My friend had bariatric surgery yesterday and before he did it I researched it on the Cleveland Clinic website (a very reliable source).

What I found there is what this article gets around to in a meandering way. Apparently modern medicine knows that diet and exercise mostly can't change a person's body from their set weight. The stats bear it out.

Both the clinic and the article conclude that surgery helps the body learn a new weight by removing the stomach tissue that signals hunger and changing the person's overall hormone levels.