Article written for Actif’s Mag, Digestive Health special issue #2, published in 2024.
Irritable bowel syndrome (IBS) is a common functional gastrointestinal disorder. It is characterised by recurrent abdominal pain associated with intestinal transit disorders, without any physical or chemical abnormality being detected.
Four subtypes have been identified based on stool experience:
- IBS-D = with diarrhoea
- IBS-C = with constipation
- IBS-M = mixed (alternating diarrhoea and constipation)
- IBS-U = undefined (normal stools)
Its worldwide prevalence is estimated at 15%, with a higher incidence in women and young adults (1). Although IBS does not lead to serious long-term complications, it significantly affects quality of life and incurs substantial medical costs.
The underlying pathophysiological mechanisms of IBS are complex and multifactorial, involving genetic predisposition, abnormal interactions between the gut microbiota, the enteric and central nervous systems, and immune alterations.
Probiotics, defined as live micro-organisms that, when administered in adequate amounts, confer a health benefit on the host, have attracted interest thanks to their ability to modulate this complex relationship.
Intestinal microbiota & IBS: a close relationship
The intestinal microbiota, made up of billions of micro-organisms residing mainly in the colon, plays a fundamental role in health. It is involved in several key functions such as nutrient metabolism, modulation of the immune system and protection against pathogens.
Inflammation & intestinal hyperpermeability: linked to dysbiosis
Disturbances in this microbial population, known as dysbiosis, have been observed in IBS patients. For example, researchers have noted a decrease in microbial diversity, with a reduction in the Bifidobacterium, and an excess of certain pro-inflammatory species belonging to the Proteobacteria (2). These changes have been associated with impaired intestinal barrier function and aberrant modulation of the immune response, two features frequently observed in patients with IBS.
Indeed, IBS sufferers have low-grade inflammation of the intestinal mucosa, although this is less marked than in chronic inflammatory bowel disease (IBD). Several mechanisms have been implicated, including disruption of the production of short-chain fatty acids (SCFAs), particularly butyrate, which is essential for epithelial health, and chronic activation of immune cells in the mucosa (3).
Another consequence of dysbiosis in IBS is increased intestinal permeability. This phenomenon, also known as ‘leaky gut’ syndrome, occurs when the tight junctions between the intestinal epithelial cells become distended: the intestinal mucosa no longer fulfils its role as a filter as effectively as it used to, and therefore acts as a protective barrier. This allows toxins, antigens, micro-organisms and other undesirable substances to cross the intestinal barrier and circulate freely in the body via the bloodstream. The result is a disproportionate immune and inflammatory response in the intestinal mucosa (4).
The brain-gut axis implicated
The gut-brain axis is a bidirectional communication pathway between the central nervous system and the enteric nervous system, influenced by the gut microbiota. This interaction involves immune, endocrine and nervous pathways (5).
In IBS, this communication is disrupted, which exacerbates the neurovisceral symptoms of IBS, such as abdominal pain and intestinal motility disorders. In particular, modulation of the production of certain neurotransmitters such as serotonin (around 90% of which is produced in the intestine) and GABA is involved (6).
How do probiotics work for IBS?
Probiotics, mainly lactic acid bacteria and yeast, act through several mechanisms to restore the balance of the microbiota and reduce the symptoms of IBS.
Homeostasis of the intestinal microbiota
Probiotics directly influence the composition of the microbiota by encouraging an increase in beneficial bacteria, while inhibiting the proliferation of pathogenic micro-organisms.
For example, administration of the genera Bifidobacterium and Lactobacillus can increase microbial diversity, which is reduced in patients with IBS. This restoration of microbial balance could improve the function of the intestinal barrier and reduce local inflammation (7).
Immunomodulation
Probiotics exert immunomodulatory effects by regulating interactions between the microbiota and the host’s immune cells. For example, they intervene in the production of mediators involved in inflammation, reducing levels of pro-inflammatory cytokines such as TNF-α and IL-6, while increasing anti-inflammatory cytokines, such as IL-10. These effects help to reduce the low-grade inflammation seen in IBS (8).
Reinforcement of the intestinal barrier
Some probiotics improve the integrity of the intestinal barrier by modulating the expression of tight junction proteins such as zonulin and occludin, reducing the intestinal permeability associated with IBS (4).
Improvement of the intestinal motility
Disorders of intestinal motility are common in IBS, whether they manifest as diarrhoea, constipation, or an alternation of the two. Some probiotics influence intestinal motility by producing metabolites such as SCFAs, which act on smooth muscle cells and enteric neurons (9). They can reduce diarrhoea by regulating the absorption of water and sodium into the colon or improve constipation (7).
Effects on the brain-gut axis
Several studies have highlighted the psychotropic effects of probiotics, which can regulate the production of neurotransmitters such as serotonin and GABA, thereby influencing the gut-brain axis. For example, some probiotics have been shown to have anxiolytic effects in IBS patients, which may alleviate the visceral hyperalgesia associated with the condition (10).
What is the clinical evidence?
Some meta-analyses of randomised controlled trials have concluded that probiotics improve the overall symptoms of IBS, and thus the quality of life of sufferers. They reduce abdominal pain and distension, as well as the production of intestinal gas and bloating. They also help to normalise intestinal transit by improving the frequency and consistency of bowel movements (11).
Combination strains are generally more effective than single strains, due to their ability to target different aspects of the pathophysiology of IBS.
Research has shown that Bifidobacterium infantis reduces abdominal pain and bloating and regulates bowel movements in IBS sufferers. In a clinical study published in 2023, Bifidobacterium infantis significantly improved IBS symptoms after 8 weeks of treatment (12).
Other clinical trials indicate that supplementation with Lactobacillus plantarum provides relief for patients suffering from IBS, not only by acting on symptoms such as abdominal pain, flatulence, diarrhoea and constipation, but also by improving their mental well-being and quality of life (13).
In a randomised controlled trial in 2020, Saccharomyces boulardii yeast showed benefits in the management of IBS, particularly in improving stool consistency in cases of IBS-D, but also IBS-C and IBS-M. It also reduces abdominal pain (14).
Probiotics offer a promising alternative approach to the management of irritable bowel syndrome due to their ability to modulate the gut microbiota, strengthen the intestinal barrier and regulate the immune and neurovisceral response. The results of the clinical trials are encouraging, although further research is needed to identify the optimal doses, and the patient subgroups most likely to benefit from this approach. Future studies should also focus on the underlying molecular mechanisms and the specific interactions of probiotics with the gut-brain axis.
LC
References
Picture: Freepik
(5) CRYAN et al. The microbiota-gut-brain axis. Physiol Rev. 2019, 99(4) :1877-2013