>>example of how exercise can result in weight gain.

My experience is the same (n=1). My BMI was right at the borderline between 'overweight' and 'obese' - but I could sit on the bottom of the pool (fresh water) with full lungs.

>> Exercise for 250 minutes each week

I was in the water 5 hours a week, so more than that. Started with a lot of fat (both visceral and external flab; had a serious 'spare tire' going, even had a female friend jokingly ask me when I was due) Lost all that within the first 6 months, and yes, I had tried innumerable sit ups, with the only result being they got really easy to do.

I got two benefits: one, pretty immediately apparent, was that I could eat like a marathoner in training, pretty much a 'see-food' diet. The other, about 25 years in, was surviving a brain infection that my surgical team assured me I otherwise wouldn't have. (and even so, it was touch and go).

Oh, and, yeah, I got pretty good at sliding through the water, with a fairly non-standard stroke I'd come up with (sort of a cross between a new-paradigm crawl and a sidestroke) to accommodate the fact that I was a sinker. Glad to share it with any other sinkers out there...