Unlike the poor of many decades ago, today's poor seem to get sufficient caloric inputs. Far too much, in many cases. But they are eating the wrong stuff.
Don't start with the 'poor people can't afford healthy food'. For what they spend stuffing their faces with McDonalds, it is possible to eat very healthy. If one isn't lazy and makes an effort to select and prepare healthy food. So, poor people don't have the time to cook? Maybe, but try taking the pop and snack machines out of a high school and replacing them with healthy foods (thus eliminating the argument about preperation effort) and the fat bastards will raise hell.
Poor, fat people are going to be poor and fat. It's a part of their culture.