It was actually common in Franklin's time for poor people with limited diets (mainly a single grain crop) to suffer from nutritional deficiencies despite getting adequate calories. Think of conditions like pellagra, goiter, anemia, etc. Of course they didn't fully understand the root causes.

The dietary problem in the US is overconsumption of macronutrients, particularly carbohydrates, not underconsumption of micronutrients, and certainly not underconsumption (of all nutrients) as could be seen in Franklin's time.

Franklin would be amazed at how little of household budgets could go to food now.