In the second of this two-part blog on IBS, Registered Dietitian Laura Tilt looks at the research surrounding the use of prebiotics in irritable bowel syndrome to determine what we can recommend to patients.  

Targeting the Microbiome

Irritable bowel syndrome (IBS) is the most commonly diagnosed digestive condition worldwide, affecting 1 in 10 adults1. For sufferers, the symptoms (which include discomfort, bloating and diarrhoea) can be severe, causing significant distress and disruption to daily life. Alongside standard approaches to symptom management (which involve dietary and lifestyle changes, pharmacotherapy and gut-directed psychotherapy) there has been a growing interest in therapies which can modify the gut microbiome in recent years, thanks to research showing that altered gut microbiota may play a role in the condition’s pathophysiology1.

Laura

Approaches to modifying gut microbiota vary. Research into the use of minimally absorbed antibiotics (such as rifaximin) and faecal microbiota transplants (FMT) have shown some promise in regards to symptom improvement in IBS2, although more rigorously conducted research is needed, particularly in the use of FMT. In addition, these therapies are expensive and carry a degree of risk - and in the case of FMT, can not be recommended at present.  Prebiotics and probiotics are an alternative approach to modifying the gut microbiome that are considered safe, low cost and accessible. The current clinical guidelines around the management of IBS (NICE CG, 43) include guidance on the use of probiotics3, but recommendations around prebiotics are absent. So, what does the research show, and what can we recommend to patients we encounter in clinical practice?

A Primer on Prebiotics

Prebiotics - Substrates that are selectively utilized by host microorganisms conferring a health benefit

Prebiotics are essentially food for beneficial microbes. Prebiotics are not a homogenous group - they exist naturally in various foods but can also be synthesised. Naturally occurring prebiotics are typically fibres or short chain carbohydrates like inulin and fructo-oligosaccharides or FOS (both fructose polymers). These are present in a range of plant foods including asparagus, onion, garlic, artichokes, wheat, bananas, leeks and chicory root. Chicory root fructans are typically extracted for use as inulin supplements.

Synthetic prebiotics include galactooligosaccharides or GOS (galactose polymers produced through enzymatic conversion from lactose). GOS produced from lactose are known as β-GOS, whereas those found in beans and lentils are known as α-GOS. When consumed in sufficient quantities, prebiotics have been shown to increase both the numbers of bifidobacteria in the colon4 and the production of short chain fatty acids (SCFAs), which are the main energy source of energy for colonocytes. Given that studies have shown that IBS is associated with both lower levels of bifidobacteria and SCFAs, there seems to be a good rationale for their use. But does the research agree?

Prebiotics in IBS

To date, inulin-type fructans (fructose polymers) and galactooligosaccharides (GOS) have been the most extensively studied4, although it’s worth noting that the majority of research has been carried out in individuals without GI conditions.  However, in a recent meta-analysis of 11 randomised controlled trials investigating the effect of prebiotics compared with a placebo in adults with IBS and other functional bowel disorders5, the authors concluded that ‘prebiotics do not improve gastrointestinal symptoms or QoL in patients with IBS, but they do increase bifidobacteria’.

On the surface of it, this appears to be ‘case closed’ for prebiotics as a strategy for symptom management - although it’s worth noting that the bifidogenic effects may well be beneficial over a longer period. However, it’s worth digging a bit deeper as both the type and quantity of prebiotic used determines symptom response.

Inulin-type (IT) prebiotics

Inulin type (IT) prebiotics include chicory root fibre and FOS. Alongside their ability to stimulate the growth of bifidobacterium and lactobacillus at intakes of 5-15 grams4, IT prebiotics typically induce side effects such as abdominal pain, bloating and flatulence when consumed in larger quantities as a result of fermentation by resident microbes6. Whilst these effects may be tolerated in adults with no gastrointestinal (G.I.) conditions, they are likely to exacerbate existing symptoms for those with IBS, due to visceral hypersensitivity. Unsurprisingly, the meta-analysis5 found that flatulence worsened with IT prebiotics.

No studies exist to investigate the effects of increasing prebiotic intake from food, but it’s fair to assume that a prebiotic rich diet would likely result in a similar exacerbation of flatulence and bloating given that prebiotic rich foods are high in FODMAPs, which have been shown to exacerbate IBS symptoms a wide range of clinical trials7.

β-GOS-type prebiotics

Unlike IT prebiotics, β-GOS prebiotics have a high selectivity for bifidobacteria, which metabolise carbohydrates with less gas production. For this reason, there has been interest in their ability to increase levels of bifidobacteria without a concomitant increase in flatulence and bloating. In the meta-analysis5 the researchers reported that flatulence improved with non-inulin type prebiotics. Improvements in abdominal pain and bloating were also seen, but these failed to reach statistical significance. Two studies are of particular note. In a 2009 controlled crossover clinical trial8, 44 patients with IBS were randomised to receive either 3.5 g⁄day B-GOS prebiotic, 7 g⁄day B-GOS prebiotic or 7 g⁄day placebo after a 2- week baseline period. After 2 treatment weeks, patients entered a 2-week washout phase before entering the next test phase. During the 3-month intervention period, symptoms were measured at 7-day intervals. Results showed that both prebiotic treatments increased the proportions of bifidobacteria without adverse side effects. Treatment with the 3.5g/day prebiotic also resulted in a significant improvement in stool consistency, flatulence and bloating. 

A later study in 20189 compared the effects of the same prebiotic (1.37g/day of B-GOS) plus a Mediterranean diet versus a placebo supplement (xylose) and a diet low in FODMAPs in a group of patients with IBS and functional abdominal distention. After 4 weeks, both the prebiotic and low FODMAP diet groups experienced a reduction in all symptom scores, although reductions for flatulence did not reach significance in the prebiotic group. As observed in previous studies, bifidobacteria was reduced on the low FODMAP diet, but increased in the prebiotic group, suggesting a potential benefit of supplementing with a non-inulin based prebiotic either alongside a low FODMAP diet or as an alternative to dietary restriction. However, further research is required to determine the most effective intakes – a more recent study found that 1.4g/day of B-GOS was not sufficient to offset the reduction in bifidobacteria seen when following a low FODMAP diet.10

So, what can be recommended?

Based on the current research available, we can conclude that prebiotics increase faecal bifidobacteria in IBS, but both prebiotic source and quantity need to be considered when making recommendations to avoid symptom exacerbation. From the studies analysed, non-inulin type prebiotics such as β-GOS are the most likely to be well tolerated, and may benefit some individual IBS symptoms like flatulence. Lower intakes of prebiotic supplements (of less than 6g/day are also more likely to be tolerated than larger intakes. Finally, since research in this specific patient group is limited, larger studies should focus on the use of lower intakes, or non-inulin prebiotics for those with functional GI disorders, in order to better understand the impact of prebiotics on symptoms. Read part 1 to learn about the role of prebiotics and IBS management.

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