Ok, I might have asked a similar question to this before, so bear with me if I am repeating myself.
I travel to, and live in, Central America part of the year. For the people of the Central America, rice and beans
are staples because they are cheap and filling. The Maya of Belize (where I live part of the year) raise corn and consume it in several forms, (most notably in tortillas) although flour tortillas are consumed with many meals. Meat (except for chicken) is an expensive commodity, usually reserved for special occasions and holidays. All that said, these people are remarkably fit and healthy. I see wizened old men walking to the milpa, swinging a machete all day. I see workers of all sorts working hard and women doing laundry in the river, tending the household etc, all in decent shape (although Mayan women have a tendency to get round after the children are born). Their diet of what primal eaters eschew seems to not affect them in the least; many of them live to be quite old. Is it the amount of exercise and activity that helps them? This is the only thing I can think of, offhand.