Quoting from the book:
Whether it’s lactose or kale, quinoa or turmeric (curcumin), you have to ask whether these were foods common at the table when you, your parents, or your grandparents were growing up. If not, it’s best to avoid them or consume them only occasionally.
One side were farmers, the other lower class city folk who got their chickens in cans, and had to over cook pork until it would shatter if you dropped it just to be sure that it was safe to eat.
That generation grew up without refridgeration; there was very little in the way of fresh foos during winter months as everything had to be preserved in cans or dried.
I for one ame glad for our modern food supply.