Overweight, obesity and weight loss are complicated indeed. The New York Times published an article last year on some of the Biggest Loser contestants who were tracked for 6 years following the show. Their resting metabolic rates have never recovered: https://www.nytimes.com/2016/05/02/health/biggest-loser-weight-loss.html
Here's the link to the scholarly publication: http://onlinelibrary.wiley.com/doi/10.1002/oby.21538/full
Another NYT article discusses how weight loss is far from 'one size fits all': https://www.nytimes.com/2016/12/12/health/weight-loss-obesity.html?action=click&contentCollection=health&module=NextInCollection®ion=Footer&pgtype=article&version=series&rref=collection%2Fseries%2Fthe-science-of-fat
It's all pretty far from 'calories in/calories out.'
No kidding. My body most definitely processes some types of calories differently from others. I can eat a LOT more calories from vegetable fats and protein without gaining weight...in fact, I have to be careful not to LOSE weight eating like that. Swap in calories from sugar and simple carbs, or fatty calories from animal products, and wa la! This has been true for me for as long as I can remember, but it really was walloped into my head again a few months ago.
Case in point, I am at risk for diabetes, so have eaten low glycemic for years. Also, I can't digest dairy well, so mostly avoid it. I just spent 2.5 years struggling to stay over 100lbs, even though I reduced aerobic activity, glopped olive oil and nut oils on top of my standard meals, and added hemp milk, avocados, nuts, everything high calorie/low glycemic/healthy that I could handle on TOP of my standard meals. Could NOT get weight over 102, and would slip back down if I stopped adding above listed high calorie items. It was scary, honestly.
Eventually, I couldn't handle glopping extra oil, nuts, etc, on my food....I was tired of struggling and thinking about it constantly, and resigned myself to being scarily skinny. So I went back to eating my usual healthy, moderate fat, low glycemic way. So I reduced calories pretty notably. But my weight did not change AT ALL. I stayed right at 101 lbs.
Then, about 6 months ago (around holidays), I fell into
very slightly unhealthier, higher glycemic eating for about 2 months. Ate more animal products, some ice cream (maybe a serving per week), a few cookies here and there. I was still eating my normal meals, but with some additional calories this time coming from animal fats and sugar. For the first couple of weeks nothing changed at all, even though I had added calories again.
But then, a month or so in, it was like my body suddenly adjusted to my 'new' diet, and BAM! I gained 9 lbs in 2 weeks! It was absolutely astonishing. I was thrilled but didn't want to keep eating crap, so I eventually tapered off the cookies and ice cream and saturated animal fats. I went back to eating my usual way, lower calorie, lower glycemic.
Except now, several months later, my weight hasn't budged. NOW, my body seems happy to sit at 110/111 lbs.
It's crazy! No wonder weight loss is such a puzzle to people. Calories in/calories out, my ass! Literally!