Probiotics in capsule form: what do large clinical trials show?

Probiotics in capsule form: what does the clinical evidence actually show?

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Abstract

Probiotic capsules are often marketed as a universal solution for everything from bloating to mental well-being. Looking at randomised controlled trials (RCTs), the reality is more nuanced. The effect is not uniform, but strain-, dose-, combination- and indication-specific: evidence for one strain cannot be extrapolated to other strains or to probiotics in general. This article reviews the evidence across the most studied areas and the professional guidelines that assess it.

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Introduction

Our gut hosts billions of microorganisms that play a role in our health. When we take probiotics in capsule form, we add live bacteria to this ecosystem. But there is a big difference between taking a supplement to "support health" and having documentation that a specific strain, at a specific dose, works for a specific condition. A product without a stated strain designation cannot be linked to strain-specific documentation.

See also our overview of gut microbiome, probiotics and IBS.

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Clinical results: what does the evidence show?

1. Protecting the gut during antibiotic treatment

Antibiotics affect both disease-causing and beneficial gut bacteria, which often leads to diarrhoea. Studies suggest that specific strains - particularly Lactobacillus rhamnosus GG or the yeast Saccharomyces boulardii - may reduce the risk of antibiotic-associated diarrhoea when taken alongside a course of antibiotics. Whether they also prevent infection with Clostridioides difficile is less certain: meta-analyses suggest a possible effect in high-risk patients, but the studies are heterogeneous, and neither European nor American guidelines recommend probiotics routinely for this purpose.

2. Irritable bowel syndrome (IBS)

Irritable bowel syndrome is one of the most common reasons people buy probiotics. Here the evidence is heterogeneous, and the major societies assess it differently:

• AGA (2020) recommends probiotics for IBS only within the context of clinical trials, because the overall evidence is judged too uncertain for a general clinical recommendation.
• ACG (2021) suggests against probiotics for the treatment of global IBS symptoms. The recommendation is conditional and based on very low quality evidence.
• BSG (2021) concludes that probiotics as a group may be effective for global symptoms and abdominal pain, but that no specific species or strain can be recommended. It considers it reasonable to try probiotics for up to 12 weeks and stop if symptoms do not improve. The recommendation is weak and based on very low quality evidence. Findings for one strain cannot be transferred to others.

The disagreement between the guidelines is not evidence of a class effect for probiotics or support for any particular product [2,3].

There is no single strain that can be named a general first choice for IBS. Multi-strain combination products have been studied in some trials, but results cannot be generalised to all multi-strain products.

3. Chronic constipation

Some studies have examined the effect of probiotics on intestinal transit time and bowel movement frequency in chronic constipation. Results vary considerably between strains and products, and there is no basis for a general, fixed-figure effect that applies to probiotics as a group. Any effect should always be interpreted in relation to the specific strain and dose tested in the study in question.

4. Chronic inflammatory bowel disease (IBD)

For the serious chronic inflammatory conditions of the bowel, probiotics are not a documented standard treatment:

• Ulcerative colitis: Some studies have examined specific probiotic strains as an adjunct to standard treatment in mild to moderate disease, but the evidence is limited and not sufficient to recommend probiotics as standard treatment.
• Crohn's disease: Placebo-controlled trials have generally not shown a reliable effect of probiotics.

5. The gut-brain axis: mental health

Research into so-called "psychobiotics" examines whether specific bacterial strains can affect symptoms of anxiety, depression or sleep. A few meta-analyses have found statistical associations, but the field is marked by substantial heterogeneity in strains, doses and study quality, and large, long-term trials are lacking. These findings should be seen as an evolving research area - not as evidence that probiotics can be used to treat mental health conditions.

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The technical corner: what affects a capsule's effectiveness?

Two factors influence whether the bacteria in a capsule reach the gut:

1. Acid resistance: Stomach acid is strongly acidic (pH approx. 1.5-3.5) and can kill unprotected bacteria. Some capsules are therefore enteric-coated (acid-resistant), so they only dissolve in the more neutral environment of the intestines.
2. CFU (Colony Forming Units): CFU indicates the number of live bacteria and is only meaningful in relation to the specific strain, dose and shelf life examined in a given study. A high CFU count is not in itself proof of quality or effect, and there is no general dose recommendation that applies to probiotics as a group.

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Safety

Probiotics are generally well tolerated in healthy people. Specific risks have been described in people with severe illness, significant immunosuppression, a central venous catheter or a compromised gut barrier, where the bacteria have in rare cases been linked to infection. In these situations, use of probiotics should be assessed individually with the treating physician.

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Conclusion

Probiotics are not a single, uniform treatment, and there is no general "reference standard" recommendation that applies across strains and conditions. The evidence is strongest for selected strains in preventing antibiotic-associated diarrhoea. In IBS the evidence is heterogeneous, and AGA, ACG and BSG do not agree in their recommendations. In IBD, chronic bloating and as a general "gut health cure", probiotics are not a documented standard treatment. Choosing a specific product should be based on the specific strain, dose and indication that have been studied.

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Assessment and treatment at Kirurgen.dk

We work up gastrointestinal symptoms with colonoscopy, gastroscopy, coeliac screening and faecal calprotectin. See also: Gut microbiome, probiotics and IBS, Chronic diarrhoea, Chronic constipation, SIBO, Low-FODMAP diet for IBS.

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References

1. American Gastroenterological Association (AGA). AGA Clinical Practice Guidelines on the Role of Probiotics in the Management of Gastrointestinal Disorders. Su GL et al. Gastroenterology 2020;159:697-705.
2. American College of Gastroenterology (ACG). ACG Clinical Guideline: Management of Irritable Bowel Syndrome. 2021.
3. Vasant DH, Paine PA, Black CJ, et al. British Society of Gastroenterology guidelines on the management of irritable bowel syndrome. Gut 2021;70(7):1214-1240. PMID 33903147. doi.org/10.1136/gutjnl-2021-324598 - gut.bmj.com/content/70/7/1214
4. World Gastroenterology Organisation (WGO). WGO Global Guidelines: Probiotics and Prebiotics. 2023.