I take the view that wheat is a low-level drug, everybody is an unwitting addict, and they are unwittingly acting as pushers. No wonder everyone brings stuff to work. No wonder they get angry when we say no. It's exactly like those old anti-drinking and anti-smoking films where the drinkers and smokers get all defensive.
For an excues, definitely use an immediate bad effect like celiac or acne or even Crohn's. Don't bring up the "eventual" diseases like diebates or obesity; that just shines the light back on them. My tried-and-true is heartburn: Since I gave up wheat, I haven't needed to take Prilosec.
5'0" female, 44 years old. Started Primal October 31, 2011, at a skinny fat 111.5 lbs. Low weight: 99.5 lb on a fast. Gained back to 111 on sugar cheat. Currently fighting off sugar/candy cravings with bulletproof cocoa and a little rice.
I (try to) follow by-the-book primal as advocated by Mark Sisson, except for whey powder and a bit of cream. I advocate a two-month strict adjustment for newbies. But everybody is different and should tweak Primal to their own needs.