Probiotics: what they are, what they do and when to take them

What are probiotics, prebiotics and fermented foods?

A probiotic is a live microorganism that provides a health benefit in an adequate amount. The scientific definition requires that benefit: a live bacterium alone is not enough [1]. The microbiota, by contrast, is the community of microorganisms that already live in our bodies. Read our gut microbiota guide to learn more about caring for it.

A prebiotic is a substance that certain microorganisms selectively use and that provides a demonstrated benefit [3]. Some fibres meet this definition, but not all. Inulin and certain oligosaccharides are examples that have been studied [3]. So seeing “with fibre” on a label does not automatically mean “prebiotic”.

A fermented food is made through the action of microorganisms that transform the food [4]. Yoghurt and kefir are familiar examples. Fermentation can change taste, texture and digestibility. It does not automatically make every food a probiotic: the specific benefit of its microorganisms would still need to be demonstrated [4].

“Functional food” is a broad term for foods said to have an effect beyond providing nutrients. It does not guarantee effectiveness for a disease. It is worth asking what effect is claimed, at what amount and on what evidence.

What are probiotics used for?

Antibiotic-associated diarrhoea is one situation in which some formulations have shown preventive value. The World Gastroenterology Organisation guideline reports a moderate effect in adults and children taking antibiotics [2]. This does not mean that everyone taking antibiotics needs to add a product.

If diarrhoea has already started, the question changes. Its duration, hydration and any warning signs need to be assessed. Buying any product to “restore the flora” skips that assessment.

For irritable bowel syndrome, studies of some strains have found mixed results for symptoms such as bloating and gas [2]. The AGA does not recommend their routine use outside clinical trials [5]. If a healthcare professional considers a product, they should check the strain and amount studied for that indication.

A strain studied for one symptom does not make all probiotics a solution for that symptom.

A doctor makes the diagnosis when symptoms persist or change. In a consultation, we can review your diet, tolerance and questions about a product. The aim is to decide whether it is worth trying and how to assess the result, without stopping medical follow-up.

Can they help with weight loss, immunity or mood?

For weight loss, a review bringing together published meta-analyses found an average weight reduction of about 600 g [6]. The authors warn that confidence in this estimate is limited by the quality of the studies [6]. This does not support promising meaningful weight loss from a capsule .

“Boosting immunity” is not a precise indication either. A review in healthy adults found limited and mixed effects on immune markers [7]. A laboratory marker does not, by itself, mean fewer illnesses. We ask that any claim name a specific health outcome and the population studied.

For depression, a review of seven trials in people with a diagnosis found modest effects on symptoms and no consistent change in the microbiota [8]. Probiotics are being studied as an add-on, with no universal protocol for treating mood . If you are concerned about your mood, speak to a professional who can assess it.

The same caution applies to claims about “balancing” the microbiota based on a commercial test. One isolated laboratory result does not tell you which strain you need. First, clarify the symptom and what decision would change based on that test.

Choosing between yoghurt, fermented foods and capsules

Yoghurt is a food. It can fit into breakfast or a snack if you tolerate it; there is no need to turn it into a medical intervention. Its taste and nutritional value may be reason enough to choose it. There is no need to attribute an effect on every digestive symptom to it.

Other fermented foods, such as kefir, contain different microorganisms and have different characteristics depending on the product [4]. We cannot apply findings for one to another. If you are interested in kefir for a child, see our guide to kefir for children for questions about age and safety; we do not give children’s guidance here.

A capsule may identify a strain and a precise amount. This lets you compare it with what was used in a trial, but the format does not guarantee that it works. The choice depends on the indication, safety and product quality [2]. For general digestive discomfort, starting with foods you tolerate is usually simpler than buying several products.

How do you read a probiotic label?

Look for the full strain name: genus, species and a code that distinguishes it. The global guideline explains that clinical evidence is tied to identified strains or combinations at a studied amount [2]. If only the genus is listed, you cannot check whether it is the same strain.

CFU, or colony-forming units, indicate viable microorganisms. Check whether the number is given per dose and guaranteed until the best-before date. Microorganisms can lose viability during storage [2]. Also check the storage instructions. A huge number printed prominently does not replace research into the expected benefit.

In the European Union, health claims on foods must be authorised. Countries differ in their rules on using the word “probiotic” on its own [9]. In Spain, AESAN allows it to appear on a label but clarifies that it cannot be accompanied by an unauthorised health claim [9]. So neither the word nor a picture of a gut proves effectiveness.

If you want to compare two products, note the strain, CFU at the end of shelf life, population studied and claimed effect. If any of these details are missing, ask before buying. If we cannot link the product to specific research, the reasonable choice is not to attribute that effect to it.

Who should take extra care before using them?

People with severe immunosuppression or a significant underlying illness need medical advice before taking live microorganisms [2]. It is also worth asking if you have a central venous catheter: an official clinical fact sheet reports infections in vulnerable people with this device [10]. Safety findings in healthy adults cannot be assumed to apply to them.

For premature babies, the neonatal team should decide whether to use them. The US health authority has warned about invasive infections associated with probiotic products in this group [11]. If the baby is in hospital, any decision should be made with the neonatal team. Do not improvise a plan for other children based on adult studies either.

If you take medication, have an active illness or have been advised to undergo a digestive test, bring the product to your medical appointment. A professional can assess whether it might interfere with your plan or whether it is better to wait. Label instructions do not replace an individual review.

What mistakes do we see in consultations?

• Assuming that more strains or more CFU mean a stronger effect. Without a studied outcome, these numbers only describe the product.
• Taking them after every course of antibiotics, just in case. Prevention has been studied during treatment and with specific strains [2].
• Thinking that natural means risk-free. These are live microorganisms, and for some people the decision is medical.
• Continuing to eat a fermented food that makes you feel unwell because it appears on a list of healthy foods.

We also see people buying products because of a vague “dysbiosis” label, without knowing which symptom they want to improve. Agreeing on a specific question is more helpful: is there diarrhoea while taking antibiotics? Is there bloating alongside a diagnosis of irritable bowel syndrome? If a new or severe symptom appears, finding its cause comes first.

There is not enough evidence to recommend probiotics for SIBO, candidiasis or bloating on its own. For symptoms in the upper digestive tract, our gastritis guide explains when to discuss the cause with a doctor.