[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.

[Frumious Bandersnatch]

Ironically, we have a less-varied diet here in the states today than we did at the turn of the twentieth century. We had a far more varied diet in fruits, vegetables and meats than we do now. This is not to say that we have access to less now than we did then, it just means that people are eating less of a variety now than they did then.

Of course, what we do eat, we have far more of on a per capita basis. But I daresay that the average Tenochitlan marketplace (that so impressed Cortez and his men) had far more of a variety of produce than does the modern supercenter.