BIOLOGISTS are shining a scientific spotlight on previously neglected inhabitants of our bodies. The 100-trillion bacteria that populate an average adult, living mainly in the guts, should be treated with more respect, recent research shows, because they play a vital role in maintaining human health.

The latest study by Chinese scientists provides new evidence that the different types of microbes in the gut can help to explain why some people grow fat and others don’t. The microbiome, as it is known, turns out to be a risk factor for obesity, alongside eating too much, exercising too little and having the wrong genes. Our digestion has evolved to work with the help of a kilogram or so of microbes; the way we extract calories from food and how fast our appetite is satisfied — determinants of how much weight we put on — depend on the balance between the 200-300 bacterial species that live in a typical human gut.

As several recent studies have shown, this balance depends critically on what we eat. People living on a modern western diet have a very different microbiome from their pre-industrial ancestors. And a study of elderly people in Ireland, published in the journal Nature recently, showed a direct relationship between diet, bacterial diversity and wellbeing. Although more research is needed to prove the point, evidence is growing that a varied diet, including plenty of unprocessed fruit, vegetables and cereals, promotes the growth of beneficial bacteria in the body.

The message for the holiday period, as you eat your festive foods, is to remember that you are feeding not just yourself but all the bugs in your body. Treat them to a varied diet — and do not poison them with unnecessary medication.

London, December 21.