For those on a diet, how do you refuse certain foods that are offered to you?

You know what the worst thing someone can say to you if you are on a diet? “But it’s Christmas!” or “Just one won’t hurt.”

Food that is either full of sugar, full of fat, or both can be detrimental to a diet, even if it’s “just one”... But you often get relatives or friends, people that you don’t want to offend, offering you things you know you shouldn’t eat. People you know will get offended if you refuse. They often tell you “Just one won’t hurt” or “But it’s Christmas/your birthday/etc”.

My question is… How do you politely refuse these people without offending them?