Gut bacteria
What are probiotics? A complete guide
Probiotics are live microorganisms that, when taken in adequate amounts, provide a health benefit. Their effects are specific to the strain and to the condition. The evidence is good in a few situations, such as preventing diarrhoea linked to antibiotics, and weak in many others. Healthy people do not need a daily probiotic. Fibre and fermented foods do more for the gut’s own bacteria.
In short
- A probiotic is defined by its strain, its dose and the evidence for a benefit.
- Different strains do different things. A result for one does not apply to another.
- Use a probiotic for a specific purpose, for a defined period, and stop if it does not help.
- People with very weak immunity or who are critically ill should not take them without medical advice.
How are probiotics, prebiotics and postbiotics different?
| Term | Meaning | Example |
|---|---|---|
| Probiotic | Live microbes with a demonstrated benefit | A named strain of Lactobacillus or Saccharomyces in a capsule |
| Prebiotic | Food for beneficial microbes | Fibres in onion, banana, oats and pulses |
| Synbiotic | A probiotic and a prebiotic together | Combination products |
| Postbiotic | Non-living microbes or their components with a benefit | Heat-treated bacterial preparations |
| Fermented food | Food transformed by microbes | Curd, buttermilk, kanji |
See prebiotics, postbiotics and fermented foods.
How is a probiotic named?
By genus, species and strain, for example Lactobacillus rhamnosus GG. The strain is the part that matters, because benefits shown for one strain cannot be assumed for another of the same species. A label that lists only “lactobacillus” tells you very little.
Where is the evidence reasonably good?
With antibiotics
Certain strains reduce the chance of antibiotic-associated diarrhoea and of a serious gut infection that can follow antibiotics.
During H. pylori treatment
Certain probiotics reduce the side effects of the antibiotic course.
Pouchitis
A specific multi-strain combination helps to prevent inflammation of the pouch created after surgery for ulcerative colitis.
Premature babies
Specific strains reduce a serious bowel complication in preterm infants, under hospital care.
Where is the evidence weak or mixed?
- Irritable bowel syndrome: some trials show benefit and others none. Guidelines do not recommend probiotics for overall IBS symptoms; a four-week trial of one product is reasonable.
- Acute infectious diarrhoea: results differ between studies and regions.
- Crohn’s disease: no clear benefit.
- General health, immunity, weight and mood in healthy people: not established.
How should a probiotic be used?
Have a reason
A condition for which a particular strain has been studied.
Choose by strain and dose
The label should state the full strain name and the number of live organisms present up to the expiry date, not only at manufacture.
Give it four weeks
If there is no benefit by then, stop. Trying a different strain is reasonable; taking several at once is not.
With antibiotics
Take the probiotic at least two hours apart from each antibiotic dose, and continue for a week or two afterward.
Store it as directed
Heat and humidity kill live organisms.
Who should avoid probiotics?
For most people they are low-risk, with gas and bloating the usual side effects. Rarely, the organisms have caused bloodstream infections. For that reason they should not be used without specialist advice by people who are critically ill, have a central venous line, have severely weakened immunity, or have a damaged heart valve.
Do I need a probiotic if I eat curd?
For general health, no. Curd, buttermilk and other fermented foods supply live bacteria, and fibre feeds the ones already living in your gut. A probiotic product is a targeted tool, not a daily necessity. See is curd a probiotic?
When should I see a doctor?
See a doctor for symptoms that last more than a few weeks. Using probiotics for months to treat undiagnosed bloating, diarrhoea or pain delays the diagnosis.
Questions patients ask me
Should I take a probiotic every day?
Not unless there is a specific reason. Benefits last only while the product is taken, and most healthy people gain nothing measurable.
Can probiotics cause bloating?
Yes, especially in the first week, and in people with bacterial overgrowth. If it persists, stop.
Are more strains and higher counts better?
No. What matters is whether that strain, at that dose, has been shown to work for your condition.
Can children and pregnant women take probiotics?
Commonly used strains have been given to both groups in studies without significant problems. Check with your doctor before starting.
Do probiotics settle in the gut for good?
Usually not. In a study that sampled the gut lining directly, an 11-strain probiotic settled in some people and not in others, and finding the bacteria in the stool did not show which. Its effect on the gut was temporary. That is why any benefit generally lasts only as long as the product is taken.
Sources
This page is written for patients and is based on the guidelines and studies below. It is for education and does not replace a consultation.
- International consensus statement on the term probiotic. Hill C et al. Nat Rev Gastroenterol Hepatol. 2014. PubMed 24912386
- American Gastroenterological Association guideline on probiotics in digestive disorders. Su GL et al. Gastroenterology. 2020. PubMed 32531291
- American College of Gastroenterology guideline on irritable bowel syndrome. Lacy BE et al. Am J Gastroenterol. 2021. doi:10.14309/ajg.0000000000001036
- Maastricht VI/Florence consensus on Helicobacter pylori infection. Malfertheiner P et al. Gut. 2022. PubMed 35944925
- International consensus statement on fermented foods. Marco ML et al. Nat Rev Gastroenterol Hepatol. 2021. PubMed 33398112
- Study of whether probiotic bacteria settle in the human gut lining. Zmora N et al. Cell. 2018. doi:10.1016/j.cell.2018.08.041