The upper class (doctors, lawyers) consumed more food then the working class because they could afford it, yet there was no obesity.
It's not about calories.
It's not about sugar.
It's the PUFAs making us fatigued and sick.
PUFAs are the evolutionary signal for the coming winter and they reduce our metabolic rate which ultimately leads to sickness.
All mammals seek PUFAs when they want to go into hibernation and if you give them a PUFA free diet they will burn through their fat-stores within days.
Listen to this for the quote in the subject: https://youtu.be/QmNzV15Wads?list=PL_7Y5WbwJV_uFMcCRNFbEhmEfV9bLyw2m&t=2142
Then listen to the whole conversation or start at 24:40 to skip the first topic which is specifically about kids.
There are two great channels: "David Gornosksi Fan Favorites" and also "Danny Roddy".
These are the people they have lot's of conversations with:
Brad Marshall (Ex-Keto. Cook, Pigfarmer, Geneticist.) https://fireinabottle.net/
Dr. Ray Peat (Ph.D. in Biology) http://raypeat.com/
Georgie Dinkov http://haidut.me/
Until recently i would have dismissed that and it's still difficult (i did keto and carnivore for the last 10 years!).
What they say is that starch turns into 100% saturated fats in your body (which is not saying it makes you fat) and the less PUFAs you have in your body the leaner and healthier you get. Not because of calories but because it increases your metabolic rate.