The upper middle class in USA is fatter than it used to be. Very few people actually lean as opposed to having much less fat. The class disparity is real but not the only explanation.
people assume it's sugar. and a lot of it is and it's the first thing people should cut back on.
but people are surprised zero-calorie artificial sweeteners and crystalized versions of stevia and monk fruit do.
people aren't even aware that omega6s PUFAs (which we're supposed to have a 1:1 ratio with omega3s, and are supposed to have both in fairly low levels) also impact insulin resistance
activity does, sleep does, things impact it positively like a wide range of plant foods or teas.
but more simply - without going into the whys or the science, if generally speaking people cut out A. Sugar and B. processed foods (thus removing a ton of salt and omega6s) people would be a million times better off.
The obvious include: high availability of energy-rich, cheap food, lower amount of physical activity, food designed to be addictive (usually with lots of sugar),...
There is also a strong correlation between obesity and low socioeconomic status, that can be also explained by "obvious" factors, like irregular meals, less physical activity, cheap, energy-rich, nutrient-poor food and poor education regarding healthy habits.
https://www.endocrine.org/topics/edc/what-edcs-are/common-ed...