Probiotics, and why the strain matters

What a probiotic is

A probiotic is a live microorganism that, given in adequate amounts, benefits the health of the person taking it, which is the definition agreed by international experts [1]. Each one is named by its species and its strain, and the strain matters because reviews weigh strains and doses separately and find their results differ [2].

A useful label gives the full name, with a species followed by a strain code, and the number of live organisms in colony forming units (CFU) per daily dose, guaranteed to the end of its shelf life.

Where the evidence is strongest

The best evidence is for preventing diarrhoea caused by antibiotics. In a Cochrane review of 33 trials in 6,352 children, diarrhoea affected 8% of those given probiotics compared with 19% of those who were not [2]. The review found a credible benefit at high doses of at least 5 billion CFU a day, naming Lactobacillus rhamnosus and Saccharomyces boulardii as examples that larger trials still need to confirm [2]. Probiotics also shortened diarrhoea by almost a day on average, and no serious side effects in the included trials were put down to them [2].

For Clostridium difficile, a gut infection that can follow antibiotics, a Cochrane review of 31 trials and 8,672 people found it in 1.5% of those given probiotics and 4.0% of those who were not [3]. The benefit was seen in trials where the starting risk was above 5%, mostly in hospital, and was not shown where the risk was lower [3].

Where the evidence is weaker

For many common reasons people take probiotics, such as general gut comfort or immunity, the evidence has not been strong enough for an authorised health claim. Every claim worded as probiotic on the Great Britain register is listed as non-authorised, most often because the product was not characterised well enough to be assessed [4].

The one authorised claim for live bacteria is that live cultures in yoghurt or fermented milk improve digestion of the lactose in that product, for people who have difficulty digesting lactose [4]. UK guidance treats the word probiotic itself as a health claim that needs an authorised claim beside it [5].

Who should not take probiotics without advice

For most healthy people, short courses of probiotics alongside antibiotics appear safe, and the side effects seen in trials were mild, such as cramping or soft stools, and as common in those who did not take them [3].

Serious infections caused by the organisms in a probiotic have been reported in people who are severely ill or have a weakened immune system, including those with a central venous line [2]. The US regulator has warned that probiotics can cause invasive, potentially fatal infection in preterm babies in hospital [6].

Speak to a doctor first if any of these apply.

• You have a weakened immune system, for example from chemotherapy or transplant medicines
• You have a central venous line, or you are seriously ill in hospital
• You are considering a probiotic for a premature baby
• You have ongoing gut symptoms that have not been explained

When to seek care urgently

Diarrhoea with blood, or with signs of dehydration such as passing very little urine, needs prompt medical care. If you feel very unwell, call 999 in the UK or the local emergency number.

How to choose a probiotic with your doctor

A doctor can help you decide whether a probiotic suits your reason for taking it and, if so, which strain has been studied for that use. Starting one alongside a course of antibiotics, where the evidence is clearest, is a different decision from taking one every day for years without a defined aim.

• Match the exact strain to the use it was tested for
• Check the dose in CFU, and that it is guaranteed to the end of shelf life
• Follow the storage advice, as some products need a fridge
• Take it for a set time with a clear reason, and stop if it is not helping

How The Wellness can help

A GP appointment is the place to talk through gut symptoms and whether a probiotic has a role for you. For people who want a deeper look, our gut microbiome testing is explained on its own page, and a doctor can help you weigh what it can and cannot tell you.

Every doctor is GMC registered, and our clinics are on Baker Street in Marylebone and Eccleston Street in Belgravia.