I didn't mention fast food, it's grocery store food I was referring to. If you're in a poor area your nearest grocery store is unlikely to have a good selection of fresh produce and similar healthy but perishable items. You're poor so you probably rely on mass transit, that means most foods requiring refrigeration such as fresh dairy can be off the menu due to the travel time to get back home. Maybe you're lucky though and you do have a car and a nearby store with a good fresh food and produce selection. In an urban area it is still more expensive to purchase those fresh perishable items instead of boxed generic mac n cheese to make a quick meal that costs less than one bell pepper. Maybe you're even luckier and you have the car and nice enough store and you can afford the healthy perishable items. Well you're working 1 full time job, 1 part time job, and you have all the other responsibilities life throws at you. Yeah you can buy the healthy stuff but then you have to spend an hour making dinner and you don't have an hour to spare in your day when you make a fast meal that is cheaper anyways and then you have a little bit of money left over to put in your savings account so hopefully you can work your way up to a better life. On top of all that, if you are living in poverty it is likely that you've never been very well educated on eating healthy or taught how to cook healthy meals. There are a lot of reasons the poor easily eat badly and become obese, and it's rarely because they eat McDonalds all the time because like you said, it's expensive, it's that in much cheaper in terms of cost, time, travel, and education to buy unhealthy and fattening but calorie dense boxed food and similar items.