The more dominant that guidelines become, the more dangerous they are As a child of the 1950s I grew up knowing that margarine, though nearly as nasty as cod liver oil, was good for you. Doctors had been pushing it for decades, or so the advertisements told us. As early as 1915 Sainsbury’s advertised its “Nuts and Milk Double Weight” margarine under the slogan “Doctors recommend it,” although no doctors were named. As late as the 1990s hard margarines containing trans fats were sold as a healthy alternative to butter, their claims backed by a medical consensus. Two weeks ago the BMJ published a study by Pelham Barton and colleagues on the effectiveness of cardiovascular disease prevention programmes.1 It concluded that eliminating trans fats from the diet would, over 10 years, prevent 27 000 deaths, gain 570 000 life years, and return a cash benefit of £2bn (€2.3bn; $3.3bn) in England and Wales alone. Its effects far outweighed a 5% reduction in cholesterol concentrations or in systolic blood pressure or a reduction in salt intake of 3 g a day. Far from being “heart smart” (as one product claimed), hard margarines were heart stupid. Equally stupid—it now appears—was the 1990s mantra that the route …