This used to be my response to anyone who offered me sweets. Then they usually say "What?" I add, "God has called me to give up sweets." Response: "you are so strong, I don't think I could ever do that" or "you are stronger than I am."
Unfortunately, the conversation dies off after that since others don't want the guilt of someone else exercising self-control successfully. Although I tried to give God the glory, I felt it was laid on my feet because of the way I presented the conversation. I do not deserve any glory, so things had to change.
Self-control is a fruit of the Spirit. This is not of my spirit, but the Holy Spirit. And believe me, it is something I tried for years to do concerning food. I could not do it on my own efforts and definitely was not motivated for my health. I could not do it for me.
God called me to do it for Him, so that sweets no longer come before Him. It is only through His enabling do I do anything good. I have no self-control when it comes to food on my own. Only through Him who strengthens me.
So my response is to start with "No thank you," and leave it at that. If pressed, then I can present it to the person, hopefully in a way to bring God glory. In doing that, I must remove myself from the equation and point to Jesus. I can't have a canned response, but respond to each person with the words the Spirit gives me.
Giving up sweets is from God to me and not to be imposed by me on someone else. I do hope to encourage anyone is in bondage over food, that with God's enabling, you can do it to. I was a mess and God cleaned me up. He's still cleaning me. He's turning my mess into my message. Self-control doesn't just happen, it is developed by choosing God's way at each choice. I didn't do it perfectly, but I made progress and that's what matters.
To God be the Glory.