I honestly have no idea if I have excess bodyfat (not weight; at last check I was well under 140Lbs, which makes me lighter than some decidedly not overweight people I know, some of whom are shorter than me), but if I did and wanted to get rid of it... I have quite a few obstacles, the biggest being financial and Akrasia-from-Hell. Mostly that last one, because lack of akrasia = more problem-solving power = better chances of escaping the wellfare cliff. (I only half apply Akrasia to diet and exercise; it's rather that my options are limited. Though reducing akrasia might increase my ability to convince my hindbrain that cooking implements other than the microwave aren't that scary.)

So, personally, all my problem-solving ability really needs to go into overcoming Hellkrasia. If there are any circular problems involved, well, crap.

But I'm assuming you've encountered or know of lots of fat rationalists who can totally afford professionals and zany weight loss experiments. At this point I have to say that no one has convinced me to give any of the popular models for what makes fat people fat any especially large share of the probability. Of course I would start with diet and exercise, and would ask any aspiring rationalist who tries this method and fails to publish their data (which incidentally requires counting calories, which "incidentally" outperforms the honor system). Having said that, though, no one's convinced me that "eat less, exercise more" is the end-all solution for everyone (and I would therefore prefer that the data from the previous hypotheticals include some information regarding the sources of the calories, rather than simply the count).

(I'm pretty sure I remember someone in the Rationalist Community having done this at least once.)