In the USA, obesity is more common among lower income groups because they make bad decisions and it is true that high carbohydrate fattening foods are cheaper and easier to prepare.

I've had more than one health food nut prove to me, usually in long lectures, that the "cheaper and easier to prepare" stuff is mostly not true.*

People eat high carbs and fats because, generally speaking, they taste good and most people like them better. It's that simple, but like in economics, nobody likes the simple answers.

I have noticed that tastes are changing, though. Every fast food outlet in this area offers low-fat options, usually salads. And not just the cheesy little one-tomato lettuce jobs, either. In the past year McDonalds, Jack-in-the-Box, and Wendy's have fielded some damn good multi-ingredient salad plates, sometimes combined with roasted chicken slices, that are meals in themselves.

When people want veggies, the food guys will sell veggies.

*As the family cook, I have found that "cheap" and "easy" are often opposed to each other. A scratch meal is cheap, but takes time. Frozen entrees and fast food are easy, but expensive. Even the price of a meal at McDonalds for four could buy two or three grocery store meals. I admit I still shoot for "easy" too often.