One thing I've learned is that providing an excuse invites a conversation about why you don't want it. I just say "No, thank you." If pressed, I repeat, "No, but I appreciate your offer/concern."
I'm currently out of a job and it's a good thing too! The customers at the bank I used to work at bring in plates of home made cookies, fudge, chocolates, and pizzas for us. They'll stand there and watch you eat it to see if you appreciate their gift. While I miss all the awesome customers, I certainly don't miss the sweets and sugars they gave me!