It is a requirement of modern politics that political leaders show off their fitness. No Prime Minister before Tony Blair thought it necessary, except for Edward Heath, who revelled in being a world class yachtsman.
Others thought it good for the image to have unhealthy habits, such as Winston Churchill with his cigars, and Harold Wilson with his pipe. But lean and healthy is the way to go now. Whether this picture flatters David Cameron is a matter of opinion, but he is not in bad shape – albeit for a man of 45 with a desk job.