As plenty of frustrated middle-aged readers will know, exercise does not always lead to a drop in waist size. But a new paper from a group of researchers at Lund University, in Sweden, offers some grounds for optimism. For even if the flab obstinately stays on, it appears that exercise alters the way in which fat cells work, presumably for the better. Specifically, it does so by altering how those cells express their genes, making it the latest discovery in a rapidly developing and exciting bit of biology known as epigenetics. But what exactly is that?

Generally speaking (and ignoring still-experimental technologies such as gene therapy), the genes an animal is born with are those it is stuck with. Collectively, those genes encode every protein the animal produces. One common analogy is that the genome is a bit like a recipe book, with the genes (and the proteins they encode) mixed together to create a complete organism. Skilled chefs, though, are able to vary the taste of a dish by choosing precisely what quantities of individual ingredients go into the pot, and that, more or less, is what epigenetic mechanisms do. By adding specific chemicals to parts of the DNA, or by controlling how the DNA is packaged, an organism can adjust the degree to which particular genes are turned into proteins (the extent to which they are “expressed”, in the jargon).