Stories about what foods are good and bad for you, which foods are linked to cancer and which have beneficial qualities are always popular online and in the news. But how do experts know what people are eating? Tim Harford speaks to Christie Aschwanden, FiveThirtyEight’s lead writer for science, about the pitfalls of food surveys. She kept a food diary and answered nutrition surveys and found many of the questions were really hard to answer – how could she tell all the ingredients in a restaurant curry; and how many tomatoes did she eat regularly over the past six months?