Diet doesn't seem to be a one size fits all proposition.

Different people have different metabolisms and different gut flora profiles.

It's more likely that everyone has a perfect diet that would be just right, but just for them. The particular bacteria in your gut might be very adept at turning lactose into calories, and not so in others. Things like this would help prescribe foods, and in certain cases where your flora are too helpful, maybe a fecal transplant from a skinny person.

When we have enough calories to meet our needs every day, we have to find ways to help people whose only way to stay thin is to walk around feeling starved, which used to be adaptive but now isn't.