First, I take issue with the cited article because the study referenced in the opening paragraph because they do not provide any discussion over limitations of their study. In particular, that members of the "Biggest Loser" show may have also been more extrinsically motivated by money and fame than intrinsically motivated to become healthy. Thus, when they failed to earn said money/fame, they returned to their normal behaviors.

> people who aren't willing to massively restrict their diet and live in discomfort for the rest of their lives

I also take issue with this statement. Many fitness motivation images on the Internet talk about rejecting temptation to work towards your goals. As I was thinking about this comment, I was reminded of Ben Stiller's pizza scene in the movie Dodgeball [1]. Jokes aside, the scene is a hyperbole to how even fit people are susceptible to temptation and take actions to not giving in. Not to Stiller's degree, but I can distinctly remember times where I just stare at the ice cream aisle with grocery shopping. It is my own mental discipline to not buy it. Quitting World of Warcraft to get in better shape is also a part of my past. Shifting to an active lifestyle is hard, it has a difficult initial start, but persevering through it has helped me become the healthier person I am. I also recall Jesse Shand's transformation [2], where I believe one of the phases of his weight loss was simply reducing the number of fast food meals he'd consume in a single sitting (instead of 8, only eat 7, etc).

[1] https://www.youtube.com/watch?v=PPmX2gNIqE8

[2] https://www.youtube.com/watch?v=jpy1FlGKYz0

There's a big difference willpowering yourself to stay at your natural state of skinnyness and willpowering through hunger-regulating systems in your body being permanently broken. Here's another reference if you didn't like the other study. https://www.latimes.com/science/sciencenow/la-sci-sn-weight-...

You still seem to confound skinniness and willpower and again, the second link I showed someone well into 40+ BMI changing. Secondly, this article has a few issues as well. The participants are older, which we know is difficult to lose weight at (which does support your claim, but not for the same reasons). The article also does not appear to compare the participant's appetite increases to those of "healthy" individuals. What does an "8" represent, and would any person show the same signs?

I will assume that formerly obese people have higher cravings. No doubt a former heroin addict has cravings. I will accept that it is harder for these individuals. But again, that does not mean you shrug your shoulders and say "that's the way it is".

I’ve never known anyone who was “naturally fit”, defining fitness as the ability to work out at a certain level of intensity for a given amount of time.

I’ve known people to be naturally skinny or even tone but not “fit”.