I wonder if it is that healthy food costs more and cheaper foods are high carb, unnatural and generally less beneficial. A lower income diet deals with more preservatives and additives, cheaper product, cheaper food, cheaper results. Plus, inactive lifestyles and vwoom?
I can understand having genes that make you gain weight more easily being a problem. I've had girls where we ate the same things and she gained weight much faster than I did. Not everything is even, but the general rise in obesity has to be coming from somewhere.