[Frumious Bandersnatch]

The diets in the U.S. from before the inception of the nation was more than sufficient to support what we needed as individuals and then some. That's not to say it was necessarily equally distributed (it rarely is). Nor does it say that there weren't cases of malnutrition.

I do agree with your point about peoples not reaching their full biological potential due to the lack of sufficient nutrients. I've noticed, however, that there is a tendency amongst some to put this down solely to natural selection rather than a change in diet.

[Question_Assumptions]

I think you need to bear in mind that before the advent of refrigeration and mass food transportation, the diets of people weren't nearly as varied or consistent as they are today and while it's obvious that people had enough to eat during many periods before today, the additional calories needed to support an active pre-modern lifestyle along with the inconsistent consumption of meat, changes in the quantity and variety of food as the seasons changed, food shortages caused by poor harvests, and the interruptions caused by war all took a toll on the diet of people, especially when they were children. Of course diseases and parasites could also take a toll on health and nutrition. You won't find many modern Americans with, say, scurvy or rickets.