There’s no doubt that exercise improves health, but it is hard to say by how much. A study of eighteen thousand runners in Denmark concluded that people who jog regularly can expect to live five to six years longer on average than non-joggers. But is that because jogging truly is that beneficial, or is it because people who jog tend to lead healthy, moderate lives anyway and are bound to have improved outcomes over us more slothful types, with or without sweatpants?
