> We can't put a bunch of humans in cages for their entire lives and feed
> each set a different number of calories and see how it comes out 120
> years later.

It looks several people are trying thing of their own will.
They dont look particularly happy.
I gone on weight control diets for long periods of time in the sub-2K
range.
After a couple weeks of initial discomfort, it gets easy. AT that
point you
switch to a low-metabolism mode and can even stop losing weight.
Its difficult to continue this for a long time.