One of the hottest topics in 2017 was the microbiome, and I’m predicting that gut microbes will continue to stir the emotions of scientists and consumers in 2018.


For those who are not familiar with the more intimate content of your bowels, the gut microbiome is a term that covers the one to two kilograms of assorted microbes that live in your guts and are essential to your health. There are at least 1,000 different species down there, made up of trillions of different cells.

Although that is a big number, in the past it was wildly exaggerated. We are not ‘90 per cent bacteria’ and ‘10 per cent human’, as many books and articles have claimed, but more like 50:50. In fact, one researcher who helped explode that particular myth claimed the proportions are so similar that “each defecation event may flip the ratio to favour human cells over bacteria”.

As a medical student, I was taught that the main role of our gut microbes was to protect us from dangerous invaders and synthesise a few vitamins. Now we know they do far more than that. Among other things (like influencing our mood and weight), those little microbes help regulate our entire immune system. Big claims? Certainly.

Over the last half-century we have seen a massive rise in allergic diseases, such as asthma and eczema, caused by an overactive immune system. We have also seen a surge in autoimmune diseases, ranging from inflammatory bowel disease (IBD) to type 1 diabetes, which are primarily caused by an immune system that has got out of control. One of the reasons for this rise seems to be that over time we have laid waste to a particular population of microbes that live in the gut and are known to immunologists as the ‘Old Friends’. They’ve been given that name because they have evolved with us over millions of years, and are vital for our health. Without enough of your Old Friends around, so the theory goes, your immune system behaves like a drunken teenager, smashing up its own home. A recent study, for example, found that one of the Old Friends, a gut bacteria called Bacteroides, helps prevent IBD by recruiting white blood cells to kill a cell of the immune system that can trigger IBD.

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So why did it all go wrong? What has happened to the Old Friends? Well, a diet of antibiotics and junk food hasn’t been good for their long term health. We also know that children who are born by Caesarean section (which is increasingly common) are far more likely to develop allergic diseases later in life, possibly because they are less likely than those born vaginally to inherit their mother’s Old Friends.

The good news is that it’s never too late to try and give your Old Friends a bit of a boost. I am now a big fan of home-made fermented foods like sauerkraut, which are rich in living bacteria. I have also switched to a diet that has more of the foods that will help my microbiome thrive (mainly those that contain plenty of fermentable fibres).


I’m also steering clear of the prebiotics, probiotics and supplements that are sold in the shops. From what I’ve learnt, few have credible science behind them and most of what is sold is based on hype. That may change, though. Watch this space.

Have you tried a prebiotic or probiotic diet? What effect did it have on you? Let us know by tweeting us, letting us know on Facebook or emailing us.


Dr Michael Mosley is former medical doctor, health writer and BBC presenter. He’s best known as presenter of Trust Me I’m a Doctor on BBC Two but has also written a number of bestsellers about personal health and medicine, including The Fast Diet, Fast Asleep and Fast Exercise.