I think it is safe to assume that a rapid change in the human genome is not responsible for skyrocketing obesity rates around the world. From what I have read it seems that dietary shifts towards more processed and simple-carbohydrate rich food are the main culprit. Increasingly sedentary lifestyles are accomplices.
These effects are on a statistical and societal level. Also, they don't imply that and individual can just change their diet and easily loose weight. It is possible once someone gains weight their body irreversibly changes so that losing weight becomes much more difficult (I mean irreversible to imply hysteresis, not impossibility).