It's probably the sugar. Just anecdotal evidence here, but then most of the primal / paleo evidence is just anecdotal. My boy has to be allowed to see his father on occasion...grrr... Anyway, I got myself a fat little Indian boy 3 years ago when I married his mother. I put him on a primal type eating program just like I am. The boy is now full of energy, feels great all the time, is slender and athletic, strong as any 9 year old should be...and doing great...much better than the sluggish, lazy, sleepy, fat 6 year old I met. Of course parental involvement, taking him out to play, taking him to run sprints and making a game of it, playing ball, etc... didn't hurt at all either.
But back on topic... When he goes to his father's house for a weekend he comes home sick. The culprit? Pasta, pizza, burritos, and ice cream. The same reason his grandparents no longer get to see him...we were tired of them bringing him back sick as in nausea, vomiting, and diarrhea for 2-3 days. When I remind his father how all humans, especially Native Americans, Indians, or whatever the proper word is these days should be eating... he says, "oh yeah, right, I forgot that's probably why all my family died from diabetes." Then does better for the next couple of visits.
Then back to eating bad and getting a sick child home a couple of months later.
I mean if the man wants to eat that stuff and croak himself over sooner...just better for me and my family...then I don't have to share my boy (hey, he is mine now)... But I do wish he wouldn't do this to my kid... But since it's his kid too, I can't stop him. But the anecdotal evidence for how the body rejects the "bad" foods is pretty clear in him.
*NOTE* When my Checz wife eats that stuff, after being primal with me for a year, it's not so rough on her, even though she doesn't feel well for a day afterwards. But my daughter (Scotch Irish, Cherokee, and Seminole like me) gets ill when she eats it, the boy child does too, and so do I.