Yeah, but if there is an intervention, then it is not purely an observational study. The big problem with nutrition science is that most of the long term studie

Yeah, but if there is an intervention, then it is not purely an observational study. The big problem with nutrition science is that most of the long term studies are purely observational. There is no intervention at all. All they are measuring is how certain observed measurements correlate (or not) with other observed measurements. These are the types of studies I'm talking about.