Do probiotics work?

Mint

19-07-2025

A DAZZLING menagerie of microbes live inside the human gut—by some counts a few thousand different species. Most residents of this gut microbiome are not the disease-causing kind. In fact, many do useful jobs, such as breaking down certain carbohydrates, fibres and proteins that the human body would otherwise struggle to digest. Some even produce essential compounds the body cannot make on its own, like B vitamins and short-chain fatty acids, which help regulate inflammation, influence the immune system and affect metabolism.
As awareness of the microbiome has grown, the shelves of health-food shops have become stocked with products designed to boost good bacteria. These usually fall into two categories: probiotics, capsules containing live (but freeze-dried) bacteria that, in theory, spring back to life once inside your gut; and prebiotics, pills made of fibres that beneficial bacteria feed on.
There are good scientific reasons to tend one's microbiome. Having a diverse assortment of gut bugs, with plenty of the good kind, seems to confer broad health benefits. A varied microbial population can fend off pathogens like E. Coli, by competing with them for nutrients and space. Reduced diversity, by contrast, has been linked to obesity, type-2 diabetes and irritable bowel syndrome. There is also evidence that having greater microbiome diversity, as well as a higher total number of microbes, are associated with improved recovery from gastrointestinal surgery.
The microbiome's influence stretches well beyond the gut. A diverse microbiome seems to be important for brain health too: people with depression have less microbial variety in their guts than those without do, for example. One study from 2023, published in the journal Brain, found that transplanting the microbiome of patients with Alzheimer's into a rat caused the rat to develop memory problems. An off-kilter microbiome has also been linked to respiratory infections: mice with fewer gut microbes are more likely to catch pneumonia or influenza.
To keep the microbiome healthy, diet is key. Microbes thrive on foods rich in fibre and digestion-resistant starch, so munching on fresh fruit, vegetables, legumes and nuts is a good place to start. Fermented foods and drinks, such as yogurt, sauerkraut and kombucha, also contain friendly micro-organisms like Lactobacillus. Avoiding unnecessary antibiotics is important, as they wipe out good bacteria along with the bad.
Supplements seem equally appealing, but because they are not regulated as medicines, many have not been rigorously tested. 'It is absolute cowboy territory in terms of marketing", says Ted Dinan, a psychiatrist at University College Cork who studies the influence of the microbiome on mental health. Fortunately for consumers based in America, Britain and Canada, academics in those countries have developed apps (each called The Probiotic Guide) that can be used to search for probiotic products and check what scientific evidence, if any, backs them up. Nothing so comprehensive exists for prebiotics, as yet.
Taking the wrong product may not do much good, but it probably won't do much harm either. 'You really cannot overdose on probiotics," says Glenn Gibson, a microbiologist at the University of Reading. Taking too many prebiotics, however, could temporarily disrupt the microbiome. The likely side effect? 'Gas", he says. 'But that's more just antisocial than anything else."
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