Once upon a time I spent most of my research time reading about obesity and I remember from back then that people in the field were disagreeing on what causes obesity and how to tackle it.
The concept of a weight "set point" was floating around since people noticed that it is very hard to lose weight in a stable manner, but once a new setpoint was reached and stabilized, the body would defend that setpoint. Similar to temperature regulation (the main topic in my current lab).
The idea is, that something changed in the past 20-30 years that lead to a global change of the setpoint. This thread (and associated book) go through some of the possibilities that caused this change.
I've recently read "The Obesity Code", by Jason Fung and have applied and bought into much of Fung's theory of insulin resistance driving much of the world's obesity. The idea of a chemically manipulated set point and insulin resistance may not be mutually exclusive either. Pollution in our environment could also cause disruption in insulin and other hormones that regulate satiety. I've shed 14 pounds since April by adhering to a low carb (50 or fewer grams daily) and avoiding any low- fat foods ( fat usually replaced by refined carbs) and I'm feeling great. Caveat, I'd gained nearly 20lbs. from Jan 8th to early March 2021, as I was prescribed a decent dose of the cortico-steroid Dexamethasone for several weeks. I'd had brain surgery to stereo-tactically ablate brain cancer cells. Dexamethasone, given to prevent brain swelling, disrupts the hormone insulin, My blood sugar levels became pre-diabetic, and I woke each morning with hunger that I'd never known before. I ate like crazy, craving refined carbohydrates. I now buy into the idea that in our world of food-abundance, regulating our insulin production and preventing insulin-resistance is pivotal to staying fit.
My insurance company sent out a letter saying "hey, wanna diet? We'll pay for it" which is the second time they've done that. The first one was basically "chew your food more, bitch" but at least they sent a pack of peanuts. The second one was "pick from this handy smorgasbord of programs" and I went with the gut health one in no small part because they didn't advertise it as a diet, they played up "chronic disease" and I am still dealing with various'n'sundry post-COVID bullshit. Also free Withings scale (which is kinda rad) and free Fitbit (which is a chunk of shit). Their basic approach is eat hippie chow until you have "reset your metabolic base point" at which point you add foods back in slowly. I'm at the "add foods back in slowly" thing Monday. Me and their nutritionists have gotten in a couple fights because I refuse to say that I enjoy eating hippie chow. They called me unprompted on Friday to say "it's not forever! You can eat like everyone else soon!" and I said "okay how many 'items' am I going to have to add back in before I can have a grilled cheese sandwich" and they were all "those uhh aren't whole foods" and I said "yeah so a grilled cheese sandwich is on the never menu and you're wondering why I say your food is aggressively uninspiring and that I hate eating now." On the plus side, I'm almost back down to the weight i was at when I finally beat the last nutritionist over the head with Okay fuckers you consistently whine about how my BMI says 'obese' despite the fact that you keep complementing my muscle tone so here you go, fux, show me what you got and they came back with obviously Mr. Bl00 you need to eat more leafy shit and protein for your gut health and I gained 20 lbs in 3 months. There's maybe nobody I hate more than nutritionists. "Ohhh, it's not your fault, still you should definitely feel guilty for your lifestyle choices!" This last plan you take pictures of your food and they were all "uhhh you're only supposed to eat quinoa once a day" and I came back with "fuckers I've been shitting pesto for three days bakdafukup" and then they decided maybe I could have a slice of Ezekiel toast every now and then. Meanwhile the eating habits I had as a teenager, which allowed me to drop 100lbs in three months, that were called "exercise bulimia" and, according to my insurance, have made me ineligible for anything but inpatient psychiatric treatment due to the health risks, are now called "intermittent fasting" and they're all the rage.
I think it's the lack of things like fiber and proteins and so on in the processed American diet. Fiber and proteins and fats make people feel full, and thus stop eating. I've done fairly well for myself sticking to a semi-paleo diet (I allow quality cheeses and 100% whole grain breads). But what I find is that I can chow down on processed versions of real foods a lot more than I can of the whole version of the same thing.
Another suggestion not raised here is that the increase in obesity is driven by rising carbon dioxide levels (example discussion). That would also potentially explain why higher altitude locations have lower weight gain.
Just found out that there is even more details on this on the blog, look for parts 1 till 10