A new way of finding out how diet affects gut microbes

AS THE world’s fatties clock up the kilos, their excuses for being that way have piled up, too. Big bones, junk foods, genes or poor parenting—there are plenty of directions in which to point a chubby finger. In the past few years, a new potential culprit has emerged: gut bacteria. Human guts are full of bugs that help digestion and also stop their disease-causing counterparts from invading. In this age-old symbiosis, some bacteria are better than others at providing food to their human hosts—and also seem, by mechanisms yet unknown, to encourage those hosts’ bodies to store that energy as fat and to keep the fat on.

In the past, when food was in limited supply, these bacteria would have been valuable allies. In an era of plenty, though, they are problematic. In particular, work on mice suggests obesity is associated with having a high proportion of bacteria called Firmicutes, whereas the lean favour another group, the Bacteroidetes. Such work has also raised the suggestion that transplanting “lean mouse” microbes to fat mice can make them thinner—for a while. …