Carbs...plain and simple.
This is alos the reason that blacks and American Indians, specifically, have a genetic predisposition to obesity and alcoholism. Carbs.
Think about it.
For hundreds of years, generation after generation of impovrished minority group in this counrty (in this case blacks and American Indians) have endured a lifestyle consisting of, shall we say, slim pickings. Food supplies were limited, and luxury foods (i.e. meat, etc.) were all but non-existant. To survive, these groups had to sustain themselves on the most calorically dense foods available, as well as foods that would keep during long periods of drought (climactic, financial, or otherwise). This means tens of generations (at least) developing a genetic NEED for, you guessed it--fat and carbohydrates. Carbohdrate rich foods tend not only to keep well for long periods of time, but are also far more inexpensive than protein rich foods (expensive / perish quickly).
The genetic adaptivity of the human body is such that we can affect change in our body in a matter of weeks and months through the mere manipulation of calories (volume and compositon). We all know that if we eat nothing but carbs for extended periods of time, we begin to lose our visible muscle tone and develop a noticable layer of subdermal fat. This is in jsut a matter of weks. Now...take that caloric habitualization and extraoplate it over a year...now, a lifetime...now, 50 lifetimes. What do you have?
You have entire generations of people who have adapted in such a way that their bodies "need" a certain amount of calories, and "need" a certain percentage of those calories through fat and carbs. As food sources became more redily available and droughts virtually non-existant, there were no longer extended periods of caloric deficit during such times as long winters, etc. So, high carb, high fat diets continued unabated, year-round. Therefore, needs were being met well above the "need" level. The net result...caloric surplus.
Of course, the incomes of these minority groups did not ride with the national economic mean, and as a result, they were forced to continue to search for inexpensive sources for nutrients. Lack of money coupled with a pre-disposition for carbohydrate rich foods leads to one simple solution...rice, pasta, bread. Food that is high in calories and low in cost. The cycle continues. The cycle continues to this day.
It is also one of the reasons that blacks and American Indians have such a predisposition to alcoholism. Carbs!
The most effective means by which to deliver carbohydrates into the bloodstream (something their bodies crave on a genetic level), is via alcohol. Ta-da! Generations of, poor, overweight, alcoholics.
It is really a sad story, and it was nice of the U.S. government to be able to tell it to them for hundreds of years. It is a story that continues to this day.