Analyzing anything that has to do with food. You basically need to lock up people and monitor them eating if you wish to determine any effects at all. It‘s a bit like studying any behaviour: sure people say they do something, but if you wish to link lettuce to any benefiz, your first challenge will be to measure actual lettuce eaters and not only people saying they eat letzuce (who may or may not actually eat lettuce, eat less, say lettuce when they mean cauliflower or spinach etc).