BearFotos/Shutterstock.com Standing by the counter at the pharmacist waiting to pick up my prescription, I couldn’t help noticing the prominent display of probiotics on the counter. It was two years ago, and I was reading everything I could find on microbiomes and probiotics – whether in books, journals or in shops – in preparation for writing my book The Microbiome: What Everyone Needs to Know . For days I had focused just on probiotics and here they were, temptingly in front of me, ready for me to buy. The packaging was so glossy and it’s claims so intriguing, I found myself picking up the box to see what they were saying. “Supporting gut health.” “Friendly bacteria.” I was about to get antibiotics for my tonsillitis. Should I get some probiotics? I’d heard they might help replace the “good” gut bacteria that antibiotics can wipe out. The pharmacist knew me by sight, partly because he had just looked down my throat and prescribed them for me and partly because I’m a local GP. He nodded encouragingly and pointed at the display. “These are very popular,” he said. I turned the box over. The packaging did best when describing what it contained. Thirty capsules to be taken every day, each containing 5 billion live cultures. I compared it with the others on the shelf. Some contained 2 billion, some 10 billion. One contained 25 billion bacteria per capsule. It was a huge number and a huge dosage range. Were these dosages safe? It wasn’t so clear on what live cultures were exactly, describing them variously as “trusted” or “friendly”. Higher-dose brands described themselves as “diverse” or “powerful”, sounding more like the boardroom of a Fortune 500 company than a dietary supplement. When it came to what they did, things became vague. Apparently, probiotics are there to “complement your natural gut bacteria” or alternatively to “complement your everyday life”. It took a bit of time for the pharmacist to package up my medication and label it, so I carried on and read the small print. Each brand was very confident its ability to survive the stomach acid: they were also confident on the research. “Most researched live culture.” “Highly researched strains.” I had no difficulty in believing this, it was the lack of claims to efficacy that baffled me. Finally, I found the actual ingredients. Each listed their various combinations of bacteria, some containing up to 15 different sorts, but always including several versions of lactobacilli and bifidobactera. Lactobacillus acidophilus I knew as a bacteria needed to make yogurt. Bifidobacteria are also often used in the food industry. Both are typical residents of our guts, known to account for about 12% of our usual gut bacteria . So why do probiotic products all seem to contain the same bacterial species? And why are their claims always so deliberately vague? Almost one in 20 adults are taking probiotics: typically those of us with higher educational levels, higher incomes and better diets. If we just knew a bit more about microbes, would we still want to take them? Stomach acid – the great destroyer It is normal to consume a lot of bacteria on our food. Even with freshly washed or cooked food, on a typical day we consume 1.3 billion bacteria a day either on or in our food. As soon as our food hits the stomach, our high levels of stomach acid kill or injure almost all the bacteria we consume. Only a few ever reach the colon and those few probiotic bacteria that survive usually only ever stay a few days. But to swallow a probiotic capsule containing 25 billion, is 20 times the number of bacteria our body is used to handling: a huge microbial load. Even “friendly” probiotic bacteria can cause a serious infection if they get in the wrong place, such as the blood stream. It’s true that most people can manage this huge microbial load fine because of our innate gut defence systems. But probiotics should be avoided by those with weak immune systems, who may be less able to keep these bacteria contained and are at higher risk of them spreading and causing infection. The reason that out of all the millions of bacteria available in the world, probiotic brands always home in on exactly the same microbes is because these are all bacteria that are known to be safe or used in the food industry since before 1958. If a microbe is officially designated “Generally Recognized As Safe” , then the producer need undertake no further research. And if the producer then sticks to general claims of efficacy – what’s known as a “qualified health claim”, they don’t even have to prove it works. Generally Recognized as Safe explained. But even with no efficacy claims at all, the probiotic industry still seems to get its message across – and, as I handled the box of probiotics, I still had a strong feeling that this product was good for me, would make me healthier and that I should buy it. I held the box uncertainly. “Do you want these as well?” the pharmacist asked. I checked the price: £17.99 for 30 probiotic capsules (low dose) for something I already had inside me from eating ordinary food. I decided to stick to the antibiotic prescription only, for £9.90. So, do probiotics work? I have learned to equivocate when asked this, because people who ask me – usually enthusiastically and with a smile – are invested in the concept of probiotics and have often already been taking them. To avoid upsetting people I now usually say: “Well, they probably haven’t done you any harm.” Apart from the cost. Berenice Langdon is the author of: The microbiome: What Everyone Needs to Know, published by Oxford University Press.