Dietary strategies for irritable bowel syndrome: A narrative review of effectiveness, emerging dietary trends, and global variability

3.1 Traditional dietary advice (TDA)

TDA follows dietary recommendations established by the British Dietetic Association (BDA) and the National Institute for Health and Care Excellence (NICE) [10,40]. In 2016, the BDA provided the first fully comprehensive evidence-based first-line dietary and lifestyle guidelines for the dietary management of IBS with advice related to healthy eating practices, the consumption of alcohol, caffeine, fluids, fat, fibre, dairy, spicy food, and the use of probiotics as summarised in Table 1 [40].

Evidence suggests that irregular meal patterns, low fruit and vegetable intake, and excessive fast-food consumption may trigger IBS symptoms [10,40]. Therefore, monitoring dietary habits and promoting a balanced diet alongside healthy eating and lifestyle practices is recommended.

Both the BDA and NICE recommend TDA as a first-line treatment prior to consideration of more restrictive diets like the low fermentable oligosaccharide disaccharide monosaccharide and polyols (FODMAP) diet (LFD) [10,40]. Although TDA is widely implemented in clinical practice, the supporting evidence is relatively limited. Most of the recommendations are informed by clinical experience, observational findings, or expert consensus rather than a robust foundation of randomised controlled trials (RCTs). One notable RCT by Eswaran et al. [41] included TDA as a comparator to the LFD and concluded that while TDA may offer symptom improvement for some individuals, the LFD resulted in significantly greater symptom relief. However, the existing lack of large-scale, well-designed trials evaluating TDA as a standalone intervention restricts our ability to draw strong conclusions about its efficacy. Despite these limitations, TDA remains a practical, cost-effective, and nutritionally balanced approach, especially for individuals with milder symptoms particularly when delivered by a skilled dietitian as part of a structured and individualised care plan.

3.2 The low-FODMAP diet

FODMAPs are a group of poorly absorbed carbohydrates that can contribute to the symptoms of IBS. These compounds vary in chain length, with short-chain FODMAPs (lactose, fructose, polyols) being rapidly and incompletely absorbed in the small intestine, where they exert osmotic effects that draw water into the bowel, potentially leading to diarrhoea and bloating. In contrast, long-chain FODMAPs (oligosaccharides) are not absorbed in the small bowel and instead reach the colon intact, where they undergo fermentation by colonic bacteria, producing gas and leading to symptoms such as bloating, flatulence, abdominal pain, and altered bowel habits [42,43,44]. These mechanisms are further detailed in Table 2.

The dietitian-led LFD is recognised as the only recommended second-line treatment for managing IBS when initial dietary interventions are insufficient [40]. This evidence-based approach is designed to alleviate IBS symptoms by reducing the intake of fermentable carbohydrates that are poorly absorbed in the gut and is implemented in three distinct phases – restriction, re-introduction, and personalisation – each playing a crucial role in identifying and managing food triggers specific to the individual [42]. As summarised in Figure 1, these phases are structured to gradually ease IBS symptoms while ensuring long-term dietary sustainability.

Figure 1

The three phases of the LFD.

The initial restriction phase concerns the global restriction of high-FODMAP food sources for 4–8 weeks with the substitution of suitable low-FODMAP sources to ensure nutritional adequacy. This serves as a diagnostic test for FODMAP sensitivity with a 50–80% positive symptom response. If the patient is non-FODMAP sensitive, alternative dietary interventions are trialled. The second phase reintroduces single FODMAPs over 6–12 weeks in gradually increasing doses over 3 days while the background diet remains low in FODMAPs. Specific challenges include single food containing fructans (wheat-containing bread and garlic), galactans (lentils, legumes), lactose (milk, yoghurt), fructose (honey), sorbitol (dried apricot), and mannitol (mushrooms). This phase is fundamental to prevent unnecessary dietary restrictions and malnutrition in the long term as it determines individual tolerance to specific FODMAPs [45]. In the final phase, the less restrictive personalised diet is formed for the longer term which incorporates low-FODMAP sources and tolerated higher FODMAP sources. However, dietary sources that were not well tolerated should still be re-challenged over time as symptoms may change. It is proposed that a daily threshold of 12 g reduction in dietary FODMAPs results in symptom improvements, keeping in mind individual variability [44,46,47,48,49,50,51].

Commonly consumed food, including wheat-based bread, pasta and pastries, certain fruits (such as apples and stone fruits), vegetables (such as onion, garlic, and cauliflower), legumes, pulses, and dairy are significantly restricted in the first phase. This may compromise nutritional adequacy, as limiting these foods can reduce the intake of key nutrients – calcium from dairy, iron and B vitamins from fortified cereals, and fibre from grains, fruits, vegetables, legumes, and pulses, with several studies looking at adherence to an LFD reporting suboptimal intakes for calcium, iron, magnesium, and B vitamins [5,40,46]. Others have reported a reduced diet quality but not lower nutrient intake [46,51] and reduced energy intakes despite unaffected protein and fat intakes [8]. Oligosaccharide restriction was also attributed to a 4 g/day dietary fibre reduction [40].

A greater symptom improvement is noted with the LFD [55]; however, such an effect may not always be consistent [13,52]. One RCT reported that those following the LFD consumed significantly less energy, carbohydrates, and fibre compared to those following TDA [13,51]. Nevertheless, both the LFD and TDA result in reduced intakes of energy, especially that derived from carbohydrates. After adjusting for the percentage of overall intakes, micronutrient consumption seemed sufficient except for riboflavin [51].

To maintain nutritional adequacy, individuals with IBS are encouraged to follow national dietary recommendations for key nutrients and food groups while incorporating suitable low-FODMAP alternatives. Dietitian-led restriction and personalisation phases of the LFD have been proposed to support overall nutrient intakes. Regular monitoring of body weight is essential and supplementation should be considered for those at risk of nutrient deficiencies [53,54,55,56].

The LFD currently represents the most extensively researched dietary intervention for IBS, with multiple systematic reviews and meta-analyses confirming its effectiveness in managing GI symptoms [57–64]. Evidence consistently demonstrates that the LFD significantly improves global IBS symptoms, with particularly pronounced benefits in individuals with diarrhoea-predominant IBS [62,65–69]. Its efficacy is further enhanced when implemented under the guidance of a dietitian, who plays a vital role in ensuring nutritional adequacy throughout the three-phased LFD [61,66,70–72]. Despite its high short-term efficacy, concerns persist regarding the potential long-term consequences of the diet, especially its impact on gut microbiota composition and metabolic outcomes [59,64,65,68]. Current literature underscores the need for further large-scale, high-quality RCTs to determine their long-term safety and sustainability, to assess their relative effectiveness compared to other dietary, supplemental, or lifestyle-based interventions, and to better characterise patient predictors of response, particularly during the personalised re-introduction phase [59,68,72–74].

While the LFD has shown considerable effectiveness in alleviating IBS symptoms, it is not without some limitations. The diet’s complexity requires extensive food knowledge and label reading, making adherence challenging, especially in social settings [51,53]. Another key concern is the potential impact on gut health, as some FODMAPs act as prebiotics, supporting the growth of beneficial bacteria in the gut, such as Bifidobacteria and Lactobacilli. Prolonged restriction of these fermentable carbohydrates may inadvertently reduce microbial diversity and alter luminal microbiota composition. Furthermore, although one recent systematic review and meta-analysis reports positive improvements of the LFD on microbial regulation, it highlights the need for further high-quality research to confirm these findings within IBS populations [75]. Moreover, while the fermentation of carbohydrates produces short-chain fatty acids, important for colonic health, this process also results in the release of colonic gas further exacerbating GI symptoms [51,53]. Strategies to combat these concerns such as the re-introduction phase, probiotic supplementation, and dietary personalisation may prove fundamental to minimising the effects that the LFD may exert on the microbiome in the longer term. However, further research is warranted to fully comprehend the complex relationship between diet, IBS symptom profiles, and the gut microbiome [76].

To address these challenges, adapted versions of the LFD have been proposed such as the FODMAP gentle approach. These involve a “bottom-up” approach, where only a few specific FODMAP sources are restricted based on a thorough evaluation of dietary history and symptomatology, rather than adopting a blanket restriction of all FODMAP groups [13,51,77–79]. Furthermore, potential microbiota alterations during the restrictive phases of the diet may be reversed in the final re-introduction phase or supported through probiotic supplementation. However, despite these strategies, evidence about the long-term impact of the complete three-phased LFD remains limited and warrants further research.

3.3 Gluten-free diet (GFD)

Wheat contains components such as fructans, gluten, amylose trypsin inhibitors, and other proteins, which may trigger food intolerance symptoms in 23–49% of IBS sufferers, with studies reporting a 58% clinical response to a GFD [13]. The physiologic mechanism of wheat-based fructans is explained in Table 2. Gluten has been primarily associated with symptoms such as abdominal pain, bloating, altered stool consistency, increased gut permeability, immune activation, and fatigue, suggesting that it may be a key driver of symptom onset following wheat consumption [48,80,81]. Supporting this, several studies have reported a reduction in overall symptom severity, including bloating and abdominal pain, in IBS patients following a GFD [82–85].

Nevertheless, the GFD raises dietary concerns including increased fat intake and reduced carbohydrate and fibre intake, along with observed deficiencies in iron, calcium, thiamine, and magnesium. However, data about the nutritional adequacy of GFD in IBS are rather limited [13].

Evidence supporting a GFD for IBS is still lacking predominantly due to the fact that symptom alleviation may not be solely due to gluten exclusion as a GFD also eliminates fructans [13,40,47]. In a placebo-controlled, crossover rechallenge study, patients habitually consuming a GFD in view of non-coeliac gluten sensitivity experienced symptom improvement with an LFD. No independent gluten-specific effects were noted on gluten challenges concluding that FODMAPs are responsible for GI symptoms and not gluten [86]. Moreover, no further benefits were observed when a GFD was introduced to IBS patients already following an LFD [7].

Eliminating consumption of cereals reduces dietary FODMAP content by 50% and may alleviate symptoms [13,40]. Single food challenges are warranted to determine whether such food products trigger symptoms. It remains unclear whether gluten is responsible for IBS symptoms while evidence for fructans as a predominant symptom trigger is accumulating [40]. However, one observational study also reports no correlation between GI symptoms and fructan intake [15]. Further research is warranted to determine the long-term efficacy of a GFD for IBS as most RCTs are short term [13].

3.4 Specific carbohydrate diet (SCD)

The SCD allows only monosaccharide carbohydrates while excluding disaccharides and polysaccharides. It is supplemented with homemade yoghurt fermented for 24 h to minimise the disaccharide lactose. This diet, including a number of allowed and prohibited foods as outlined in Table 3, is proposed as it may reduce inflammation and microbiota dysfunction [87].

As can be determined from Table 3, the SCD shares overlapping mechanisms with the LFD particularly in its exclusion of disaccharides in the form of lactose (found in dairy) and polysaccharides such as starch (found in cereal, potatoes, corn), which are known to be osmotic and fermentable, leading to gas production, bloating and discomfort in IBS patients. Therefore, both diets aim to alleviate IBS symptoms by reducing fermentable carbohydrate intake, but while the LFD takes a broader approach by excluding additional FODMAPs including oligosaccharides and polyols, the SCD is more focused on limiting specific complex carbohydrates. Indeed, a recent randomised non-inferiority study reported that a diet low in starch and sugar shares similar efficacy with the LFD in the reduction of GI symptoms, possibly due to the overlap between the two dietary patterns [88].

Contrastingly, a clinical single-blinded RCT noted that the LFD provided significant improvements in bloating and distension compared to the SCD. Folic acid and vitamin D deficiencies were also observed with the SCD [87]. Such findings support those previously reported in the literature [5]. Given the absence of significant improvements in symptoms, such an exclusion diet is not routinely recommended [40].

3.5 Low lactose or lactose-free diet

Lactose intolerance is common in both the general population and amongst IBS sufferers. Symptoms are triggered due to the inability to form lactase enzymes, unfavourable microbiota compositions, or history of GI disorders. Not all lactose malabsorbers experience symptoms following consumption while symptoms may be heightened at even minimal intakes with visceral hypersensitivity [50]. One systematic review reports the absence of any clear correlations between IBS and lactose intolerance and does not support routine lactose-free diets for all IBS sufferers. Further research is warranted to determine whether any specific components in dairy other than lactose are responsible for IBS complaints [89]. In light of the available evidence, low lactose or lactose-free diets for the management of IBS symptoms should be considered as part of an LFD and not on their own merit.

3.6 Low fructose diet

Over one in three adult IBS sufferers experience fructose malabsorption and its prevalence is similar amongst IBS sufferers and asymptomatic controls [50,90]. The main determinants of fructose malabsorption are presumed to be excessive fructose intake, co-ingestion of fructose and sorbitol, and fructose in excess of glucose as when fructose and glucose are equimolarly ingested, symptomatic improvements were recorded [50,90,91]. Fructose is present in equimolar amounts to glucose in bananas, strawberries, table sugar, and high-fructose corn syrup [91]. Nevertheless, a low fructose diet is rarely acknowledged as effective since fructose malabsorption is a separate disease from and not specific to IBS; it may be uncommon and due to the absence of published dietary guidelines [90]. Should a low fructose diet be warranted for the management of IBS symptoms, this should be considered part of the LFD.

3.7 Comparative analysis

A lack of consensus exists with regard to the comparative efficiency of TDA, LFD, and GFD and it remains unclear which dietary treatment is superior due to the absence of head-to-head trials. RCTs demonstrate similar efficiency between these diets with TDA appearing to be less costly and time consuming and easier to comply with [52,92]. The key uses and limitations of the dietary interventions analysed in this narrative review are compared in Table 4. Table 4 may serve as a practical resource for healthcare professionals guiding decisions as to the most appropriate dietary intervention to adopt.

3.8 Emerging dietary trends

With advancements in IBS care, novel dietary interventions show promise for symptom relief. Precision nutrition has the potential to revolutionise GI health by tailoring dietary interventions based on individual gut microbiota profiles offering promising approaches for the management of IBS symptoms [16]. One pilot clinical study compared microbiota compositions of mixed subtypes of IBS patients, as diagnosed by the Rome IV criteria, against that of a control group. Following 6 weeks of either AI-based personalised nutrition or a standard IBS diet, it was concluded that AI-based personalised microbiome modulation through diet improved IBS symptoms and induced favourable microbiome changes, but further large-scale RCTs with long-term follow-up are necessary [17]. A multicentre RCT comparing the effectiveness of microbiome-based AI-assisted personalised diets against the LFD for IBS symptom management concluded that the AI-assisted personalised diet approach demonstrates significant symptom improvements, improved quality of life, and enhanced microbiome diversity [18].

In their hypothesis-driven article, Black and Ford propose a novel framework for the management of IBS that moves beyond traditional symptom-based subtyping. Using latent class analysis, seven distinct patient clusters were identified and characterised by unique constellations of GI symptoms, extraintestinal manifestations, and psychological comorbidities. This classification model demonstrated predictive value for healthcare utilisation, treatment response and quality of life outcomes. By outlining tailored first- and second-line treatment strategies for each cluster, the authors offer a personalised, mechanistically informed approach to IBS care. This model underscores the heterogeneity of IBS and presents a compelling argument for shifting towards stratified treatment pathways, warranting validation through prospective clinical trials [95]. Further three studies support the integration of precision nutrition in IBS care while encouraging future long-term and larger-scale RCTs to fully determine the effectiveness of such digital and personalised dietary interventions [96–98]. As our understanding of the genetic underpinnings of IBS and nutrient interactions expands, precision nutrition may become a cornerstone in the comprehensive management of this complex disorder.

The role of PBDs in managing IBS symptoms has garnered increasing attention, although the existing evidence remains preliminary and warrants further investigation. PBDs may confer GI benefits through the increase in gut bacterial diversity [19,20,21]. One cross-sectional study investigating the association between PBDs and IBS in a large French cohort concluded that long-term PBDs may be associated with IBS but further long-term studies are necessary to confirm this association [22]. Additionally, one systematic review and meta-analysis suggests that PBDs may influence the circulating levels of inflammatory markers and reports a strong association between c-reactive protein, an inflammatory marker, and PBDs. Nevertheless, further investigation is necessary [23]. While PBDs are generally considered beneficial for gut health, some individuals with IBS may experience symptom exacerbation due to increased fibre intake, particularly from high-FODMAP plant foods. Therefore, personalised dietary approaches, possibly integrating elements of PBDs with low-FODMAP principles, may offer a balanced strategy for managing IBS symptoms.

The MD characterised by high consumption of fruits, vegetables, wholegrains, legumes, nuts, seeds, olive oil, and moderate intake of fish and poultry has garnered attention for its potential benefits in managing IBS symptoms. A recent pilot RCT concluded that the MD improves abdominal symptoms for diarrhoea-predominant IBS and mixed subtypes IBS but warrants larger-scale trials to evaluate the effectiveness of the MD against that of LFD and TDA [28]. In agreement, a 6-week RCT involving adults with Rome IV-diagnosed IBS demonstrated that participants adhering to the MD experienced significant reductions in GI and depressive symptoms compared to controls [27]. Similarly, a network meta-analysis identified the MD as one of the most effective dietary interventions for improving IBS symptom severity and quality of life. Nevertheless, it still warrants future large-scale RCTs with head-to-head comparisons to fully comprehend the effectiveness of the MD for IBS populations [29]. These findings are supported by studies indicating that the anti-inflammatory properties of the MD, attributed to components like polyphenols and omega-3 fatty acids, may modulate gut microbiota composition, potentially enhancing gut health and immune function as well as reducing inflammation in IBS patients [24,25,26,27]. Ultimately, while currently available evidence supports the efficacy and feasibility of the MD in IBS symptom management, a combined LFD and MD, or an IBS-modified MD, is recommended for further study and to establish clinical guidelines [99].

The current evidence on IF as a dietary intervention for managing IBS remains preliminary and inconclusive. IF may offer therapeutic benefits through gut microbiota modulation [30,31]. A systematic review of human studies on IF and gut microbiota found that IF can improve gut microbiota richness and alpha diversity which may potentially offer beneficial effects for IBS patients. However, this systematic review highlights the need for further research to clarify and confirm such effects in light of the substantial heterogeneity in results and bacterial strains determined to be statistically significantly influenced by this dietary pattern [30]. While IF shows promise in modulating factors associated with IBS, its clinical application remains uncertain and more rigorous, long-term studies are needed to determine its efficacy and safety in IBS management.

The role of histamine in IBS has garnered increasing attention in recent years, particularly in relation to the emerging concept of histamine intolerance [100]. Although the pathophysiology of IBS is multifactorial, evidence suggests that histamine has the potential to influence gut motility, visceral hypersensitivity, and intestinal inflammation triggering symptoms [101–103]. A positive correlation is identified between mast cells, which release histamine, and IBS symptoms particularly abdominal pain and bloating [104]. Accepted dietary guidelines for a low histamine diet are yet not available, but generally, a low histamine diet involves an elimination-type diet reducing the consumption of hard and semi-hard cheeses, oily fish and shellfish, raw fermented meat products, chicken eggs, fermented soy products, pickled vegetables, fruit and vegetables triggering the release of endogenous histamine, mushrooms, chocolate, wine and beer [103]. However, the role of histamine in IBS is not fully elucidated and the current body of evidence remains limited making it difficult to establish clear clinical guidelines [105]. As such, while a low histamine diet may offer therapeutic potential for certain IBS subtypes, its routine use in clinical practice warrants further investigation supported by high-quality research.

The ketogenic diet, characterised by high fat, adequate protein, and very low carbohydrate intake, has been explored for its therapeutic potential beyond epilepsy and weight management, including in GI disorders such as IBS. The proposed mechanisms through which a ketogenic diet may benefit IBS include modulation of the gut microbiota due to ketone body activity [106–108]. However, the current evidence supporting its efficacy in IBS remains sparse and largely preclinical. Most available studies have been conducted in animal models, particularly rats, demonstrating beneficial changes in gut permeability, inflammation, and microbial composition [106,109]. A limited number of human case studies have suggested that very low carbohydrate diets may reduce symptoms such as bloating, abdominal pain, and altered bowel habits, especially in individuals with diarrhoea-predominant IBS [110]. Moreover, concerns regarding the diet’s restrictiveness, potential nutritional deficiencies, and its impact on gut microbial diversity raise questions about its safety and sustainability for IBS management [108]. While the ketogenic diet may offer symptomatic relief for select patients, there is insufficient high-quality evidence to support its widespread use in IBS treatment and further RCTs are needed to clarify its role.

3.9 Dietary intake of international IBS populations

Nutrient intakes of IBS populations differ across geographical regions with some studies agreeing that IBS does not adversely affect nutrient intakes. A recent systematic review and meta-analysis reports no significant differences in dietary intake between IBS and non-IBS sufferers except for suboptimal intakes of fibre, calcium, and vitamin D [32]. Additionally, a cross-sectional study targeting an English population reported a low risk of nutritional deficiencies and stated that adequate substitutions and supplementation were being practised if specific foods were being excluded [33]. In agreement, a Swedish study reported no significant differences between the dietary intake of IBS and non-IBS sufferers with IBS participants consuming higher dietary fibre intakes compared to the general population. The only suboptimal intakes observed were those for vitamin A, riboflavin, calcium, and potassium mainly due to limited intakes of dairy [34]. An additional RCT also reports nutrient intakes comparable to the general population and even suggests that the LFD may improve overall dietary intake. Vitamin B12 was higher compared to habitual diets, possibly due to increased intakes of eggs and fish [35]. Furthermore, a North American study concluded that IBS sufferers consume significantly higher energy intakes compared to non-IBS counterparts [111].

However, the following studies contraindicate this by arguing that IBS sufferers practise significantly more dietary avoidances. A study performed in a large French population concluded that IBS sufferers consume less milk, yoghurt, and fruits and greater intakes of non-sugar-sweetened beverages [36]. A Norwegian study also reports dairy avoidance in view of lactose malabsorption and intolerance with calcium, potassium, zinc, and B vitamins deficiencies as a consequence [37]. Both studies report a limited consumption of fruit and vegetables due to their potential to generate gas upon fermentation and laxative effects further depleting levels of water-soluble vitamins and minerals. Higher intakes of water and carbonated beverages might be observed in an attempt to compensate for fluids lost through diarrhoea or to prevent constipation. Tea may be avoided as salicylate content may cause constipation and gut symptoms while coffee is related to diarrhoea. A higher intake of carbonated beverages may worsen symptoms due to the potential presence of caffeine. Alternatively, it may be consumed in addition to water with the aim of substituting milk with other fluids to meet fluid requirements as recommended for the management of constipation and diarrhoea. Although alcohol is known to exacerbate symptoms, its consumption may persist in an attempt to relieve severe symptoms [36,37]. Vitamin D levels may also be depleted through the avoidance of fortified dairy [38].

Cultural diets, food availability, and strength of healthcare systems play significant roles in shaping dietary intake and adherence to dietary interventions among IBS populations. Cultural dietary patterns, including staple foods, preparation methods, and meal structures vary widely across regions and often influence the types of food consumed by individuals with IBS. For instance, in regions where high-fibre foods are common, such as in Mediterranean or East Asian diets, IBS patients may experience different symptom management outcomes compared to those in regions with lower fibre intake or a reliance on processed foods such as the Western diet. Moreover, access to food may hinder the IBS patients’ ability to follow specialised diets, such as the LFD or GFD, which may be difficult to adhere to without access to specific food or ingredients. The health system also plays a critical role in IBS management, as its effectiveness in providing education and access to the relevant healthcare professionals, especially dietitians, influences adherence to dietary recommendations. In countries with well-established healthcare infrastructures, IBS patients may have better access to tailored dietary counselling, while in regions with less comprehensive healthcare systems, patients may face challenges in obtaining the necessary support to manage their condition effectively. These factors collectively contribute to variations in nutritional profiles and symptomatology, as cultural, economic, and healthcare-related barriers impact both the quality of the diet and the ability to adhere to tailored dietary interventions.

Table 5 summarises dietary findings of IBS sufferers from different geographical regions and may serve as a useful practical clinical tool for healthcare professionals as it provides an outline of possible dietary deficiencies. It also serves as a valuable resource for improving patient care and facilitating tailored management strategies.