One simple explanation is protein. Modern diets have terribly low protein levels, and much of that protein comes from sources with extremely poor digestibility [1], meaning the effective protein consumption levels are even worse than they seem.

Protein is a major appetite suppressant. I'd read this but never really realized how true it is until starting on a protein heavy diet. I now eat near 500g of chicken breast a day, and by the end of the day I'm basically having to force feed myself. Yet 500g of chicken breast is less than 800 calories! And the other stuff is just to balance out my diet.

And I love eating as much as anybody else. But you simply cannot eat that much when you're on a protein heavy diet. Finding data on protein consumption rates over time is difficult, but I'd hypothesize we were probably much higher on effective protein consumption in the past. I know absolutely nobody that regularly had something like 'steak and eggs' for breakfast, yet it seems that was indeed a thing at some point.

[1] - https://en.wikipedia.org/wiki/Digestible_Indispensable_Amino...

Meat consumption per capita has almost doubled since 1960: https://ourworldindata.org/grapher/per-capita-meat-consumpti...

Meat consumption is associated with higher BMI: https://pubmed.ncbi.nlm.nih.gov/32443920/ (summarized in: https://www.news-medical.net/news/20200615/Vegetarians-have-...)

That's world, not USA. And the obesity inflection point was in the mid to late 70s. Total USA meat consumption since 1980 is up about 9%, with an interesting shift in distribution (as per the ourworldindata link): -11kg beef/year, -4kg pork, and +31kg poultry. It also says nothing about protein. Those numbers should suggest an increase in protein, but that's not entirely clear due to changes in the way food is consumed and the way it's measured. For instance a 10 piece set of McDonalds chicken nuggets weighs around 163 grams, but contains only 23g of protein. By contrast 163 grams of chicken breast contains just about exactly 50 grams of protein. The more processed something is, the more it tends to drift from its nominal nutritional value, and people are certainly eating more processed junk than ever before.

Observational studies on diet are completely irrelevant because they invariably end up comparing some group of people opting into some diet or another, versus the population at large. An apples to apples comparison would be something closer like comparing those on a long-term vegetarian diet to those on a long-term carnivore or keto diet. The idea is to just make sure you control for the "I care about my diet and have sustained dietary self control" variable.

That's world, not USA. And the obesity inflection point was in the mid to late 70s. Pew has an excellent study on American diets from 1970 to 2010 here. [1] In short our consumption of meat, in terms of calories, is pretty much flat-lined, and at an abysmal 400. But grain products, fats/oils, and sweetener consumption has skyrocketed.

Something you have to keep in mind in modern times is also food processing. A 10 piece set of McDonalds chicken nuggets weighs around 163 grams, but contains only 23g of protein. By contrast 163 grams of chicken breast contains just about exactly 50 grams of protein. The more processed something is, the more it tends to drift from its nominal nutritional value, and it's safe to say that a much larger chunk of people's "meat" is also coming from these sort of heavily processed meat-like foods.

As for the studies, you have a common bias they suffer from. You're comparing some group of people opting into some diet or another, versus the population at large. An apples to apples comparison would be something closer like comparing those on a long-term vegetarian diet to those on a long-term carnivore or keto diet. The idea is to just make sure you control for the "I care about my diet and have sustained dietary self control" variable. Unfortunately actually controlling studies means you'd get far fewer exciting headlines and bullet points for our CVs.

[1] - https://www.pewresearch.org/short-reads/2016/12/13/whats-on-...