The low FODMAP diet in the management of irritable bowel syndrome

Irritable bowel syndrome (IBS) is a functional bowel disorder in which abdominal pain is associated with defaecation or a change in bowel habits. Disordered bowel habits are typically present, eg. constipation, diarrhoea, or a mix of constipation and diarrhoea, as are symptoms of abdominal bloating and distension. The worldwide prevalence of IBS is 11.2% based on a meta-analysis of 80 studies involving 260,960 subjects. In western countries, IBS has an estimated population prevalence of 40% accounting for up to 60% of outpatient referral to gastroenterology clinics.¹,²

Diagnosis of IBS

The diagnosis of IBS requires a thoughtful approach, limited diagnostic tests and careful follow-up. Because a number of conditions have symptoms that can mimic IBS, eg. inflammatory bowel disease, coeliac disease and microscopic colitis, testing may be required to accurately distinguish these disorders. When diagnostic criteria for IBS are fulfilled and alarm features are absent, the need for diagnostic tests should be minimal.¹ 

Medical treatments for IBS

Medical treatments for IBS typically target the patient’s most bothersome symptoms and in rigorous, randomised controlled trials offer therapeutic gains of 8-20% over placebo and make from 18% to 51% of patients feel better (18-51%).³ The medical community has only recently started to focus attention on the role of food in the pathogenesis of IBS, though the association between food and gastrointestinal symptoms has been recognised by patients for decades. 

According to Prof Willliam Chey, professor of gastroenterology, we are currently experiencing what he calls a ‘diet renaissance’. During Gastrodiet 2015, Prof Chey reported that there is mounting evidence that food exerts effects on changes in motility, visceral sensitivity, microbiome, permeability, immune activation and gut-brain interactions – all key elements that have been implicated in the pathogenesis of IBS. However, he also reported results of some unpublished data by Ferch, Shaw and Chey et al, indicating that although 1,511 gastroenterologists reported that diet therapy is as good (45%) or better (46%) than other therapies, 36% never refer to a dietitian, 43% sometimes refer, 16% usually refer and 5 % always refer.

Role of food

Up to 90% of people with IBS identify food as having an important role in the generation of symptoms and many patients report a preference for dietary management rather than a reliance on medications.⁴ Guidelines for the management of IBS state that diet and lifestyle should be first-line considerations.⁵ Advice regarding eating at regular intervals and appropriate eating behaviours form the basis of first-line dietary guidelines.

FODMAP and IBS management

There is growing interest on the effects of restricting short-chain fermentable carbohydrates, collectively termed FODMAP (fermentable, oligo-saccharides, di-saccharides, mono-saccharides and polyols), for the management of IBS.⁴ There are now several comparative and randomised controlled trials of varying design providing convincing evidence that the low FODMAP diet has efficacy in IBS. Whether it is ready for primetime as first-line therapy is still being debated. 

At Gastrodiet 2015, Prof Peter Gibson of Monash University, reported that the relationship between the ingestion of poorly absorbed short-chain carbohydrates and development of functional gut symptoms is not new. 

Many studies have investigated the effects of individual FODMAP on gastrointestinal symptoms in patients with IBS over the years including lactose as many as 50 years ago and fructose in the 1980s, but it was in 2005, that it was hypothesised that all the poorly absorbed short-chain carbohydrates be considered together (applying the acronym, FODMAP) and that, to get maximal symptomatic benefit in IBS, restriction of all FODMAP, not just single groups of them, would be needed. 

The low FODMAP diet was subsequently described and an evaluation in patients with IBS showed a high degree of efficacy. The concepts have continued to evolve since then. Research suggests that the low FODMAP diet is effective in improving overall symptoms in up to 70% of patients in randomised controlled trials and up to 94% in patients in uncontrolled work.⁴

Effects of FODMAP on IBS

Although FODMAP are often referred to as a single group of carbohydrates, it is important to understand that they are a diverse group of carbohydrates that exert different effects in different parts of the gastrointestinal tract.³

FODMAP are small molecules that may be poorly absorbed in the small intestine. These then arrive into the large intestine, which is populated by the gastrointestinal microbiota. Luminal bacteria rapidly ferment these to oxygen, carbon dioxide and short-chain fatty acids. The major source of gas in the lumen is via bacterial fermentation, the principal substrate being carbohydrates. How rapidly these are fermented is dictated by the chain length of the carbohydrate; oligosaccharides and sugars are more rapidly fermented compared with polysaccharides such as dietary fibre. Gas, bloating and distension may occur due to rapid fermentation.⁶

Small sugars are highly osmotic. When these are not absorbed in the small intestine, they are delivered into the colonic lumen, together with water due to their osmotic effect. If sufficient short-chain carbohydrates reach the colon, gastrointestinal function may be disturbed via effects on motility and/or via the effect on the osmotic load. Thus, short-chain carbohydrates are reasonable candidates for dietary triggers of luminal distension and irritable bowel symptoms.⁶

Sub-classification of FODMAP

Types of FODMAP

Fructans are chains of the sugar fructose of varying length. Major sources of fructans include: 

• Wheat and rye

• Various fruit and vegetables 

• Added ingredients such as fructo-oligosaccharides (nutritional supplement), oligofructose or inulin sometimes termed ‘prebiotics’.

Galacto-oligosaccharides are chains of the sugar galactose of varying lengths. Major sources of galacto-oligosaccharies include pulses and legumes.

Polyols are sugar alcohols. Sorbitol, xylitol and mannitol are the polyols most commonly found in the diet. Major sources include various fruits and vedgetables and sugar-free chewing gums.

Fructose is a single unit sugar (monosaccharide). Major sources include various fruits, more than 100ml of any fruit juice or smoothie and honey and agave nectar.

Glucose, a different sugar unit that is easily absorbed, helps fructose to be absorbed so you can eat some foods that contain fructose. The issue arises when fructose is in excess of glucose.

Lactose is a double unit sugar (disaccharide) found in animal milk (cow, sheep, goat). Major sources include milk and yoghurts.

Implementation of the low FODMAP diet

As with any medical nutrition therapy, implementation under the guidance of a trained dietitian is paramount. Chey³ emphasises that a one-page handout is not sufficient to implement the diet. During Gastrodiet 2015, Prof Gibson discussed the important role of the dietitian as gatekeeper for those who should not be started on the low FODMAP diet.

Food lists are a useful way to start, but individualised dietetic counselling will aid nutritional adequacy and compliance throughout the trial period which typically lasts for four weeks. The initial consultation must include a thorough assessment of the patient’s history to ensure relevant tests have been completed and other possible diagnoses have been excluded. Living situation, age and cooking skills of the patients, as well as the type and severity of symptoms, need to be considered to determine the level of restriction likely to be required. All FODMAP sub-types will be explained and high FODMAP foods will be pointed out, with considerable time spent discussing low FODMAP alternatives that fit in with the patient’s current food preferences and patterns to ensure compliance and nutritional adequacy. Additional limitations need to be given attention including other restrictions such as a vegetarian diet.⁷ 

During the four week elimination phase, bloating and abdominal pain are the most likely symptoms to respond and diarrhoea is more likely to respond than constipation.³ It is critically important to understand that the full elimination low FODMAP diet is not intended to be a diet for life. 

Once the four-week FODMAP elimination trial period is completed, responders should be instructed by their dietitian on how to re-introduce foods containing individual FODMAP in order to identify their individual triggers. This helps to minimise the level of restriction they require going forward, allowing diversification of their diet. 

How to use the low FODMAP diet safely

Exclusion diets carry a risk of nutritional inadequacy. It was recently shown in a large randomised controlled trial that advice from a dietitian regarding the low FODMAP diet, broadly maintained nutrient intake in 51 patients four weeks after low FODMAP diet advice versus baseline habitual diet. However, the proportion of patients meeting UK calcium DRV was lower than baseline (NS).⁴

Extensive dietary modification, particularly to carbohydrate intake, has a pronounced impact on the gastrointestinal microbiota. Recent data suggests a four-week low FODMAP diet reduces luminal bifidobacteria concentration in the absence of changes to total bacteria abundance,⁴ while a three-week low FODMAP diet leads to reduced total bacteria abundance with reduced concentration of other bacterial groups.⁸ Whether low FODMAP diet-induced microbiota alterations can be prevented with probiotics is being investigated. 

Conclusion

A growing number of high-quality studies support the use of the low FODMAP diet in the management of IBS. Thus far, the low FODMAP diet has only been evaluated as a dietitian-taught intervention. Compliance, which is influenced by patient motivation, but also careful advice from an experienced dietitian, is vital to its success. The low FODMAP diet is not a diet for everyone and it is not intended to be a diet for life. Based on the current data, there is no benefit for low FODMAP in healthy controls. It would seem that this is the beginning of the road, not the end and we have much more to learn on this subject. 

Gastrodiet 2015 was the first of a planned series of biennial international conferences addressing diet in the pathogeneis and treatment of gastroenterology conditions.. The focus of the first meeting was FODMAP and was hosted by the Australian Monash team, in Prato Italy November 1-3, 2015

References

1. Lacey, BE, Mearin, F, Chang, L et al. Bowel Disorders. Gastroenterology, 2016; 150: 1393-1407
2. Farmer, AD, Aziz, Q. Irritable Bowel Syndrome Pathogenesis. In: Advanced Nutrition and Dietetics in Gastroenterology; 2014
3. Chey, WD. Food: The main course to wellness and illness in patients with Irritable Bowel Syndrome. The American Journal of Gastroenterology; 2016 
4. Staudhacher, H., Parkes, G. Irritable Bowel Syndrome dietary management. In: Advanced Nutrition and Dietetics in Gastroenterology; 2014
5. McKenzie, YA, Alder, A, Anderson, W et al. British Dietetic Association evidence-based guidelines for the dietary management of Irritable Bowel Syndrome in adults.  Journal of Human Nutrition and Dietetics, 2012; 25: 260-274
6. Shepherd, S. Short-chain fermentable carbohydrates. In: Advanced Nutrition and Dietetics in Gastroenterology, 2014
7. Barrett, J. Extending our knowledge of fermentable, short-chain carbohydrates for managing gastrointestinal symptoms. Nutrition in Clinical Practice, 2014; 28(3): 300-306
8. Halmos EP, Christopherson, CT, Bird, AR et al. Diets that differ in their FODMAP content alter the colonic luminal microenvironment. Gut, 2015; 64, 93-100