In the middle ages, only royalty and elites were fat because they were the only ones who had access to the abundance of food that made it possible. It was seen as a good thing to be fat. Hell, in today's society the "Health At Any Size" movement portrays morbid obesity as no big deal at best, and a good thing at worst.

> But you can see in the stats that obesity hit an inflection point in the mid-1970s

This is about where mass production of hyper-processed foods hit critical mass of cheapness and availability.

Americans have easier and cheaper access to nearly infinite amounts of food that middle age kings could only dream of.

My bet is that a significant chunk of this is the push of populations out of cities and into car-focused suburbs. Matches up quite well timeline wise as second-order effect of the post-war suburban boom.