I don't think it's genetics...I've seen Mexican folks living on their diet and never get fat...but get them into the US, and they become blimps (pardon the imagery). I think it's the massive sugar intake...nearly every foreigner that enters American complains about how overly sweet things are in the US...Krispy Kreme Anyone? And you don't just get a 150 ml of coke to down them donuts, you get your 2 gallon jug of your favorite sweet drink at 7-11, and don't forget to super size it at McDees, or get one pizza free when you buy two extra large size....the pattern I see is over consumption...not satisfied with Japanese portions of food....you gotta supersize everything