I’m not sure there is such a thing as the “standard American diet”, and I’m not sure any of us know a whole lot about how, exactly, people become obese.
One who lives on “most” fast foods is likely to become obese, due to the high fat and calorie content of them, and if they eat a lot of it. But not all fast foods are like that, and therefore, not all eaters of fast foods are eating the same kind of the thing other eaters are.
Some people eat a lot of “take out” from fast food or “near-fast” food places. Some of that is not very good for a person. Some people eat a lot of “near-ready” stuff from grocery stores. Some of it is generally regarded as “unhealthy”. Some isn’t.
Some people eat very reasonably, but become obese because they don’t get any exercise. Some people eat reasonably and get reasonable exercise but become obese because they gobble twinkies or something on the side, or simply eat too much “healthy” food. I have known some pretty chubby vegetarians in my time. On the other hand, I had an aunt who ate two ribeye steaks, two eggs, toast and butter every morning for breakfast and ate just as generously the rest of the day. She was skinny all her life and lived into her nineties. She had a sister (neither married) who lived with her and ate like a bird, but stayed a bit heavy.
Some people eat a lot of high-fat foods. Some people eat too much of foods that are not high in fat.
Some people are skinny. Some are at about the right weight. Some are a little obese, and some are a lot obese. I don’t think we really know, for sure, what all the differences are.
Look at Renaissance paintings; how rolling fat so many of those people, particularly the women, are. They didn’t have McDonald’s or highly processed foods. Their meat-eating was doubtless less than ours, because the only really big meat-eaters at the time were the Brits. But by our “ideal” standards today, they are quite obese. Obviously, the Renaissance painters didn’t think they were grotesque or unusual. That’s probably what their models looked like, and they thought them “normal”, even “ideal”. Even many of the men; the “heroes” and the “athletes” have “handles” men would be embarrassed to show at the swimming pool today. Late Nineteenth century paintings have a lot of fat people in them. The “beauties” of the time would not be given a second look today, because they’re so fat.
It’s interesting to look at old World War II photos of soldiers. They are a lot “fatter”, by and large, than are soldiers today. It’s really remarkable. Yes, if they were fighting on Iwo Jima for two months, and not eating half or more of the time, they won’t be fat. But look at the photos of soldiers who have not been out in some jungle for a month. Even their faces are fat. No doubt their food was packed with fats and calories, to keep their energy up. (So are MREs. I have tried to eat them, and can’t come close to finishing one.) But still, they weren’t eating a “modern” diet as we consider that today. They certainly got a lot of exercise. Were they that fat when they were drafted, while they were eating cornbread, ham and beans at home? We don’t know.
When I was a kid, I picked strawberries in the spring. That will take a lot out of you. Up, down, up, down, bend over, straighten up, twist this way, twist that way. Day-long calesthenics in the blazing sun. Some of my fellow pickers were rail thin. Some were fat and stayed fat. None of them ate what, to us now, would be a “modern diet”. Some of them were really poor, but none of us were wealthy, and not one of us ate processed foods. I remember people whose noon lunch was a slab of cornbread slathered with bacon grease, and were skinny.
I’m willing to stipulate that many Americans are overweight; probably to the detriment of their health, and that many eat awful foods. But, as I said, I truly don’t know that we really do know why given individuals are heavy. And I don’t know that we can say a particular heavy person is unhealthy or that a thin person is necessarily healthy.