Most diets require that you accept being hungry....really hungry. Yes, you will lose if you create a calorie deficit by only eating the prescribed weight watcher meals...a fate worse than death.

I can eat bacon and eggs for breakfast, steak and salad for lunch, and similar for dinner, and still slowly loss or maintain at a good level. No hunger (craving now and again, but that is not hunger).

I was skeptical when I 1st tried keto 15 years ago, but it worked great.

As someone stated, the only real way to do this study would be to put the subjects in cages for a year, and feed them measured amounts of food. Any study that does not have 24/7 monitoring of the subject is invalid, because the subjects will cheat and lie.