So what you're saying there's a great business opportunity in people paying to get locked in a diet-cell?
I've heard of other attempts at things like this. Generally you are not locked into a cell, you are removed to a very remote location by bus so that if you want to leave you have to go through a formal withdrawal process - while waiting for the bus - during which they convince you to stay). They then not only control your diet they also give you exercise (often lead by military drill instructors) thus being a healthier environment than a diet cell. I have no idea how much money they make.
Doing studies with humans ist just hard!
You can of course study these people, but the only study anyone is interesting in is how different changes in conditions affect how much people lose and how much money you can get out of them.
That's what I always thought about the kind of research RFK Jr is always talking about. Normally it's not ethical to do food / medicine trials with prisoners, but these would be trials like giving regular food to one set of prisoners and food without dyes or chemicals to the other. The "test group" would just be getting healthier food.
Seems like just radically measuring portion sizes might fit into the same kind of thing. And you could probably measure activity level more easily, too.