We talk about an “obesity epidemic” and yet we cannot usefully test for its presence like chlamydia or hepatitis, nor is it possible to “infect” others with it. If one of the famous fat men of yore, Jack Falstaff, had written “If sack and sugar be a fault, God help the wicked!” in a Saturday column, he would have been met with stern looks and po-faced letters to the editor: “Don’t you know how much lifestyle-related conditions cost the NHS?”