (NSAIDs), use of alcohol, use of immunosuppressive agents systemically, poorly controlled psychiatric diseases, presence of fissures, hemorrhoids, and microscopic colitis on biopsy specimens and colonoscopy. Usage of any drugs that affect bowel function four weeks before the study, such as broad-spectrum antibiotics, and probiotics.	Reduction in IBS-SSS in 73.3% of patients with 3.3% having >60% and 53.3% having 30-60% reduction, increased level of unfavorable gut microbiota Bacteroidetes (11.69%- 26.65%), reduction in Firmicutes (31.59%- 22.17%), increased relative abundance of Bifidobacterium and Lactobacillus at the genus level, significant reduction in Firmicutes to Bacteroidetes (F/B) ratio (2.6:1 – 0.8:1), reduction in the level of FC	Small sample size and patients predominantly being IBS-D
Saadati et al. [60]	Randomized single-blinded controlled clinical trial	50	Adults who meet Rome IV criteria, aged 18- 80 years. Exclusion criteria: the presence of psychological disorders, major abdominal surgery, diabetes mellitus, pregnancy, making dietary changes during the study, unwillingness to continue, following a GFD or low FODMAP diet in the past six months,	Significant reduction in pain severity and frequency. Significant reduction in the anxiety levels. Improvement of the quality of life.	Small number of patients, limited follow-up time
Barone et al. [62]	Randomized double-blind placebo-controlled crossover trial	30	A diagnosis of IBS according to Rome IV criteria, no serious symptoms, no use of any drugs for the abnormalities in bowel habits in the previous three months. Exclusion criteria: Use of a GFD in the previous six months, presence of any of the following diseases; celiac disease or wheat allergy, chronic intestinal inflammatory diseases, major abdominal surgery, psychiatric problems, diabetes mellitus, pregnancy or prior anaphylactic episodes	Significant improvement in the symptoms following a low FODMAP-GFD	Limited number of patients in the sample, difficulty to exclude patients with seronegative celiac disease among NCGS patients, use of Di Sabatino criteria in the evaluation which is not routinely applicable
Nordin et al. [63]	Double-blind placebo-controlled, randomized three-way crossover trial	103	Fulfilling IBS Rome IV criteria with moderate to severe IBS, BMI 18.5-38kg/m2, age18-70 years, hemoglobin 120-160g/L, transglutaminase immunoglobulin A <7 U/ml, C-reactive protein <5 mg/L, thyroid stimulating hormone <4 mU/L, systolic/ diastolic blood pressure ≤160/≤105 mmHg	Higher IBS-SSS in FODMAP group in comparison to the gluten and placebo groups, higher frequency of abdominal pain during FODMAPs intake	Exposures limited to seven days, low impact diets were provided as dietary advice instead of ready-made meals, adherence to the diet was assessed based on self-reporting
Staudacher et al. [57]	2×2 factorial, multicenter, randomized, placebo-controlled trial	104	IBS-D, IBS-M, IBS-U patients fulfilling Rome III criteria. IBS-C excluded. Age 18-65 years	Significant symptom relief following LFD (61%), a significant increase in the Bifidobacterium in fecal samples after supplementation with probiotic	Maintenance of blinding is difficult, excluded constipation-predominant IBS patients, difficulty to identify which dietary item (collective FODMAPs or one or individual FODMAP) cause the response, problem of collinearity, dietary changes might impact indirectly on microbiota composition by changing other physiological parameters like transit time, raised questions regarding usage of a dichotomous endpoint as a primary outcome
Rossi et al. [64]	2×2 factorial, multicenter, randomized, placebo-controlled trial	93	Fulfilling Rome III criteria, without any other significant medical problems	Significant change in IBS-SSS in LFD (80%) compared to Sham diet (45%), Similar response rate for both probiotic (63%) and placebo (61%)	A novel study without data prior data, need for a larger cohort to determine the validity of the study, limited information about the mechanisms linked to VOCs since it only determines VOC patterns, results could be unclear due to 2×2 factorial design of the study
Staudacher et al. [65]	2×2 factorial, blinded, randomized placebo-controlled trial	95	Age 18-65 years, diagnosed with IBS based on Rome III criteria	Increased abundance of Lactobacillus and Streptococcus with probiotic supplementation, mitigation of the effect of low FODMAP on Bifidobacterium	Collection of dietary data using food records, cut off for inclusion of operational taxonomic units for analysis together with adjustments for multiple comparisons could result in a type 2 error and mask true diet-microbiota relationships
Wilson et al. [66]	Randomized placebo-controlled three-arm trial	69	Fulfilling Rome III criteria, age 18-65 years	Significant symptom relief of 67% in LFD/B-GOS group, lowering of Bifidobacterium concentration, actinobacteria proportion and fecal butyrate in LFD/B-GOS group	Unclear effect of prebiotic, low dose of prebiotic in the LFD/B-GOS group to increase the Bifidobacteria
Abhari et al. [67]	Randomized controlled trial	50	Fulfilling Rome IV criteria	Significant improvement in IBS-SSS in LFD/ Bacillus coagulans group, higher frequency of patients with improved IBS-SSS in the same group	Presence of inulin in probiotic capsules which is a FODMAP
Al-Biltagi et al. [72].	Prospective cross-sectional case-controlled study	100	Children and adolescents diagnosed with IBS according to the Rome IV criteria, aged between 12-18 years	Mediterranean diet was found to be safe and well-tolerated among IBS patients. In the group following the Mediterranean diet, the IBS-SSS score decreased from 237.2 ± 65  to  163.2 ± 33.8, Mean IBS-QoL improved from 57.3 ± 12.9 to 72.4 ± 11.2 and the mean total IBS score increased from 24.1 ± 10.4 to 28.8 ± 11.2 by the end of the study. However, no significant improvement was seen in the group following the regular diet.	It is a cross-sectional study therefore it is unable to conclude the causality and the same was from a single center therefore cannot generalize the study results
Zito et al. [73].	Prospective study	1134	Age between 17-83 years	The study found significantly low adherence to a Mediterranean diet in the groups with IBS and FD symptoms (0.57 ± 0.23, and 0.56 ± 0.24 respectively) when compared to the control group (0.62 ± 0.21). Females had significantly low adherence scores to a Mediterranean diet in both IBS and FD groups, while for males it was only significantly low in the FD group. The study also showed that the adherence score was significantly low in the age groups 17-24 and 25-34 for FD and IBS when compared to the control group.	The study was performed only in one region
Chen et al. [74].	Retrospective cross-sectional study	214	IBS patients diagnosed according to Rome III/IV criteria and health control participants	"There was no difference in the aMED and MEDAS scores between IBS and HCs. Standard MD did not increase IBS symptoms although certain MD foods were associated with an increase in IBS symptoms. Adherence to a symptom-modified MD diet showed a decrease in harmful bacteria such as Faecalitalea, Streptococcus, and Intestinibacter and an increase in beneficial bacteria such as Holdemanella from the Firmicutes phylum suggesting that a personalized MD diet may benefit patients with increased IBS symptoms.	Participants were not randomized into specific dietary interventions and the results only demonstrated the association and not the cause. The DHQ II which assessed diet was based on recalling diet in the past year and did not involve lifestyle components of the MD. The study was based in Los Angeles which resulted in a below-average MD score when compared to the Mediterranean population, and the study population consumed less olive oil when compared to MD studies.
Russo et al. [77].	Pilot study	16	Patients diagnosed with IBS-D according to the Rome IV criteria aged between 18-65 years.	TBD reduces the IBS-D symptoms by reducing intestinal permeability, decreasing levels of markers of integrity, decreasing mucosal inflammation, and fermentative dysbiosis.	The cohort is too small to draw strong conclusions and the finding of fermentative dysbiosis was not supported by data since the bacterial population of the GI tract was not analyzed.
Riezzo et al. [78].	Prospective cohort study	18	Patients with IBS-D according to Rome III/IV criteria between ages 18-65	The IBS-SSS “Intensity of abdominal bloating” was reduced with an improvement in the anthropometric profile. An altered psychological profile was also seen with a reduction after TBD. A correlation between the intensity of abdominal pain and anxiety was also reported. However, there was no correlation between “Intensity of abdominal bloating” and “Abdominal circumference”.	The study was not a double-blind controlled design and there was no control group to evaluate the subjective responses. The anthropometric and BIA parameters evaluated in this study were not influenced by a placebo and are in line with each other, the symptoms, and the previous study they conducted. The study did not investigate whether the psychological factors precede abdominal symptoms.
Russo et al. [75].	Randomized-controlled trial	72	Patients with IBS-D according to Rome III/IV criteria	Both TBD and LFD improved GI symptoms and QoL, which was demonstrated by a reduction in the IBS-SSS. However, both diets did not alter the micronutrient content during the study period.
Caponio et al. [76].	Randomized-controlled trial	38	Patients with IBS-D according to Rome IV criteria	Significant changes in the fecal volatile organic compounds (VOCs) were seen with increased levels of decanoic acid in the TBD group and increased ethanol and nonanal in the LFD group. SCFAs were found to be reduced in both groups.	Small cohort size and short duration of study.
Rej et al. [79]	Randomised controlled trial	114   (TDA ¼ 35, LFD ¼ 33, GFD ¼ 33)	Adults aged   18 years with Rome IV IBS-D or IBS-M, and an IBS-SSS of >75.   Additional inclusion criteria included being English literate, able to travel to the hospital, and having telephone or internet access	The study group concluded that TDA was cheaper & easier to follow. The individuals who followed the LFD diet had a significant improvement in depression than those who followed the TDA diet.  The individuals who followed LFD also had a significant improvement in dysphoria compared with TDA and GFD. However, changes in anxiety, somatization, and IBS QoL did not differ across all three groups.	The food frequency questionnaire tool used (CNAQ) was based on the Australian diet.
Zhang et al.  [80]	Randomised controlled trial	108	Adult patients meeting Rome III criteria for IBS-D who had no abnormal results in blood or stool tests with a normal colonoscopy within the prior 2 years.	In the LFD group, FODMAP intake was reduced to a similar extent in responders and non-responders, whereas FODMAP intake in the TDA group remained at baseline amounts.