Stronger by Science covered this in a podcast episode maybe a year ago or so. There does seem to be reasonably good evidence that basal metabolic rates have dropped over the last century without any conclusive idea why, though a case was presented that it has something to do with toxin/pathogen exposure - the body works less when it doesn't have to fight infection. Another case was made for indoor temperature control, which another comment brings up.

It is nowhere near enough to "explain" the rise in obesity, though. It also doesn't sharply drop out of nowhere around 1980 or so. I've been using MacroFactor now for about two years, weighing myself every morning and weighing and measuring everything I eat. Looking at the log right now, it seems I've averaged 3,348 calories a day for the past 30 days. The weight estimate for me was 172.2 lbs 30 days ago and 172.0 lbs today. Seems I'm managing to use the large amount of energy I eat, in spite of being part of a population with decreased basal metabolism. If you're not using that much energy, then also eat less.

The Internet badly needs to stop galaxy-braining this stuff. Sure, obesity is multifactorial like anything else that happens to an animal body. Potentially specific foods are less satiating in the short run than others or simply take up less space in the gut. Some are easier to eat, literally taking less time and effort and even energy, than others. Some sort of endocrine disruptor or what not maybe messes with ghrelin and leptin levels. But all energy consuming systems are still subject to basic thermodynamics. If you use less energy, then eat less energy. There may be a variety of reasons you find that hard to do, but that is nonetheless what you need to do if this matters to you.

And if you're going to propose some kind of cause specific to your region, your country, even just to humans, to me that isn't enough. I've got four cats, all of whom are over a decade old now, and I've had them their entire lives. Among the varying personality traits are food motivation. You can see it very easily. Two of them have stayed exactly the same size no matter what. They often don't finish a meal and never eat more than they need. Two are constantly begging no matter what, always hungry. One of them is so motivated by food that he has figured out how to open a refrigerator, a screwtop Talenti container, a locked cabinet, which took him six months but he still figured it out.

They're not eating sugar, heavily processed foods, not subject to marketing aimed at humans, yet we still see the same differences. Given effectively infinite access to food, some of them will still only eat what they need and some of them will eat until they puke and then eat the puke. It's an innate personality difference. The humans of 1909 did not have near infinite access to food. Humans of today do. And we're still mammals. Whatever difference exists in appetite coupling to energy usage and food motivation in cats exists in us as well. I guess I'm lucky. I only eat what I need. Many people don't. I don't think the difference is going to be solely down to microplastics in the water or whatever that somehow I magically managed to avoid. It's just a spectrum along which animals differ.