I just spent the last six months dieting down to 6% bodyfat and I haven't once gone under 2000 calories. Other than women, I know zero bodybuilders who can sustain anywhere near 1200 a day. Hell, Neggy Shelton's death was just exposed in the Washington Post from extreme dieting and she was doing about 900 calories a day, plus 2.5 hours of steady state cardio, and she was a 124 lb woman.

Those Minnesota starvation study guys you're referencing had to use an extreme deficit to get down to concentration camp weights like that over the course of a year. It doesn't say in there, but that article with Michael Rea was written in 2006, and finding other stuff on him, he seemed to have been doing calorie restriction since at least 1999. A small calorie deficit can add up when you keep at it for a very long time. Even if his TDEE was 2200 or so, that would drop 30 pounds or so in a year eating 1900.

For whatever it's worth, I've been that size, and it was not on purpose. I grew very quickly in middle school and was extremely active. By 8th grade, I was 6'2" 120 lbs and it took many years before I got much bigger than that, and I used to buy entire boxes of donuts, Little Debbie snacks, and family-size bags of potato chips on the way home from school and eat them in addition to the pizza and cheeseburgers they served at the school. And when I got a car and started driving all my friends home, we'd usually go by McDonald's every day, and in addition to just the actual food, I'd get a McFlurry, pretty much every single day.

But unlike everyone else who seems to be wondering how that was possible and what happened when they were skinny in youth, it's not a mystery to me at all. I sometimes played basketball for 18 hours in a day. When I was driving my friends home, that was usually after cross-country practice, running for up to three hours after school. I walked or rode my bike everywhere. I almost never sat down. I rarely even slept. I could eat basically anything because it may as well have been the Michael Phelps Olympics diet. I was insanely active and using a ton of energy every day.

And yeah, when life normalized in my mid-20s, I too gained a bit for a short while, then I adjusted and ate less, and now I'm very lean again, but also bigger because I started lifting and learned to eat on purpose, in an intentional, measured way calibrated to actually meet my energy needs, not just ad hoc having whatever I crave at any given minute and not thinking about it. I understand why people don't want to do this. Even though I'm not restricting like him, at least not permanently, my diet habits are basically like Michael Rea's. I weigh everything, prepare all my own food, and eat pretty much exactly the same thing every day. It probably sounds like a slog and people want to just free graze and wonder why we can't live like humans must have lived for most of the past 300,000 years, when diets and hunger levels seemed to calibrate to energy needs automatically, nobody thought about it, yet almost nobody was fat except a tiny number of super rich idle nobility. But that isn't the environment we live in any more. People are extremely sedentary and randomly selected food from anywhere is utter trash loaded with extra calories in every possible form for no good reason. Unless you have the activity level of a middle schooler from the 90s or an Olympian, you need to actually try.