> we should put them in a sterile room and study them

We have.

https://news.ycombinator.com/item?id=49319339

> Body adapts significantly over time when subjected to weight loss.

> There are compensatory physiological adaptations that favor weight regain, such as changes in energy expenditure and appetite.

> Even with sustained effort, maintaining the initial weight loss may become progressively more difficult because of those changes.

> This shows up in research as results like mean weight loss of ~9% at year 1 and ~5% at year 8.