Overview

Irritable bowel syndrome (IBS) is a common condition that can severely disturb gastrointestinal (GI) function, most commonly presenting as recurrent diarrhoea, constipation, and abdominal pain.¹ IBS can sometimes result in excessive weight loss with serious consequences to overall health. Individuals with IBS may require beneficial dietary regimes and appropriate measures to overcome nutritional deficiencies.²

Irritable bowel syndrome is a gastrointestinal disorder characterised by recurring intestinal dysfunctions, including abdominal pain, diarrhoea, and constipation. IBS is prevalent in 5-20% of the general population. However, the National Institute for Health and Care Excellence, NICE, notes that many individuals with IBS symptoms who do not seek medical advice may remain unaccounted for.

IBS can be distinguished from inflammatory bowel disorders (IBD), colon cancer, or microscopic colitis due to the absence of visible colonic lesions and microscopic markers of inflammation. As such, diagnosis is usually exclusively based on symptoms after other bowel diseases have been ruled out.¹

An important complication of IBS is excessive weight loss, which can occur due to various reasons, such as poor appetite, diarrhoea causing malabsorption of nutrients, or deliberate food avoidance to prevent symptoms.^2,3 Weight loss can have many detrimental effects on one's health, including loss of muscle mass, metabolic and hormonal changes, insecurity about body image, and increased risk of premature death.⁴ As a result, maintaining good nutritional status and adhering to dietary and therapeutic interventions is an important part of the management of IBS.

Causes and mechanisms of IBS

The exact causes and pathophysiology of IBS are poorly understood. However, researchers hypothesise that IBS may involve an interplay between abnormalities with the gut neurotransmitters, the intestinal microbiota, and immune-mediated inflammation.

The chemical serotonin—a neurotransmitter—plays an important role in gastrointestinal motility and pain sensitivity in the gut. Higher or lower peripheral serotonin levels have been associated with diarrhoea- and constipation-predominant IBS.¹ In addition, an overexpression of serotonin receptors (to which serotonin binds) can result in heightened visceral sensitivity, contributing to abdominal pain.

Other studies suggest the influence of inflammatory cytokines, such as TNF-alpha, which can activate pain-sensing nerve fibres and alter normal bowel function.⁵ Finally, individuals with IBS have differences in intestinal microbes, which are important for digestion and motility. Individuals with IBS, particularly, have less diversity of bacteria, with excess growth of certain species relative to others.⁶

Different genetic and environmental factors can promote the development of IBS, including:^7,8

• Having certain genetic variants, like in the genes encoding sodium channels in the peripheral nervous system, makes the gut oversensitive, leading to IBS symptoms 
• Previous gastrointestinal infection
• Stressful life events and childhood trauma
• Vitamin D deficiency

In addition, IBS is usually diagnosed under the age of 50, most commonly in women. Being previously diagnosed with certain mental illnesses such as depression, anxiety, and chronic fatigue syndrome also increases one's risk.

Symptoms and diagnosis of IBS

Symptoms

Irritable bowel syndrome can cause a wide range of symptoms. Patients may experience diarrhoea, constipation, or alternating episodes of both. These symptoms can be accompanied by:

• Abdominal pain
• Nausea
• Loss of appetite
• Flatulence
• Bloating
• Mucus in stool
• Feelings of incomplete bowel emptying

Many people find these symptoms to arise in response to known triggers, typically certain foods and emotional stress. Moreover, individuals with IBS have an increased risk of being diagnosed with anxiety and depressive disorders as well as insomnia and chronic fatigue syndrome.²

Diagnosis

Diagnosis of IBS mostly involves ruling out other intestinal conditions such as acute infection, IBD, coeliac disease, or cancer. Therefore, IBS diagnosis typically requires colonoscopy (with and without biopsy) and imaging studies such as abdominal computed tomography and ultrasound, which should be normal in IBS.

Additionally, the patient history must meet the following diagnostic requirements:

• Recurrent abdominal pain, on average, at least one day per week in the past three months, associated with two or more of the following:¹
  • Related to defecation
  • Change in frequency of stool
  • Change in the form (appearance) of stool
• Symptoms should have started at least six months ago

Connection between IBS and weight loss

IBS can cause weight loss in different ways. Diarrhoea can cause food to go through the digestive system too fast, potentially leading to malabsorption of nutrients. Constipation, on the other hand, can lead to loss of appetite or feeling full with just a few bites.

Furthermore, patients sometimes perceive symptoms like abdominal pain and bloating as worsened by eating, causing them to skip meals to avoid episodes.^2,3 Additionally, mental illnesses such as depression and anxiety are more prevalent in individuals with IBS, which may be due to genetic risk or caused/aggravated by frequent debilitating gastrointestinal symptoms. These can greatly affect eating habits, causing patients to neglect their nutritional needs.

Complications of weight loss in IBS

Rapid weight loss and food deprivation can greatly affect metabolic, hormonal, kidney, and psychiatric health, leading to harmful nutritional deficiencies and even mortality. Anaemia due to vitamin deficiencies is possibly a fatal complication. Moreover, it can cause one to lose not only fat but also bone and muscle mass, increasing the risk of physical disability, fractures, and osteoporosis.⁴

The ideal Body Mass Index (BMI) should be between 18.5 and 25, with research indicating health risks associated with being both over and underweight. The latter can weaken bones and muscles, slow down skin healing, and cause hair loss and hormonal imbalances that can lead to infertility.

Additionally, it can make individuals more susceptible to infections, hypothermia, and psychiatric problems, including problems with body image. Most importantly, however, research shows having a BMI below 18.5 is associated with increased risk of acute cardiovascular events—such as stroke and myocardial infarction— and mortality.⁹

Management of IBS-associated weight loss

Management of IBS-associated weight loss must address both the underlying symptoms of IBS and the nutritional needs of the patient. Dietary modifications to avoid trigger foods are fundamental. A diet low in poorly digestible fermentable carbohydrates, known as FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols), can improve IBS symptoms and reduce gastrointestinal discomfort.

Working with a registered dietitian is crucial for creating personalised meal plans that prevent deficiencies and promote weight maintenance or gain. 

Lifestyle modifications and psychological support, such as cognitive-behavioural therapy, can help manage stress and anxiety, which often exacerbate IBS. Medications may be prescribed to manage specific symptoms, such as diarrhoea, constipation, and abdominal pain. These can include antispasmodics, laxatives, or anti-diarrheal agents, depending on the individual’s predominant IBS symptoms. Sometimes probiotics and long-term medications like antidepressants can also be beneficial.

Regular monitoring by healthcare professionals is necessary to track weight, nutritional status, and overall health, allowing timely adjustments to dietary plans and treatments. In severe cases, nutritional supplements or enteral nutrition might be required to ensure adequate calorie and nutrient intake.

FAQs

What is IBS?

IBS is a gastrointestinal condition that causes abdominal pain and abnormal bowel movements, which can involve constipation, diarrhoea, or both.

What causes IBS?

The exact cause of IBS is poorly understood, but genetic and environmental factors such as gastrointestinal infections and traumatic stress can cause alterations in bowel function. These include altered gut serotonin signalling, the release of inflammatory cytokines, and changes in intestinal bacteria.

What is the connection between IBS and weight loss?

IBS can lead to weight loss in several ways, such as malabsorption due to diarrhoea, poor appetite, food avoidance, and a predisposition to mental illness.

What are the dangers of excessive weight loss?

Excessive weight loss and being underweight can cause a vast array of complications, such as vitamin deficiencies, anaemia, kidney damage, poor wound healing, muscle wasting, osteoporosis and fractures, hair loss, psychiatric illness, recurrent infections, hypothermia, and increased risk of cardiovascular events and mortality.

What is the treatment for IBS-associated weight loss?

The management and prevention of IBS-related weight loss focus on minimising symptoms and ensuring appropriate nutrition through diet and possible supplementation. Some patients find improvements by avoiding fermentable carbohydrates (low FODMAP diets) and other trigger foods. Symptoms can also be managed acutely with laxative, antidiarrhetics, and antispasmodic medication. Finally, some evidence suggests antidepressants can be beneficial for normalising gastrointestinal functions. 

Summary

Irritable bowel syndrome is a potentially debilitating condition that causes persistent gastrointestinal discomfort related to bowel movements, which can be associated with diarrhoea, constipation, or alternating instances of both. Episodes can often be triggered or worsened by certain foods or emotional stress, which should be avoided, alongside full adherence to dietary and therapeutic plans.

In addition, a common complication of IBS is excessive weight loss, usually due to factors such as poor appetite, diarrhoea-related malabsorption, or even deliberate food avoidance to prevent symptoms. Rapid weight loss and having a low body mass index can cause nutritional deficiencies and infertility, affect bones and muscles, and increase the risk of infections and cardiovascular mortality. As practitioners should be aware of this risk, it is fundamental to encourage a diet, lifestyle, and potentially pharmacotherapy that ensures good nutritional status and symptom prevention.