We read almost every week of new research into the deleterious effects of sugar on our bodies. In the US, the latest edition of the government’s official dietary guidelines includes a cap on sugar consumption. In the UK, the chancellor George Osborne has announced a new tax on sugary drinks. Sugar has become dietary enemy number one. ... This represents a dramatic shift in priority. For at least the last three decades, the dietary arch-villain has been saturated fat. When Yudkin was conducting his research into the effects of sugar, in the 1960s, a new nutritional orthodoxy was in the process of asserting itself. Its central tenet was that a healthy diet is a low-fat diet. Yudkin led a diminishing band of dissenters who believed that sugar, not fat, was the more likely cause of maladies such as obesity, heart disease and diabetes. But by the time he wrote his book, the commanding heights of the field had been seized by proponents of the fat hypothesis. Yudkin found himself fighting a rearguard action, and he was defeated. ... In 1980, after long consultation with some of America’s most senior nutrition scientists, the US government issued its first Dietary Guidelines. The guidelines shaped the diets of hundreds of millions of people. Doctors base their advice on them, food companies develop products to comply with them. Their influence extends beyond the US. ... We tend to think of heretics as contrarians, individuals with a compulsion to flout conventional wisdom. But sometimes a heretic is simply a mainstream thinker who stays facing the same way while everyone around him turns 180 degrees. When, in 1957, John Yudkin first floated his hypothesis that sugar was a hazard to public health, it was taken seriously, as was its proponent. By the time Yudkin retired, 14 years later, both theory and author had been marginalised and derided. Only now is Yudkin’s work being returned, posthumously, to the scientific mainstream.