(I wrote this as a comment reply, but decided it should be its own post)
History so far:
Pre-20th century: Most people ate a diet low in carbs and high in animal fats. Heart disease and diabetes are almost non-existent.
Early 20th century: Seed oil is marketed as replacement for animal fat. Crisco and margarine are king.
Mid 20th century: Heart disease has become a thing. A sensible person would say, "shit, must be these vegetable oils!" Nope. American Heart Association declares that animal fats are the problem. Seed oils become even more popular.
Late 20th century: Heart disease has accelerated. In 1977 the United States Government declares that fats are the problem (hey, at least they are half right which is better than the government usually does). Low fat/high carb is recommended.
Later 20th century: Starting in 1980 the obesity and diabetes rates are a hockey stick (and you don't even have to merge to completely different data sets to get it). A sensible person would say, "shit, it must be the low fat/high carb!" Nope. In 1992 the food pyramid is released and it recommends low fat/high carb.
Early 21st century: Obesity and diabetes continue to soar! Obesity is the new bitcoin! To the moon! 2005 revised pyramid. low fat/high carb. 2011 Pyramid is trashed for circle. Low fat/high carb. Changing the shape of the recommendation didn't help strangely enough.
Future: Who knows what horror this Beyond Food and bug burger future holds for us. Just like the early 20th century push for seed oil was a successful attempt to sell agricultural bioproducts as food so is the 21st century push for plant protein. The pea protein in Beyond burgers is a byproduct of glass noodle production in China.
My point is NEVER EVER EVER trust "the experts" on nutrition because they will always be in favor of whatever is profitable (and make your own mayonnaise).
Don't forget BMI. If you fit these arbitrary numbers you are this or that. No tests for strength or exercise, or even body fat.
BMI is solid enough for 90%+ of the population. Unless you're a trained distance runner or a heavy weight lifter BMI is a quick, accurate-enough measurement.
One can argue that BMI understates the number of people that should be considered obese. You can have a 'healthy' BMI while still being rather unhealthy, especially if you have a sedentiary lifestyle.
I'm not making excuses, but the history of food has been muddled since the dawn of man!
People thought that potatoes caused a disease that could only be cured by a king, tomatoes were deadly but that turned out to be because of the cutlery, corn flakes were meant to stop masterbation and Nutella was helped in creation by the Napoleonic wars.
I'll give credit to the 20th century lot, they generally seem to trying to kill us compared to previously where we just kept betting a guy to eat shit and if he didn't die it was safe for us too.
That "stopping masturbation" crap needs to stop. People who use that sneeringly tend to leave out the fact that the Kelloggs were working in an insane asylum. They weren't "just masturbating", these were sickos who couldn't stop masturbating/would do it in public or wherever they felt like. He felt that perhaps diet could help with that.
Unfortunately that's something that's been mixed up in history, at least they didn't build a giant statue on a literal throne in your capital to it...
It isn't just diet. People flat out used to walk more. They didn't necessarily ride everywhere. Of course, shops were generally closer than they tend to be now (thanks to cars), but they wouldn't think of getting on a horse just to go from one end of a one-street town to the other.
They also did more physical labour, in all kinds of contexts. Pretty much everything was inherently more labour-intensive than they are now, in the age of electricity, and natural gas. We don't even need to chop wood to cook or keep warm all winter, except perhaps as supplemental heat.
This is the other side of the coin as to why fatness used to denote the tribal chieftain in certain cultures. It wasn't just that they got all the food - they also didn't have to lift a single finger to do anything.
While physical activity is important I think it is only a tiny piece of the puzzle compared to diet. Humans don't need all that much physical activity to stay healthy. A laborer may burn 4000 calories per day and a professor may burn 2000 but if they are both insulin resistant with a carb heavy diet they will both likely be too hungry and overconsume calories. If they are insulin sensitive with an animal fat heavy diet then they will both likely be satiated and not overconsume calories. It's just the laborer will naturally eat twice as much to fit their need.
Have to take issue with this. You have to go back to hunter/gatherers for this claim. With the adoption of agriculture and the rise of settlement the human diet went crazy for grains. Look, for one example, at the peasant diet of medieval England. It's virtually all bread and ale.
It's not so much carbohydrates per se that lead to inflammation and vascular/heart disease but the explosion of the use of refined sugar and some seed oils that began in earnest with industrialization of the food supply.
Couldn't they just find a way to turn it into booze like normal people?