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, 42 years old.
Started Primal October 31, 2011, at a skinny fat 111.5 lbs.
Current weight: 101.5. lbs and holding steady. Spring yardwork here we come!
Co-worker 1: Needs to lose ~50. Now he wants to start Mayo Clinic Diet. Yeesh. Give it up, man.
Co-worker 2: Needed to lose ~55. Lost 20 from stress. Started Primal in Sept, lost 20 more, but gained 10 back on a carb spree. We're working on it.