IMO, it's the fast food (high in saturated fats), and heavily sugared soda, pop sold in large sizes, 20 ounces or more. Heck, USA has exported fast foods to world, KFC, McDonald's, Taco Bell, etc., etc.
I remember long ago, when soda pop was sold in 6 and 8 ounce bottles, now people will "jumbo size" their drinks, which often have about 1/4 cup of sugar in large sizes.
Likewise too much of our food has corn syrup in it, which is even less healthy than sugar.
But, the real problem often is not what people eat, but the general lack of exercise, with few people walking, or doing regular work outs on a daily basis. Too many jobs are sedimentary, sit-down at computer, or desk jobs.
We don't burn many carbs when answering questions on Yahoo answers, lol