The first paragraph alludes to the fattening of America in the recent past. Something has changed.

It's worth noting, in the context of this whole discussion, that the increasing homogenization of Americans' eating habits because of franchise restaurants and mass-produced supermarket food is a possible change that is consistent with the theory behind the Shangri-La Diet.

The message that I'm taking home from your post is that if we wish to be "less wrong" we should avoid sarcasm.

YES. And not just sarcasm, but a whole variety of devices of speech that fall under "not speaking with a straight face". I've seen arguments couched in sarcasm with real rhetorical force, that simply could not be stated in straight-face language without looking obviously fallacious; and that's leaving aside the effect you discuss, which is that it makes it harder for other people to discuss what you've said, or for that matter the straightforward way it makes the heat/light ratio worse.

I think that as we move from rationality as a lonely art to rationality as a group art, speaking with a straight face is one of the things we should be promoting. Though I'm sure I haven't kept to this rule myself...

Hypothesis: at least some fraction of the weight gain is a result of dieting. A fair number of people regain more than they've lost.

As a result of the will-power and obligatory virtue model, some people go through the cycle many times.