No such thing as "The American Diet" and even if there was, it would not make anyone obese.
Obesity is solely caused by taking in more calories than you expend. Indians (Feather, not dot) genetically do not need as many calories to survive as do other races and they also posess more fat cells at birth (a good thing in evolutionary terms given the potential effect of famine) which resulted from environmental dietary natural selection thoughout bottlenecked populations.
I hope this helps. (somehow I don`t think it will though)
Indians were taller than their Euro brethen due to the high protein diets. Ever been to a colonial house and had to duck to get in? Euros were smaller until they adopted a diet rich in protein, so in a sense, there is such thing as a American diet, one that originated here.
Also, the current diet that the people of United States have, is considered a American diet, for all the fast food shit originated here.
Ever seen a 6 foot Asian, or a Asian girl with big breasts? I have, and it's mainly due to the high protein diet. There are studies in Japan that show the more Japanese adopt a Western or American diet rich in protein, the women's breasts have become larger, and so has the breast cancer rate.