How to decline politely

Leading on from what I was saying yesterday, I was wondering whether other people find it difficult to decline sugary foods and other things that are bad for the blood sugar in a polite way. I often have people saying 'one won't hurt' and 'you should let yourself have a treat' without understanding that even a small amount of sugar can make a really big difference to your blood sugar. I feel that some people think you are rude to say no to food when it is offered even if you have a medical condition that means you can't have it. Maybe its because people with diabeties so often don't worry about their diet.