G-flux is fantastic stuff. Dr. John Berardi started writing about it on T-nation back in 2016: https://www.t-nation.com/diet-fat-loss/g-flux and https://www.t-nation.com/diet-fat-loss/g-flux-redux
On the opposite end of the spectrum, you can be extremely obese on a minimal amount of calories. I'll never forget the patient who was 300 pounds and ate 400 calories a day. As you can imagine, her physical activity was extremely limited -- less than a few hundred feet of ambulation each day.
When she got nutritional counseling through her physician, they recommended she eat about 1100 calories a day -- which actually would have caused her to gain another pound of fat a week for a while, since her metabolism was so destroyed. No one had actually taken a dietary history with her. She reported a history of dieting "effectively" to lose weight by consuming only black coffee with minimal other food for a few days.
Ultimately, the misunderstanding of the metabolism as a "calculator" that's always the same instead of a "thermostat" that rapidly adjusts to starvation conditions is what causes diets to fail (and perpetuates the weight loss industry). After all, contestants on the Biggest Loser are famous for putting the weight back on -- of course they do, because their metabolisms thought their was a famine that was going to kill them.
How would a smart body respond to adequate nutrition after a famine? Well, it would store as many calories as possible as fat to prevent the next one. (See: hibernation in mammals.)
And how would a smart body respond to a consistent surplus of calories (compared to resting needs) balanced by a ton of physical activity? It would be as lean as possible, since it requires relatively less calories to move a lean body than a fat one.
G-flux is the exact opposite of a famine state, and it's what we see in indigenous hunter-gatherer populations around the world: normal-weight physiques, physical activity, and slow-digesting foods that are rich in protein (thus minimizing insulin, which results in obesity in the absence of sufficient muscular glycogen depletion). For example, the Pirahã people, who don't even store food: http://www.documentarytube.com/articles/how-the-piraha-people-became-the-happiest-tribe-in-the-world
Of course, we also see a lot of excellent physiques in CrossFit gyms, and CrossFit "work capacity" workouts burn a ton of calories. Coach Christian Thibaudeau talks about the G-Flux concept (aka Hypermetabolism) here: https://www.t-nation.com/training/destroying-fat
Thanks for sharing!