I remember watching a documentary on IFC about Woody Hayes from 1974 and he was boasting about having the "biggest offensive line I've ever had" (up to that year), averaging 265lbs per lineman, and I'm guessing they had only a little fat.
Also, several years ago, I watched a documentary from 1975 on the making of the movie "Jaws". There were several scenes showing all of the extras and scenes of many other people just walking around the boardwalk and other open areas by the beaches and almost all of them looked anorexic compared to people in America today.
It was at about this time that the American woman became fully "liberated" from all of the "oppressive" responsibilities and duties that she had to "endure" for many millennia, including family-centered tasks such as making good, wholesome, home-cooked meals for her husband and children. Fast food joints started to slowly crop-up in more and more places as a substitute food source as the now career-empowered American woman was simply too busy with her new Corporate responsibilities and duties to even bother with it anymore, if she even remembered or learned how to do it in the first place.
As the years passed, giant-sized SUV's replaced Station-Wagons to help accommodate their now Super-Sized asses as there was now a fast-food joint on every corner. Going to the grocery store now became a burdensome task as it would involve getting out of the giant-sized SUV and actually walking into the store and around the isles to find the kid's favorite brand of Doritos, hot dogs, and soda, and, eventually, walking back to the giant-sized SUV and climbing back in...such an energy-sapping task after working all day at the office. Getting to the couch and not missing a minute of American Idol, The Bachelor, or CSI sounds so much better....priorities, priorities. So, just take the damn kids to McD's, Burger King, Taco Bell, Wendy's, Pizza Hut, or whatever and get them some fries to go along with their mystery meat sandwich and then head right home and to the couch.
There's more to it, but that helps explain some of the "how"...at least for White people.
For blacks, it's much easier to explain...just eventually drive away all of the good grocery stores from where you live by shoplifting, looting, robbing, and soiling them so that the profit margins are just too small and the insurance premiums are just too high to keep them there. Then, when you become hungry, your only alternative is to go the local convenience store or McDonald's or Taco Bell to satisfy it over and over and over again.