I remember watching a documentary of a woman who had been obese but lost the weight and was thin and healthy at an older age. She said she measured the amount of calories she actually burned vs. what a person with a normal metabolism would burn when exercising and it was something like she'd burn 2 for every 5 she should have been burning, because her body wanted to get her back up to obese because that had become the new normal. I'm a lot more inclined to believe this about someone who had overcome it, even if it meant devoting a significant portion of their day every day to keeping it off, than a fatty who just uses it as an excuse not to try. Also, there's evidence that you can have mutations in genes that cause your metabolism to be slower, or even just epigenetic profiles that lead to this effect.
There is no real "normal metabolism" it varies quite a bit with height, weight, muscle mass (gender) etc.
What she was probably referencing, in a pseudo-scientific way, is that yes, as you lose weight, your metabolism slows down as your body simply needs less energy at 160lbs as it does at 320lbs. There's a reason you can safely consume 1K calories a day for months, even years, when you're super obese, because the body is burning off the stored fat to provide energy. If a starving person tried that, they'd be dead within that time frame as the body would have no stored energy to consume and would start cannibalizing critical organs and systems.
If she was really onto something and could prove it -- that when exercising she burned only 40% of the calories as say Joe Schmoe Marine Grunt in basic training you'd have every military in the world racing to provide such a logistical advantage to their soldiers, not to mention every pro sports team and bodybuilder out there trying to duplicate her magic, er, results. Tour De France riders would probably literally kill for a trick that could reduce their energy consumption by 60%.
There are disorders that affect a vanishingly small amount of the population (under 1%) that can affect BMR, satiety (food craving) and in general "muh cundishans" -- but unless you believe the genetic profile of Americans has changed incredibly in only 50 years you can't blame the incredible fact that nearly 40% of adults are now clinically obese.
What changed? It wasn't "muh cundishans" it's the access to incredibly cheap empty calories via Starbucks, soda, fast food, gas station chips, etc. These empty carbs and sugar laden foods and drinks lead to increased blood sugar, low satiety levels, and if abused long enough serious issues like brittle bones, fucked up endocrine systems (makes men more like women with increased Estrogen and women more like men with increased T) and loss of lean mass.
Most of the fad diets like paleo, gluten free, keto work because they trick people into following CICO without actually thinking about CICO. Like it's really fucking hard to overeat on bacon and burgers and avocados. You can, but it's entirely easier to eat 2.5K calories via a bag of Doritos then sitting down and consuming 2lbs of ground beef or eating 6 avocados in one sitting.
Part of the problem was / is the demonization of fat in the diet, fat provides satiety in a way that sugar and carbs do not. This is another reason I rec most super obese people try a low carb diet like paleo or try to do Keto.
