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, 45 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 115(!) on SAD chocolate, potato chips, and stress. Currently 111.