BLOG BY Katie Avis, BSc Hons

What Is Irritable Bowel Syndrome

The term IBS has become fairly mainstream but what do we mean by IBS, what causes it, and what treatment options are available?

Irritable Bowel Syndrome (IBS) is the most common type of functional gastrointestinal disease. Many reports suggest a global prevalence of 10-15%, whilst more recent epidemiological studies suggest that the occurrence could be lower, around 2-6%. The difference in figures is likely due to different diagnostic and research criteria. IBS is more common in females and, unlike a lot of illnesses, incidence decreases with age.


Symptoms differ between patients and they can change over one’s life time.  However common symptoms include:

• Recurrent abdominal pain

• Bloating

• Change in bowel habits such as constipation and/ or diarrhoea

The pathophysiologic mechanisms behind IBS are still unclear but research suggests that they are multifactorial and patient dependent.


Without a gold standard tool and the way in which IBS can mimic other disorders, diagnosis can often be challenging. However, clinicians tend to work using four key components. They assess patient clinical history which helps to identify any additional pathologies, as well as ruling out other forms of disease. This is accompanied by a physical examination to identify patient symptoms and morbidities. In some cases, laboratory tests and colonoscopies are needed to confirm or further the diagnosis. The Rome IV criteria are used to classify the IBS subtype; IBS-C (constipation predominant), IBS-D (diarrhoea predominant), IBS-M (mixed bowel habits) and IBS-U (unclassified; when it does fit into the other categories).


Since IBS is so variable, each patient will respond differently to each treatment. Available therapies include pharmacological agents such as; antibiotics, antispasmodics, antidepressants, laxatives and antidiarrheal drugs etc. Cognitive behavioural therapy (CBT) and other psychological interventions are also available. However, the treatments are by no means a cure and since this is a multi-modal condition, they often only target one part of the disease. Moreover, with an unknown aetiology, these treatment options can be non-specific.

Patients often turn to dietary management which can involve elimination or reduction of trigger foods, with patients often avoiding lactose, fructose and or gluten.  However, if deemed appropriate, patients are advised to undertake the low Fermentable, Oligosaccharides, Disaccharides, Monosaccharides and Polyols diet, known as the low FODMAP diet (LFD). 


FODMAPs are a group of carbohydrates that humans cannot digest. There are many foods that fall into the FODMAP category which contain one or more of  these carbohydrates including; apples, avocado, pears, mango, cauliflower, soya beans, rye, chorizo and so on. A full list of FODMAPs can be found here.

So how do FODMAPs cause symptoms?

Symptom severity is patient dependent and the mechanisms behind FODMAP symptom production is not fully known, but there are two main hypotheses;

• The ‘small bowel hypothesis’ describes how FODMAPs are osmotically active molecules which causes an increase in intraluminal water content in the small intestines. This leads to distension, bloating and discomfort.

• The ‘large bowel hypothesis’ suggests that the FODMAPs increase colonic bacterial fermentation as well as gas production. This results in bloating, flatulence and discomfort.

It is also thought that low-grade digestive tract inflammation, psychological factors, and an altered brain-gut axis may also play a role.

The low FODMAP diet

The LFD is the most widely implemented dietary therapy and one that has proven to improve patient quality of life. There are also a number of studies that have proven its efficacy in symptom improvement. It's not just a restrictive diet, but one that allows individuals to establish what foods they can or cannot tolerate, allowing them to lead a personalised diet and lifestyle.

Implementing the LFD takes 2-3 appointments with a specialist dietician.  This is done on a one-to-one basis or in group education sessions. The initial appointment involves going through the FODMAP restriction phase. This is a period of 4-6 weeks where patients must restrict all FODMAPs from their diet. At the end of this period they should see symptom improvement. At the next appointment (short term follow up) FODMAP reintroduction is discussed. Over the following weeks patients begin to re-introduce FODMAPs into their diet. This is done systematically by adding one food at a time, from one of the FODMAP groups, back into the diet at increasing quantities. This allows the patients to identify their symptom threshold for each of the FODMAPs. Over time this will allow patients to personalise their diet; add back in the FODMAPs that don’t cause symptoms and reduce or eliminate those that do, to stay in control of symptoms whilst maintaining a nutritionally adequate diet. Patients may also attend a long term follow up appointment with their dietician.

Round up

IBS is not uncommon and can have huge impacts on everyday life.  Although research suggests that there is not one single cause, there are a variety of treatments that can help improve symptoms and thus quality of life. If you think you are experiencing symptoms then speak to your GP.  It should be noted however, that you should only undertake the LFD if you have been advised to and have the support of a trusted healthcare professional.

This post was written by Katie Avis (BSc Hons) who has just finished her Clinical and Public Health Nutrition MSc at UCL. Katie’s thesis explored the impacts of restrictive diets on the efficacy of the low FODMAP diet. Katie is the cofounder of e.k. nutrition, an evidence based platform which aims to make nutritional information accessible to everyone. Follow her on Instagram @e.k.nutrition for up to date research and fun recipes.


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