Almost every interesting and serious study regarding lifestyles is ethically and practically unable to be instantiated as a controlled experiment over the long term. Like many things, from studies of the economy, to social structures, to the climate. You can try to do a multivariate analysis, but ultimately you're not going to have much to go on. You have to take everything with a grain of salt, but it doesn't mean there isn't some valuable data in there. In addition, long term results are probably more interesting and important than short-term results; so much so that I would argue longitudinal studies are probably more useful than controlled studies, so long as one is careful and rigorous and vigorous with multivariate analysis after the fact.